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Background: Neuromuscular deficits at the trunk and hip may contribute to dynamic knee valgus and anterior cruciate ligament injury mechanisms. However, comprehensive examination of neuromuscular patterns and their mechanical influence is lacking., Objectives: To investigate the influence of lumbar spine joint rotational stiffness (JRS) and the gluteal musculature contribution to hip JRS on dynamic knee valgus., Methods: In this cross-sectional study, 18 university-aged women completed a drop vertical jump while we measured kinematics, kinetics, and 24 channels of electromyography (EMG) spanning the trunk and hip musculature. We classified each limb as high or low valgus, based on frontal plane knee displacement magnitude. We used anatomically detailed, EMG-driven biomechanical models to quantify lumbar spine JRS and muscle contributions to hip JRS., Results: Low-valgus limbs generated greater gluteus medius frontal JRS ( P = .002; effect size, 1.3) and gluteus maximus transverse JRS ( P = .003; effect size, 1.2) compared to high-valgus limbs. Participants with bilateral high-valgus collapse had substantially reduced lumbar spine sagittal JRS compared to the group with low valgus on both limbs ( P = .05; effect size, 5.1). Those with low valgus on both limbs also had a peak lumbar spine flexion angle of 24° ± 4°, compared to the bilateral high-valgus group's angle of 38° ± 10° ( P = .09; effect size, 1.8)., Conclusion: Participants who avoided high medial knee displacement had greater proximal JRS. Increased JRS at the lumbar spine and greater JRS contributions from the gluteal musculature are linked with preventing high medial knee displacement. J Orthop Sports Phys Ther 2019;49(8):601-610. Epub 26 May 2019. doi:10.2519/jospt.2019.8248 .

Objective: The purpose of this study was to evaluate the ability of a pneumatic decompression belt to restore spinal height lost following an acute bout of exercise that induced compression., Methods: This study implemented a test-retest repeated measures design in which twelve participants (male = 10, female = 2) age, 21.5 ± 1.0 years; height, 179.0 ± 7.70 cm; weight, 84.0 ±11.5 kg; were recruited from a university population and acted as their own control. All participants were healthy with no previous history of disabling back pain, and were frequent weight trainers. A stadiometer was used to measure spinal height at baseline, then following an acute bout of exercise and then again following the intervention (use of a pneumatic decompression belt for 20 minutes) or control (lying supine for 20 minutes). A 2-way repeated measures ANOVA was performed on the change in spinal height in order to evaluate differences between measurement phases and intervention conditions., Results: The use of the decompression belt increased spinal height gain (4.3 ± 3.0 mm) significantly more than the control condition (1.8 ± 1.2 mm) following an acute bout of weightlifting exercises known to elicit high compressive loads on the lumbar spine., Conclusion: The pneumatic decompression belt restored spinal height faster than a non-belt wearing condition in young healthy asymptomatic participants., (Copyright © 2016 National University of Health Sciences. Published by Elsevier Inc. All rights reserved.)

Purpose: To describe female lumbar spine motion and posture characteristics during coitus and compare these characteristics across five common coital positions. Exacerbation of low back pain during coital movements and positions is a prevalent issue reported by female low back pain (LBP) patients. To address this problem, the first study to examine lumbar spine biomechanics during coitus was conducted., Methods: Ten healthy males and females performed coitus in the following pre-selected positions and variations: QUADRUPED (fQUAD1 and fQUAD2 where the female is supporting her upper body with her elbows and hands, respectively), MISSIONARY (fMISS1 and fMISS2 where the female is minimally and more flexed at the hips and knees, respectively), and SIDELYING. An electromagnetic motion capture system was used to measure three-dimensional lumbar spine angles that were normalized to maximum active range of motion-a transmitter and receiver were affixed to the skin overlying the lateral aspect of the pelvis and the spinous process of the twelfth thoracic vertebra, respectively. To determine if each coital position had distinct spine kinematic profiles (i.e., amplitude probability distribution function and total range of lumbar spine motion), separate univariate general linear models followed by Tukey's honestly significant difference post hoc analysis were used. The presentation of coital positions was randomized., Results: Female lumbar spine movement varied depending on the coital position; both variations of QUADRUPED, fQUAD1 and fQUAD2, were found to use a significantly greater range of spine motion than fMISS2 (p = 0.017 and p = 0.042, respectively). With the exception of both variations of MISSIONARY, fMISS1 and fMISS2, the majority of the range of motion used was in extension. These findings are most pertinent to patients with LBP that is exacerbated by motions or postures. Based on the spine kinematic profiles of each position, the least-to-most recommended positions for a female flexion-intolerant patient are: fMISS2, fMISS1, fQUAD1, fSIDE, and fQUAD2. These recommendations would be contraindicated for the extension-intolerant patient., Conclusions: The findings provided here may guide the clinician's specific recommendations, including alternative coital positions and/or movement patterns or suggesting a lumbar support, depending on the female LBP patient's specific motion and posture intolerances.

This study investigates the consequences of carrying load in one hand versus both hands. Six participants walked carrying buckets containing various weights. The weight was either carried in one hand or distributed evenly between both hands. Electromyography, force plate and body kinematic data were input to a three-dimensional anatomically detailed model of the spine to calculate spine loading. Carrying loads in one hand resulted in more load on the low back than when the load was split between both hands. When carrying 30 kg in one hand, the low back compression exceeded 2800 N; however, splitting the load between hands reduced low back compression to 1570 N (reduction of 44%). Doubling the total load by carrying 30 kg in each hand actually produced lower spine compression than when carrying 30 kg in one hand. Balancing the load between both hands when carrying material has merit and should be considered when designing work., Practitioner Summary: Carrying a load in one hand (30 kg) resulted in more spine load than splitting the same load between both hands (15 kg). When carrying double the load in both hands (30 kg in each hand vs. 30 kg in one hand), spine load decreased, suggesting merit in balancing load when designing work.

Torso muscles contribute both intrinsic and reflexive stiffness to the spine; recent modeling studies indicate that intrinsic stiffness alone is sometimes insufficient to maintain stability in dynamic situations. The purpose of this study was to experimentally test this idea by limiting muscular reflexive responses to sudden trunk perturbations. Nine healthy males lay on a near-frictionless apparatus and were subjected to quick trunk releases from the neutral position into flexion or right-side lateral bend. Different magnitudes of moment release were accomplished by having participants contract their musculature to create a range of moment levels. EMG was recorded from 12 torso muscles and three-dimensional lumbar spine rotations were monitored. A second-order linear model of the trunk was employed to estimate trunk stiffness and damping during each quick release. Participants displayed very limited reflex responses to the quick load release paradigms, and consequently underwent substantial trunk displacements (>50% flexion range of motion and >70% lateral bend range of motion in the maximum moment trials). Trunk stiffness increased significantly with significant increases in muscle activation, but was still unable to prevent the largest trunk displacements in the absence of reflexes. Thus, it was concluded that the intrinsic stiffness of the trunk was insufficient to adequately prevent the spine from undergoing potentially harmful rotational displacements. Voluntary muscular responses were more apparent than reflexive responses, but occurred too late and of too low magnitude to sufficiently make up for the limited reflexes.

The objective of this study was to investigate 3 different rowing exercises and quantify the muscle activation of the torso and the hip musculature, together with the corresponding spinal loading and stiffness. Seven healthy male subjects from a university population were instrumented to obtain surface electromyography of selected trunk and hip muscles and to obtain spine position using an electromagnetic spine position sensor, together with video analysis to calculate joint moments. The 3 rowing exercises investigated are the inverted row, standing bent-over row, and the standing 1-armed cable row. The inverted row elicited the highest activation of the latissimus dorsi muscles, upper back, and hip extensor muscles. The lower activation of the lumbar erector spinae muscles during the inverted row corresponded to the lower-spine load measured. The standing bent-over row produced large activation symmetrically across the back but produced the largest lumbar spine load. The 1-armed cable row challenged the torsional capabilities of the trunk musculature. Some "core" exercises may be better for rehabilitation (e.g., having the training goals of modest muscle activation with low spine load), while other exercises may be better for athletic training (e.g., resulting in higher muscle activation and larger spine load). When prescribing "core" exercises, those wishing to spare the low back may choose the inverted row given the lowest spine load exercise. The standing bent-over row elicited large muscle activation symmetrically from the upper to lower back, however induced larger spine loads, but not surprisingly the highest spine stiffness. If torsional endurance or strength is the training goal, the 1-armed cable row might be considered.

Strongman events are attracting more interest as training exercises because of their unique demands. Further, strongman competitors sustain specific injuries, particularly to the back. Muscle electromyographic data from various torso and hip muscles, together with kinematic measures, were input to an anatomically detailed model of the torso to estimate back load, low-back stiffness, and hip torque. Events included the farmer's walk, super yoke, Atlas stone lift, suitcase carry, keg walk, tire flip, and log lift. The results document the unique demands of these whole-body events and, in particular, the demands on the back and torso. For example, the very large moments required at the hip for abduction when performing a yoke walk exceed the strength capability of the hip. Here, muscles such as quadratus lumborum made up for the strength deficit by generating frontal plane torque to support the torso/pelvis. In this way, the stiffened torso acts as a source of strength to allow joints with insufficient strength to be buttressed, resulting in successful performance. Timing of muscle activation patterns in events such as the Atlas stone lift demonstrated the need to integrate the hip extensors before the back extensors. Even so, because of the awkward shape of the stone, the protective neutral spine posture was impossible to achieve, resulting in substantial loading on the back that is placed in a weakened posture. Unexpectedly, the super yoke carry resulted in the highest loads on the spine. This was attributed to the weight of the yoke coupled with the massive torso muscle cocontraction, which produced torso stiffness to ensure spine stability together with buttressing the abduction strength insufficiency of the hips. Strongman events clearly challenge the strength of the body linkage, together with the stabilizing system, in a different way than traditional approaches. The carrying events challenged different abilities than the lifting events, suggesting that loaded carrying would enhance traditional lifting-based strength programs. This analysis also documented the technique components of successful, joint-sparing, strongman event strategies.

Objective: To determine the utility of the active straight leg raise (ASLR) test as a screen of lumbar spine stability and abdominal bracing (AB) ability., Design: A biomechanical study of the ASLR test as a clinical evaluation of lumbar spine stability and AB., Setting: Clinical research laboratory., Participants: Fourteen participants who were currently asymptomatic for back pain and leg pain were evaluated., Methods: Spine posture, muscle activation, and pressure distributions underneath the supine subject were determined., Main Outcome Measurements: An estimation of lumbar spine stiffness, a direct correlate with spine stability, was obtained using an anatomically detailed spine model., Results: AB during the ASLR reduced the center of pressure (CoP) movement on a strain-based pressure mat in lumbar rotation (P < .0125) as well as reducing directly measured lumbar rotation (P = .02). Active AB increased lumbar spine stiffness (P < .002). Regression analysis between stiffness and CoP movement suggested that different participants used different strategies to control torso motion., Conclusions: This study demonstrates that the ASLR has utility as a screen of lumbar spine stability and AB ability. The ASLR maneuver can assess control of lumbar rotational movements in the transverse plane. Finally, this study demonstrated that AB can measurably improve the rotational (transverse plane) stiffness of the lumbar spine.

The objective of this study was to investigate 3 different rowing exercises and quantify the muscle activation of the torso and the hip musculature, together with the corresponding spinal loading and stiffness. Seven healthy men from a university population were instrumented to obtain surface electromyography of selected trunk and hip muscles and to obtain spine position using an electromagnetic spine position sensor, together with video analysis to calculate joint moments. The 3 rowing exercises investigated were the inverted row, standing bent-over row, and standing 1-armed cable row. The inverted row elicited the highest activation of the latissimus dorsi muscles, upper-back, and hip extensor muscles. The lower activation of the lumbar erector spinae muscles during the inverted row corresponded to the lower spine load measured. The standing bent-over row produced large activation symmetrically across the back, but it produced the largest lumbar spine load. The 1-armed cable row challenged the torsional capabilities of the trunk musculature. Some core exercises may be better for rehabilitation (e.g., having the training goals of modest muscle activation with low spine load), whereas other exercises may be better for athletic training (e.g., resulting in higher muscle activation and larger spine load). When prescribing core exercises, those wishing to spare the low back may choose the inverted row, given the lowest spine load exercise. The standing bent-over row elicited large muscle activation symmetrically from the upper to lower back; it induced larger spine loads but also, not surprisingly, the highest spine stiffness. If torsional endurance or strength is the training goal, the 1-armed cable row might be considered.

Objective: This study evaluates if side posture lumbar manipulation is associated with a refractory period of the audible "crack" and if so, to quantify this refractory period across subjects., Methods: Three subjects were exposed to multiple "baseline" side posture manipulations until no further audible cracks were recorded. "Test-refractory period" manipulations were administered after a set time (ie, potential refractory period) at which point the number of audible cracks was recorded. The refractory period was declared when a minimum of 50% of the baseline audible "cracks" had recovered during the test manipulations. The study design included 2 clinicians who performed side posture lumbar manipulation on asymptomatic subjects ranging from 38 to 49 years of age., Results: The refractory period was 40 minutes for subject A, 70 minutes for subject B, and 95 minutes for subject C. The average refractory period across subjects was 68.33 minutes. The audible "crack" recovery was maintained for the remaining test days once the refractory period had been met., Conclusions: The audible "crack" heard during side posture lumbar manipulation is believed to originate from the zygapophyseal joints. This is supported by the presence of a refractory period and by the number of audible "cracks" found per manipulation.

Background: The abdominal muscles provide stiffness to the torso in a manner that is not well understood. Their unique anatomical arrangement may modify their stiffening ability with respect to the more commonly studied long strap-like muscles of the limbs. The purpose of this study was to examine stiffness inherent to the trunk, as modified by different torso, and in particular, abdominal muscle activation levels., Methods: Nine healthy male participants were secured in a "frictionless" apparatus and subjected to applied bending moments about either the flexion/extension or lateral bend axes. Abdominal muscle activation levels were modified through biofeedback from the right external oblique muscle. Moment-angle curves were generated and characterized by an exponential function for each of flexion, extension, and right-side lateral bend, at each of four abdominal muscle activation target level conditions., Findings: Stiffness measured in extension increased in a linear fashion throughout the range of motion and increased with each successive rise in abdominal activation. Stiffness in flexion and lateral bend increased in an exponential fashion over the range of motion. In flexion and lateral bend, stiffness increased with each successive rise in abdominal activation from zero to approximately 40% and 60% of the range of motion, respectively. After these points, stiffness at the highest levels of activation displayed a "yielding" phenomenon whereby the torso stiffness dropped below that characterized at lower levels of activation., Interpretation: Increasing torso muscle co-activation leads to a rise in trunk stiffness over postures most commonly adopted by individuals through daily activities (neutral to approximately 40% of maximum range of motion). However, towards the end range of motion in both flexion and lateral bend, individuals became less stiff at the maximum abdominal muscle co-activation levels. The source and mechanism of this apparent yielding are not fully understood; future work will be directed toward elucidating the cause.

Objective: To determine whether the abdominal hollowing technique is more effective for lumbar spine stabilization than a full abdominal muscle cocontraction., Design: Within-subject, repeated-measures analysis of variance was used to examine the effect of combining each of 4 loading conditions with either the hollow or brace condition on the dependent variables of stability and compression. A simulation was also conducted to assess the outcome of a person activating just the transversus abdominis during the hollow., Setting: Laboratory., Participants: Eight healthy men (age range, 20-33y)., Interventions: Electromyography and spine kinematics were recorded during an abdominal brace and a hollow while supporting either a bilateral or asymmetric weight in the hands., Main Outcome Measures: Spine stability index and lumbar compression were calculated., Results: In the simulation "ideal case," the brace technique improved stability by 32%, with a 15% increase in lumbar compression. The transversus abdominis contributed .14% of stability to the brace pattern with a less than 0.1% decrease in compression., Conclusions: Whatever the benefit underlying low-load transversus abdominis activation training, it is unlikely to be mechanical. There seems to be no mechanical rationale for using an abdominal hollow, or the transversus abdominis, to enhance stability. Bracing creates patterns that better enhance stability.

Background: Prolonged sitting is recognized as a risk factor for the reporting of low back troubles. Despite the use of exercise balls in replacement of the office chair, little quantitative evidence exists to support this practice and hence motivated this research. Given the potential for several biological effects and mechanisms this study was approached with several layers of instrumentation to quantify differences in muscle activation, spine posture, spine compression and stability while sitting on an exercise ball versus a stable seat surface. Also, differences in the pressure distribution at the seat-user interface were quantified for the different seat surfaces to provide an objective perspective on the mechanism influencing perceived comfort levels., Methods: Eight male subjects volunteered to sit for 30 min on an exercise ball and on a wooden stool. Muscle activity and spine position were used to model spine load and stability. An additional seven sat on an exercise ball and chair to examine pressure distribution over the contact area., Findings: There was no difference in muscle activation profiles of each of the 14 muscles between sitting on the stool and ball. Calculated stability and compression values showed sitting on the ball made no difference in mean response values. The contact area of the seat-user interface was greatest on the exercise ball., Interpretation: The results of this study suggest that prolonged sitting on a dynamic, unstable seat surface does not significantly affect the magnitudes of muscle activation, spine posture, spine loads or overall spine stability. Sitting on a ball appears to spread out the contact area possibly resulting in uncomfortable soft tissue compression perhaps explaining the reported discomfort.

Objective: To evaluate the neuromuscular strategy adopted during sitting balance on an unstable surface in the frontal plane., Design: Electromyographic evaluation of trunk muscles., Setting: University spine biomechanics laboratory., Participants: Seventy asymptomatic men (mean age, 34.5 y)., Interventions: Not applicable., Main Outcome Measures: "Balancers" and "nonbalancers" were identified by principal component analysis of their lumbar spine side flexion angle during sitting balance. Average electromyographic levels were used as a measure of muscle activation. Pearson correlations were used to identify coactivation versus asymmetrical muscle activation of opposite muscle groups., Results: External oblique, internal oblique, and thoracic erector spinae (TES) were most active, and most likely to be used asymmetrically, with other muscles showing low levels of coactivation. Between groups, the average electromyographic levels in the balancers was lower than in the nonbalancers (P<.05), with further differences in the symmetry of external oblique, internal oblique, and TES activation between groups., Conclusions: Sitting balance in the frontal plane appears to involve a combined feedforward-feedback strategy of muscle activation. Successful balance was characterized by low levels of muscle coactivity, along with higher levels of asymmetric activation in the global trunk muscles, specifically external oblique, internal oblique, and TES.

Quantifying the stability of the spinal column offers a perspective on the effectiveness of the motor control strategy to ensure a stable spine--and minimize the risk of injury from experiencing an unstable event. There are essentially three energy based methods of calculating a stability index for the lumbar spine. All three methods involve mathematical manipulation of an 18 x 18 Hessian matrix. The purpose of this paper was to consider the mathematical implications for the three methods of determining a single stability index, and examine the effects of biological factors such as muscle activation in each of these methods. The first approach computes the Hessian's determinant and is thought to represent a more global or "average" perspective on stability. A second approach computes the smallest eigenvalue of the Hessian matrix to determine the weakest link of the spine. The final method determines an average critical stiffness difference for the spine and is intended to effectively determines how far a human spine is from instability, and allows comparison between tasks. This study shows that the same interpretation of stability is achieved via all three computational approaches--they agree as to whether the spine is stable or not. However they appear to differ in their sensitivity to the effect of muscle activation patterns.

Study Design: Sixty-four asymptomatic participants, ranging in age from 22 to 49 years, volunteered to act as patients for the study. Twenty-eight different clinicians performed thoracic and lumbar spinal manipulative procedures. The range of clinical experience was 1 to 43 years., Objectives: The purpose of this study is to first locate the joints that produce an audible sound in response to manipulation (cavitation) during spinal manipulative procedures so that the accuracy and specificity of manipulation can be assessed., Summary of Background Data: Clinicians utilizing spinal manipulative therapy (SMT) claim to be very specific and accurate with the delivery of their dynamic thrust. It has been suggested that the clinical success of SMT is dependent on the accurate delivery of that therapy to the target spinal joints., Methods: Asymptomatic participants received SMT to either the thoracic or lumbar regions of their spine. Accelerometers were secured to the skin over the spinal column, and the relative time at which each accelerometer detected the vibration from the cavitation associated with the SMT was used to calculate the source of the vibration. The site of cavitation was then compared with the target location., Results: For lumbar SMT, the average error from target of 124 cavitations in lumbar procedures was 5.29 cm (at least one vertebra away from target), with a range of 0 to 14 cm. Of these cavitations, 57 were deemed to be accurate and 67 were deemed to be inaccurate. The average error from target of 54 cavitations in the thoracic spine was 3.5 cm, with a range of 0 to 9.5 cm. Of these cavitations, 29 were deemed to be accurate and 25 were deemed to be inaccurate. In most cases, individual manipulative procedures were associated with multiple cavitations ranging from 2 to 6., Conclusions: In the lumbar spine, SMT was accurate about half the time. However, because most procedures were associated with multiple cavitations, in most cases, at least one cavitation emanated from the target joints. In the thoracic spine, SMT appears to be more accurate.

Study Design: A systematic biomechanical analysis involving an artificial perturbation applied to individual lumbar muscles in order to assess their potential stabilizing role., Objectives: To identify which torso muscles stabilize the spine during different loading conditions and to identify possible mechanisms of function. SUMMARY OF BACKGROUND DATA.: Stabilization exercises are thought to train muscle patterns that ensure spine stability; however, little quantification and no consensus exists as to which muscles contribute to stability., Methods: Spine kinematics, external forces, and 14 channels of torso electromyography were recorded for seven stabilization exercises in order to capture the individual motor control strategies adopted by different people. Data were input into a detailed model of the lumbar spine to quantify spine joint forces and stability. The EMG signal for a particular muscle was replaced either unilaterally or bilaterally by a sinusoid, and the resultant change in the stability index was quantified., Results: A direction-dependent-stabilizing role was noticed in the larger, multisegmental muscles, whereas a specific subtle efficiency to generate stability was observed for the smaller, intersegmental spinal muscles., Conclusions: No single muscle dominated in the enhancement of spine stability, and their individual roles were continuously changing across tasks. Clinically, if the goal is to train for stability, enhancing motor patterns that incorporate many muscles rather than targeting just a few is justifiable.

Background and Purpose: Physical therapists commonly attempt to reduce and prevent low back pain by "improving" individuals' lumbar posture. To investigate the physical therapy clinical practice of attempting to "improve" lumbar posture, measures of passive tissue stiffness and angular deformation during activities of daily living were used., Participants: The lumbar spine posture of 150 university students was measured as the inclinometer angle difference between L1 and S1. Eighteen female participants (6 with hypolordosis, 6 with hyperlordosis, and 6 controls without lumbar spine impairment) were recruited from this lumbar posture database. Hypolordosis and hyperlordosis were clinically classified by physical therapists., Methods: Lumbar passive tissue stiffness was measured during sitting, standing, and walking before and after a 12-week exercise program, and estimates of lumbar passive tissue strain were calculated from those measurements., Results: The neutral zone (NZ), a range of lumbar positions of low passive tissue stiffness, was identified. Prior to training, the subjects with hypolordosis had more passive tissue strain during sitting than the subjects with hyperlordosis, and the subjects with hyperlordosis stood in extension relative to their NZs while the control subjects and subjects with hypolordosis stood within their NZs. Before and after training, subjects in all 3 groups walked with lumbar spine positions within their NZs. After training, the lumbar posture of the subjects with hypolordosis and the subjects with hyperlordosis changed toward a "mean" (mid-range) lumbar posture. After the exercise program, subjects in all 3 groups stood and walked with their lumbar spines in positions within their NZs, and they sat with their lumbar spines flexed relative to their NZs., Discussion and Conclusion: Knowing that tissue failure can be related to passive tissue strain, the results of this study support the clinical practice of attempting to change individuals' posture-related lumbar spine positions during activities of daily living. Lumbar passive tissue strain, as estimated from the NZ and angular deformation during activities of daily living, can be decreased, but can also be increased, by an exercise program.

The intention of this paper is to introduce some of the issues surrounding the role of muscles to ensure spine stability for discussion -- it is not intended to provide an exhaustive review and integration of the relevant literature. The collection of works synthesized here point to the notion that stability results from highly coordinated muscle activation patterns involving many muscles, and that the recruitment patterns must continually change, depending on the task. This has implications on both the prevention of instability and clinical interventions with patients susceptible to sustaining unstable events.

The sit-and-reach (S&R) test is often included in standard fitness tests (e.g., Canadian Physical Activity, Fitness and Lifestyle Appraisal [CPAFLA]), justified on the assumption that it is an indicator of low back health. Two issues were examined here: Is low back flexibility linked to having a history of low back disorders? And is the S&R test an indicator of low back flexibility? The relationship between S&R test scores, lumbar range of motion, and having a history of low back discomfort was examined in 72 asymptomatic (at test time) industrial workers (70 M, 2 F; mean age 35 ys; height 1.79 m; mass 84.7 kg). The S&R test, among many collected, was performed according to the CPAFLA guidelines. History of low back discomfort (LBD) was categorized based on whether or not time was lost from work. The S&R test was unable to distinguish between those with a history of LBD and those without. Specific lumbar sagittal range of motion could make this distinction. A moderate correlation (r = 0.42) surfaced between S&R and lumbar flexibility. This study suggests that the value of S&R as an indicator of previous back discomfort is questionable and there may be better indicators for inclusion in the CPAFLA.

Study Design: A clinical convenience sample of workers with low back problems was tested once for a variety of parameters., Objectives: To quantify the links between lumbar range of motion and scores obtained from functional evaluation tests., Summary of Background Data: Although lumbar range of motion traditionally has been the standard used to determine disability for the purposes of compensation, a concern arises from the fact that a discrete physical impairment associated with low back pain and diminished range of motion often is never found., Methods: This study involved 18 workers with chronic low back pain referred to a rehabilitation center for determination of compensation and fitness for return to work. Lumbar range of motion was precisely and accurately measured with a three-dimensional lumbar motion instrument. In addition, typical functional tests were performed for each patient. Comparisons were made between all the lumbar range of motion values and each of the functional ability scores., Results: The most notable finding was the lack of significant correlation between most of the lumbar range of motion values and the functional test scores. Only 9 of a possible 144 correlations were significant., Conclusions: The relation between lumbar range of motion measures and functional ability is weak or nonexistent. This has implications for clinical practice as it relates to disability determination for patients with chronic low back pain, and perhaps for the current impairment guidelines of the American Medical Association.

Introduction: General practice in many team sports is to have the athletes who do not start in a game sit on a bench while waiting to play. The purpose of this study was to examine the effect of a warm-up followed by bench rest on the stiffness of the lumbar spine in athletes., Methods: Nine varsity-level volleyball players volunteered to have their lumbar-spine stiffness measured. The protocol consisted of an initial stiffness measurement followed by a 30-min warm-up, then another stiffness measurement, then 30 min of bench rest, and finally a third stiffness measurement., Results: In general, lumbar spine stiffness increased as a result of bench rest after a warm-up. This effect was seen in both the spine extension and lateral bend axes but not in the flexion or axial twist axes. However, there was no decrease in stiffness associated with the active warm-up portion of the task., Conclusions: It was concluded that a warm-up followed by bench rest does lead to an increase in stiffness of the lumbar spine, suggesting this practice is not in the best interest of reducing the risk of back injury or optimal performance.

Study Design: Analytic cohort with a convenience sample in a research clinic., Objectives: To determine the influence of a spinal manipulation on trunk kinematics and associated trunk myoelectric activity., Summary of Background: While the mechanism of spinal manipulation is unknown, it has been theorized to influence spinal range of motion and trunk muscle activity., Methods: Trunk kinematics were measured in low back pain patients (n = 14) during simple range of motion tasks in three planes, while trunk muscle electromyogram signals were recorded bilaterally from paraspinal and abdominal musculature. Kinematics and electromyogram signals were assessed pre-post manipulation. Electromyogram activity was also assessed pre-post manipulation during quiet stance., Results: While no consistent kinematic or electromyographic changes occurred following manipulation across the population, individual changes were observed. The largest changes (> 6 degrees ) in range of motion occurred in the sagittal plane of three patients experiencing the greatest amount of pain. During quiet stance 17 muscles across all subjects exhibited changes in muscle activity following manipulation. Sixteen of those changes were decreases in muscle amplitude., Conclusions: This study offers some preliminary data on the short-term effects of manipulation on lumbar range of motion and dynamic electromyogram. The findings suggest that the response to manipulation is variable and dependent on the individual, with no change in some to the largest changes seen in the more pained patients. Relevance. Basic science investigations into the mechanisms and biomechanical influences of spinal manipulation are few. This study attempts to address issues of measureable functional change with manipulative therapy.

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible -- particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p<0.001) low back compressive loads (mean +/- SD 1698 +/- 467 N) than those experienced by the lumbar spine during standing (1076 +/- 243 N). Subjects were equally divided into adopting one of two sitting strategies: a single 'static' or a 'dynamic' multiple posture approach. Within each individual, standing produced a distinctly different spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.

Low back stability: from formal description to issues for performance and rehabilitation. Exerc. Sport Sci. Rev. Vol. 29, No. 1, pp. 26-31, 2001. The concept of stability, together with notions of design and the application of stabilization exercise, is briefly synthesized. The objective is to challenge muscle systems to achieve sufficient functional stability but in a way that spares the spine of excessive exacerbating load.

Study Design: Fiber angles of longissimus thoracis and iliocostalis lumborum at L3 were documented in vivo, using high resolution ultrasound, with the lumbar spine in neutral curve and when fully flexed., Objectives: To evaluate the effect of changes in lumbar curvature on the mechanics of these muscles., Background: Full flexion modifies the failure tolerance of the lumbar spine, determines the load distribution among muscle and passive tissues, and modulates the types of tissue damage that occur. Related to this issue are the possible changes in muscle line of action with full flexion which changes the ability of the spine to support shear loads., Methods: Nine normal men and 5 normal women were scanned in three positions: (1) an upright standing posture; (2) with the hips flexed to approximately 30 degrees and the spine fully flexed; (3) hips flexed but the spine returned to a neutral curvature., Results: Mean longissimus/iliocostalis fiber angles for upright standing, hips flexed-spine flexed, and hips flexed-spine neutral lordosis were 25. 7 degrees, 10.7 degrees and 28.3 degrees, respectively., Conclusions: Anterior shear load on the lumbar spine has been recently shown to be highly related to the risk of reporting a back injury. Bending forward allowing the spine to fully flex changes the line of action of the largest lumbar extensor muscles compromising their role to support anterior shear forces. Relevance Fiber angles of longissimus thoracis and iliocostalis lumborum were documented with high resolution ultrasound at L3, with the spine in neutral curvature and fully flexed. Full lumbar flexion changes the line of action of these muscle compromising their role to support anterior shear forces on the spine - anterior shear forces have been recently documented to be highly related to the risk of reporting a back injury.

Owing to the recent interest in torso stabilization exercises together with many questions regarding the duration of prolonged isometric holds in occupational settings, the authors attempted to assess the level of back muscle oxygenation during prolonged isometric contractions. Specifically, this study recorded relative oxygen saturation of haemoglobin/myoglobin using Near Infrared Spectroscopy (NIRS) in the L3 erector mass during prolonged isometric contractions at intensities from 2 to 30% of maximum voluntary contraction (MVC). It was hypothesized that available oxygen to these muscles is severely compromised even at moderate levels of activation observed in occupational work. Eight volunteers without a history of lower back pain or injury participated in this study. The exercise task involved isometric contraction of the lower erector spinae at five different levels of each subject's maximal voluntary contraction: 2, 5, 10, 20 and 30% MVC, presented in random order. Subjects were placed in a sitting position, with a curved plastic plate secured horizontally to the pelvis to minimize movement at the hip joint. During extensor exertions, they were restrained with a harness that was attached at chest level to a load cell. Each isometric contraction was performed for 30 s followed by 1 min of rest. All levels of contraction demonstrated reduction in oxygen. Given the concern for motion artefact on the NIRS signal, sham trials were conducted where the subjects went through the procedure of attaching the pulling cable but no active pull was performed. These trials showed no change in the NIRS signal. At this time NIRS appears to be the only non-invasive instrumentation available to indicate total available muscle oxygen during low level, prolonged work. Although the specific tissue volume sampled by NIRS cannot be positively identified, it appears that tissue oxygenation in the lumbar extensor musculature is reduced as a function of contraction intensity, even at levels as low as 2% of MVC. These data have implications for prolonged work where postures requiring isometric contractions are sometimes held for hours, and where musculoskeletal illness has been linked to prolonged contraction levels above 2%MVC--these data suggest a possible biological pathway.

Objective: To investigate whether a more sophisticated and detailed analysis of both simple and complex tasks may yield more information regarding the short-term influence of an adjustment on spine biomechanics., Methods: The study used a single-subject, before-after design. Three-dimensional spine kinematics and trunk muscle electromyography were assessed during a variety of tasks performed by a professional golfer exhibiting non-specific, chronic, low back pain. The patient received a right-to-left and left-to-right spinous pull adjustment., Results: After the adjustment, changes were seen in all 3 axes of motion during a golf swing, with concomitant muscle responses. In addition, changes in the off axes of motion were seen during simple movement tasks., Conclusions: A more detailed spine kinematic analysis, specifically analysis of motion in the nonprincipled axes, yielded more information regarding the short-term influence of an adjustment on lumbar spine motion and muscle function.

Animal models for analysis of spine injury and orthopaedic issues are common given concerns about bone integrity, disc degeneration, and controlled studies of identical specimens matched for age, weight, physical activity and genetic background. Given this asset, the question is asked: "Is the porcine cervical spine a reasonable model of the human lumbar spine?" Three porcine cervical spines (C2-C7) were assessed for geometric characteristics, with a larger cohort (N = 24) loaded to failure under compressive or shear loading. In addition, in vivo loading was estimated and compared between the human low back (biped) and the porcine neck (quadruped). Generally, the porcine vertebrae are smaller in all dimensions. The porcine vertebrae have anterior processes unlike humans; however, they possess similar ligamentous structure and facet joint orientation. Stiffness values (compression and shear) are similar, and comparable injuries resulted from applied compressive and shear loads. Given the scarcity of healthy, young human lumbar spines, porcine cervical spines may be a useful model for studying human lumbar injury because of the similarity of mechanical characteristics and the resulting injuries, particularly of the adolescent or young adult who has not experienced disc degeneration or calcified end-plates.

Objective: To establish a database of lumbar spine kinematics and muscle activation profiles for a healthy elderly population., Design: Spine motion parameters and muscle activation profiles of the elderly were identified and quantified in part by comparison with an existing database of younger people., Background: Databases are often used as a benchmark to establish what is "normal", or for developing appropriate exercise programs or diagnostic methodologies., Methods: Twelve people (average age 69 yr) performed full range of motion movements about the flexion, lateral bending and axial twist axes of the lumbar spine. Fourteen torso muscles (7 per side) were monitored with electromyographic electrodes and the signals normalized to maximum voluntary efforts., Results: The elderly were slower moving, and had a reduced range of motion in full flexion and lateral bend but not in axial twist. Furthermore there was more coupled motion evident in the twisting efforts of the elderly. Abdominal muscles appeared to become more active, earlier in the lateral bending movement., Conclusions: Loss of motion is normal with age together with subtle changes in the way the spine moves and groups of muscles are recruited around the torso., Relevance: Quantification of "normal" changes associated with aging will enable clinicians and scientists to distinguish normal changes from pathology.

Objective: The purpose of this study was to examine the three-dimensional low back loads, spinal motions, and trunk muscular activity during gait. Specific objectives involved assessment of the effects of walking speed, and arm swing on spinal loads, lumbar spine motion, and muscular activation., Design: An in vivo modeling experiment using five male participants. Thirty walking trials were performed by each participant yielding five repeats of each condition (3 walking cadences x 2 arm swing conditions)., Background: Walking is often prescribed as a rehabilitation task for individuals with low back injuries. However, there are few studies which have examined the joint loading, spinal motions, and muscular activity present when walking. Additionally, the majority of studies examining spine loading during gait have used an inverse dynamics model, commencing at the cranial aspect of the body, approach which does not include the impulsive phases of gait (i.e. heel strikes and toe offs)., Methods: Low back joint forces (bone on bone) and moments were determined using an anatomically complex three-dimensional model (detailing 54 muscles and the passive structures acting at the low back) during three walking cadences and with free arm swing or restricted arm swing. In order to assess the influence of the transient factors such as heel contact on the joint forces a bottom up (from the feet to the lumbar spine) rigid link segment analyses approach was used as one input to the three-dimensional anatomic model. Lumbar spine motion and trunk muscle activation levels were also recorded to assist in partitioning forces amongst the active and passive tissues of the low back., Results: Net joint anterior-posterior shear loading was the only variable significantly affected by walking cadence (fast versus slow P < 0.0003). No variable was significantly affected by the arm swing condition. Trends demonstrated an increase in all variables with increased walking cadence. Similarly, most variables, with the exception of axial twist and lateral bend lumbar spine motion and lateral joint shear, demonstrated increasing trends caused by the restriction of normal arm swing., Conclusions: Tissue loading during walking appears to be below levels caused by many specific rehabilitation tasks, suggesting that walking is a wise choice for general back exercise and rehabilitation programs. Slow walking with restricted arm swing produced more 'static' lumbar spine loading and motion patterns, which could be detrimental for certain injuries and tissues. Fast walking produced a more cyclic loading pattern.

Objective: To document the three-dimensional (3D) kinematics of the lumbar spine and the myoelectric activity of trunk muscles throughout the range of motion in selected tasks performed by healthy subjects., Design: A survey of a single population., Setting: A research laboratory in a university setting., Participants: Twenty-four men and women, university-age, with no history of chronic lower back pain volunteered for this study., Interventions: The selected tasks consisted of lateral bend and axial twists to the right and left plus four different flexion-extension tasks that varied a hand-held weight (10kg and 0kg), and flexion-extension velocity (free-paced and 20 degrees/sec)., Main Outcome Measure: 3D lumbar spine kinematics and normalized, linear enveloped electromyogram (EMG) of trunk muscles., Results: The motion time histories were illustrated graphically. Consistent spinal kinematics and EMG activation patterns during each task across subjects were seen. Peak displacement values between tasks, using one-way analysis of variance, showed no significant difference., Conclusion: A database of healthy spine kinematics and normalized linear enveloped trunk muscle activation patterns were developed for future comparison of elderly and pathological subjects.

Successful rehabilitation programs arise from the balance of wisdom obtained through scientific laboratory experiment, and the "art" developed from clinical experience. Specifically, choosing the best exercises is enhanced by knowledge of the resultant tissue loads to reduce the risk of injury exacerbation and to strengthen healthy supporting tissues. The following report attempts to briefly describe a technique to obtain tissue load distribution, together with examples of tissue loads during the performance of some selected tasks. For example, it appears that those persons with shear injury to the vertebral joint (including facet, neural arch damage, or spondylolisthesis) or posterior ligament damage should avoid fully flexed postures given the resultant tissue loading. Specific data are provided to guide selection of various abdominal and low back exercises. Finally, some hypotheses and opinions are offered for possible improvements in clinical success.

The purpose of this paper is to introduce some concepts of low back injury for use towards developing better injury risk reduction strategies and advancing rehabilitation of the injured spine. Selected issues in low back injury are briefly reviewed and discussed, specifically, the types of tissue loads that cause low back injury, methods to investigate tissue loading, and issues which are important considerations when formulating injury avoidance strategies such as spine posture, and prolonged loading of tissues over time. Finally, some thoughts on current practice are expressed to stimulate discussion on directions for injury reduction efforts in the future, particularly, the way in which injuries are reported, the use of simple indices of risk such as load magnitude, assessment of the injury and development of injury avoidance strategies. This paper was written for a general biomechanics audience and not specifically for those who are spine specialists.

While most existing models that predict loads on the low back for occupational risk analysis are restricted to assessing moments in the sagittal plan, a few have the ability to determine spine compression from three-dimensional (3-D) loading. The objective of this work was to find a method to estimate low-back compression forces during 3-D loading tasks from a model that contains as much biological content validity as the authors could incorporate, such as the effects of muscle co-contraction, but was simple enough to be implemented into a model appropriate for industrial use. The problem that had to be solved was how to represent the anatomical reality that a given muscle force vector contributes simultaneously to all three moments, flexion or extension, lateral bend and axial twist. Simply summing the compression components of independent equivalent muscles in each plane unrealistically assumes that muscles are uncoupled (i.e. each one works independently to support only one moment). In this study, loads in the various tissues of the low back that resulted from the simultaneous generation of moments about the three orthopaedic axes during 3-D tasks were estimated by an anatomically detailed 90 muscle model. A four-dimensional regression equation was developed to predict low-back compression from the three moments generated about the three axes (R2 = 0.94). Comparison of compression estimates from an uncoupled model showed that accounting for muscle coupling reduces compression by 22% on average. The predictive equation is presented to simplify analysis of complex 3-D industrial tasks for those who would like to incorporate it into their own models that produce moments of force in three plans of the L4/L5 level of the lumbar spine.

Study Design: This study analyzed anatomic and neural control characteristics of the trunk musculature. Subjects were exposed to external shear and compressive loads with equivalent moments to evaluate activation patterns and loading on the low back., Objectives: The migration of activity between the thoracic and lumbar erector spinae muscle groups was examined to determine whether the motor control system chooses to minimize joint loading by recognizing differences in moment, compression, and shear support requirements and assigning muscle activation in the most appropriate way., Summary of Background Data: Loads were applied either parallel or perpendicular to the low back to create compressive or shear forces. No previous study has attempted to isolate the response of the trunk musculature with the type of external load., Methods: Eleven male subjects isometrically held an external load that was altered to create either a compressive or an anterior shear load on the low back but with equal extensor (reaction) moments (experiment 1). In a second experiment four men repeated the task with an increased range of applied loads (5-25 kg) together with measurements of intra-abdominal pressure., Results: The tasks with a compressive external load resulted in significantly higher levels of activation for all seven electromyographic channels recorded. Intraabdominal pressure, compressive and shear joint forces were all higher in the compression loading method when equal loads and low back moments were compared., Conclusions: It was concluded that the motor control system does not arrange muscle activation levels in a way to minimize lumbar spinal loading at least for the relatively low levels of this study. Biomechanical models that use the objective criterion of minimum joint load may not be representative of the motor control system, at least in the low back.

The purpose of this study was to use anatomical data obtained from cadavers, and geometrical scaling data obtained from MRI scans of living subjects, to assess the line of action and mechanical function of the psoas major muscle in three dimensions about each lumbar spine level. In addition, the line of action of the psoas major was documented as a function of lordosis. A total of seven cadavers were dissected from which fibre/tendon architecture was measured, while MRI scans were performed on 15 males to obtain centroid paths and area scales of the muscle over its length. In this way, the curving path of muscle line of action was accommodated together with force and moment predictions that recognized the presence of a tendon at lower lumbar levels (up to L3 in some subjects) significantly increasing the stress. Results confirm that the mechanics of the psoas cannot be adequately represented with a series of straight line vectors from vertebral origins to insertion. Moreover, the mechanical action of the psoas major does not change as a function of lumbar spine lordosis as the muscle path of action changes in accordance with changes in spine posture. Functionally, contrary to claims, the psoas cannot act as a 'derotator' of the spine, does not impose large shear forces on the spine in any posture except at L5-S1, and cannot have major affects to 'control lordosis'. It has the potential to stabilize the lumbar spine with compressive loading and with bilateral activation, to laterally flex it, and can create large anterior shear forces but only at L5-S1.

The purpose of this study was to determine whether the same estimates of individual muscle and L4/L5 lumbar joint compressive forces result from an optimization (OPT) compared to an electromyography (EMG) assisted approach for solving the inderminate moment equilibrium equations in the same anatomical model. Four male subjects performed near maximum, isometric, ramp efforts in trunk flexion, extension and lateral bending in a testing apparatus. The EMG approach was sensitive to subject and trial differences in the magnitudes of individual muscle forces needed to produce the same reaction moment. In contrast, the OPT method converged on a similar estimate of muscle forces for all subjects and trials producing the same moment. The OPT method predicted lower L4/L5 joint compression values, on average, by 32, 43 and 23% in trunk extension, flexion and lateral bending, respectively, because, unlike the EMG method, it could not predict co-contraction of anatomically antagonistic muscles. We incorporated the OPT method's advantage of forcing an equilibrium in the reaction moments into the EMG method in a new approach we have called 'EMG assisted optimization' (EMGAO). Muscle force estimates from the EMG and EMGAO methods differed from those from the OPT method, on average, by 123% (RMS) for flexion and extension and by 218% for lateral bends. Data from the two approaches result in different conclusions about spine mechanics. We have more confidence in the EMG assisted methods because they respond to variation in muscle synergy and co-contraction patterns commonly observed in different trials and subjects for the same reaction moments.

Study Design and Methods: This study used an anatomically detailed model of the lumbar tissues, driven from biologic signals of vertebral displacement and myoelectric signals, to estimate individual muscle and passive tissue force-time histories during the performance of the "flexion-relaxation" maneuver. Eight male university students performed three trials each of the "flexion-relaxation" maneuver with six pairs of surface myoelectric electrodes monitoring the right side of the trunk musculature, an electromagnetic device to record lumbar flexion, and videotape to record body segment displacement., Objectives: To examine the loads on individual tissues during the transfer of moment support responsibility from predominantly active muscle to predominantly passive tissue., Summary of Background Data: No previous studies, to the authors' knowledge, have examined individual tissue loading during the flexion-relaxation maneuver., Results: Although most subjects were able to "relax" their lumbar extensors in full flexion, activity remained in the thoracic extensors and abdominals. Tissue load predictions suggested that while the lumbar extensor muscles were neurally "relaxed" (i.e., myoelectric silence), substantial elastic forces would assist the passive tissues in extensor moment support. On average, subjects sustained almost 3 kN in compressive load on the lumbar spine and about 755 N of anterior shear during full flexion with only 8 kg held in the hands., Conclusions: The "relaxation" of lumbar extensor muscles appeared to occur only in an electrical sense because they generated substantial force elastically through stretching. Loading of the interspinous and supraspinous ligaments, in particular, was high relative to their failure tolerance.

Force contributions from the facet complex and posterior ligaments during the generation of axial torque are a function of lordosis, and it has been speculated that these forces together with muscular contributions play a role in axial trunk twisting. This study investigated the electromyographic activity of the trunk musculature and torque-generating capacity of the lumbar spine under the conditions of normal lordosis, hyperlordosis, and hypolordosis. Eleven male subjects volunteered for this study. The subjects performed isometric twisting efforts and maximum dynamic twisting efforts at 30 degrees/sec. The myoelectric activity levels (normalized to maximal amplitude obtained from nontwist activities) were quite low despite maximal efforts to generate axial torque (for example: approximately 60% maximum voluntary contraction for latissimus dorsi and even lower for the abdominals). Furthermore, changes in lordosis did not produce any consistent changes in muscle activity, although a hyperlordotic spine produced significantly smaller axial torques, and a hypolordotic spine smaller still. Larger torques were measured during all three conditions of lordosis, as the subjects rotated toward an untwisted position, and lower torques as the subjects rotated away. The opposite trend was observed, however, in myoelectric activity of the agonistic side of latissimus dorsi, the thoracic level of erector spine, and the lumbar level of erector spinae, i.e., larger amplitudes were observed as the trunk was twisted away from the untwisted position. These data suggest that tissues other than muscle (i.e., passive tissue) contribute significantly to axial torque production and that the flexed and twisted spine is less able to resist applied axial torques, possibly increasing the risk of torsional injury.

This work describes a dynamic model of the low back that incorporates extensive anatomical detail of the musculo-ligamentous-skeletal system to predict the load time histories of individual tissues. The dynamic reaction moment about L4/L5 was determined during lateral bending from a linked-segment model. This reaction moment was partitioned into restorative components provided by the disc, ligament strain, and active-muscle contraction using a second model of the spine that incorporated a detailed representation of the anatomy. Muscle contraction forces were estimated using both information from surface electromyograms, collected from 12 sites, and consideration of the modulating effects of muscle length, cross-sectional area and passive elasticity. This modelling technique is sensitive to the different ways in which individuals recruit their musculature to satisfy moment constraints. Time histories of muscle forces are provided. High muscle loads are consistent with the common clinical observation of muscle strain often produced by load handling. Furthermore, the coactivation measured in muscles on both sides of the trunk suggests that muscles are recruited to satisfy the lateral bending reaction torque in addition to performing other mechanical roles such as spine stabilization. If an estimate of the intervertebral joint compression is desired for assessment of lateral bends in industry, then a single equivalent lateral muscle with a moment arm of approximately 3.0-4.0 cm would conservatively capture the effects of muscle co-contraction quantified in this study.

The mechanical role of the lumbar posterior ligaments during lifting tasks remains controversial. This study was designed to assess the ligament and disc contribution in resisting trunk flexion moment during extremely heavy lifts performed by national class powerlifters. Direct measurements of lumbar vertebrae kinematics in sagittal plane were obtained from videofluoroscopy utilizing multiple digitizing, correction for optical distortions and digital filtering. Four experienced powerlifters executed three trials, resulting in about 72 mA s of total radiation exposure. In the first trial, joint angles were measured when subjects fully flexed their spines to a point where the passive tissues resisted the flexor moment creating myoelectric silence in the extensor musculature. Next, two conventional deadlift style lifts were executed with the barbell load ranging from 183.7 to 210.9 kg. Four vertebral corners were digitized at a sampling rate of 30 Hz. The relative intervertebral joint angles, distance between the ligament attachment points, shearing and compressive displacements were calculated from a rigid body motion approach. Analysis revealed that except for one trial of one subject, they accomplished their lifts with an amount of lumbar flexion between 1.5 and 13 degrees less than they demonstrated during full flexion. Resultant ligament lengths at the beginning of the lifts ranged from 56.1 to 99.8% of their lengths when the trunk was fully flexed. It was concluded that ligaments did not strain sufficiently to contribute substantial resistance to the trunk flexion moment, relegating this responsibility to the musculature.

The reaction moments at the knee, hip, and L4/L5 joints, and the compressive and shearing forces on L4/L5 are documented in powerlifters competing in a national powerlifting championship. Analyses were made of 13 female and 44 male competitors. The joint moments and forces were estimated from a linked segment model (WATBAK) that incorporated functional low back extensor musculature with a moment arm of 6 cm and a line action that was oriented 5 degrees posteriorly to the L4/L5 compression axis. This oblique orientation of the extensor muscles reduced the anterior shearing load on the vertebral motion unit. Average compressive loads on L4/L5 were estimated up to 17,192 N while the highest average L4/L5 and hip moments were 988 and 1047 N.m, respectively. The sumo deadlift style resulted in a 10% reduction in the joint moment and 8% reduction in the load shear force at the L4/L5 level when compared with the conventional lifting style. Formulation of linear regression equations to predict the load lifted using reaction joint moments yielded substantial unexplained variability, though significant relationships were found. This analysis suggested that there is large variability in the pattern of loading joints among national class powerlifters.

This study was done to assess the interplay between muscular and ligamentous sources of extensor moment during dynamic lifting with various loads and flexion angles of the trunk segment for 15 subjects lifting a total of 150 loads. Ligament forces predicted from an anatomically detailed biomechanical model did not generally contribute more than 60 Nm for most of the lifts because the lumbar spine was only flexed to a moderate and constant degree for each load condition. In contrast, additional moment demands associated with increases in hand load were supported by muscle. Although the compression forces on the L4-5 intervertebral disc were fairly insensitive to the interplay between the recruitment of muscle and ligament, the shear force was significantly higher with a greater degree of lumbar flexion. The risk of injury may be influenced more by the degree of lumbar flexion than the choice of stoop or squat technique.

Many studies have examined the mechanics of the lumbar spine in various planes, but only a limited number of three-dimensional investigations have been reported. Analysis of the low back during complex, dynamic postures demands rigorous representation of the trunk musculature. The data of this study demonstrated the force and torque contributions of approximately 50 laminas of various trunk muscles to flexion-extension, lateral bending, and axial twisting torque at the L4-L5 joint. This analysis was conducted with the spine in an upright standing posture and when fully flexed (60 degrees), laterally bent (25 degrees), and axially twisted (10 degrees) together with two examples of combined postures. Maximum moment potential, muscle length excursions, and the resultant compressive, anteroposterior shear, and lateral shear forces on the joint were also computed. The results indicate that the position of the vertebrae and their orthopaedic axes, which are a function of spinal posture, are an important factor in the reasonable determination of joint compressive, lateral shear, and anteroposterior shear loads. Muscle length changes that exceeded 20% of their respective length during upright standing were not observed during a full axial twist, but were observed in portions of the abdominal obliques during lateral bending, and in some extensors during full flexion. Extreme postures tended to change the torque potential of some muscles and influence joint load. Various portions of erector spinae were observed to have appreciable potential to generate torque about all three orthopaedic axes. This observation supports the notion held by some therapists that conditioning of the erector spinae is of utmost importance.

Study of the mechanics of trunk twisting is of special interest given epidemiological evidence linking occupational twisting to increased incidence of low back pain. An anatomically detailed, three-dimensional model of the trunk (rib cage, pelvis, five lumbar vertebrae and 50 muscles), was used to predict maximum axial trunk torque. Predicted axial torques were compared with measured torques. Thirty-one (10 male and 21 female) subjects performed maximum effort isometric twisting exertions, at 0 degrees of twist and +/- 30 degrees of twist together with dynamic exertions, at 30 degrees s-1 and 60 degrees s-1. Females were able to generate approximately two-thirds of the torque of males (males, 97Nm; females 60Nm, isometric at 0 degrees). When the trunk was prerotated to 30 degrees, subjects were able to generate greater torque when the effort was toward the 0 degree position (approximately 105Nm by males and 68Nm by females). Experimental data indicated that velocity of rotation and amount of twist are important modulators of axial torque. Changes in muscle length were demonstrated to be minimal from model output as most muscle length changes during a twist from 0 degrees to 30 degrees, measured between the pelvis and the shoulder harness, were less than 1%, although some portions of the abdominal obliques underwent a length excursion of 5%. The small changes in the individual muscle force components that contribute to twist, i.e. the muscle unit vector about the axial twist axis and its moment arm that change as a function of twisted position, do not entirely account for the measured differences in torque, suggesting that additional mechanisms influence axial torque generation.

The purpose of this study was to add to the growing database of cross-sectional areas and moment arm lengths of trunk musculature using the methods of computerized tomographic scanning. An attempt was also made to estimate muscle force and moment generating capacity under various reported values of muscle force per unit cross-sectional area. The data were obtained on 13 active men 40.5 +/- 11.9 years of age, 173.8 +/- 5.9 cm tall and 89.1 +/- 11.7 kg body mass. Transverse CT scans were taken at the level of the L4/L5 disc with the subjects supine. Muscle cross-sectional areas were measured from 35 mm slides of the scans using a planimeter and moment arm length in the transverse plane were taken from the centroid of the L4/L5 disc to the centroid of the muscle section. Prior to estimating force and moment generating capacity, areas were corrected, where necessary, for fibre pennation angle to produce a physiological cross-sectional area. The physiological cross-sectional areas (cm2) for one side of the body were (mean +/- S.D.): sacrospinalis (SS) 15.9 +/- 2.5; multifidus (Mu) 4.2 +/- 0.7; psoas (Ps) 17.6 +/- 4.0; rectus abdominis (RA) 7.9 +/- 2.5; external oblique (EO) 9.4 +/- 2.7; internal oblique (IO) 8.1 +/- 2.3; transverse abdominus (TA) 2.9 +/- 1.3. The anterior posterior moment arm lengths were: erector mass (SS and Mu combined) 5.90 +/- 0.52; Ps 0.58 +/- 0.40; R.A. 10.28 +/- 2.07; E.O. (anterior portion) 5.94 +/- 1.39; E.O. (posterior portion) 2.08 +/- 1.39; I.O. (anterior portion) 6.92 +/- 1.63; I.O. (posterior portion) 3.85 +/- 1.54. The corresponding lateral moment arm lengths were: 3.26 +/- 0.36; 4.88 +/- 0.36; 4.35 +/- 1.31; 12.86 +/- 1.93; 13.95 +/- 1.16; 10.77 +/- 2.02; 12.52 +/- 1.26. The maximum force per unit cross-section that human muscles are capable of generating is not well defined. However, assuming an intermediate value of 50 N cm-2 of physiological cross-section, the erector musculature observed at the L4/L5 level should be capable of generating an extensor moment of about 118 N.m. At a muscle stress of 30 or 90 N cm-2, values also reported on human muscle, the moment would be 71 and 213 Nm, respectively. It must be remembered, however, that muscles not observable at the L4/L5 level can create moments around that center of rotation.(ABSTRACT TRUNCATED AT 400 WORDS)