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STUDY DESIGN Most studies of manual material handling have focused on stable (fixed) loads. The muscular response during unstable load handling deserves more investigative attention. OBJECTIVES To investigate muscular and postural responses during unstable load lifting, and to quantify the threat posed by such loads in the workplace. SUMMARY OF BACKGROUND DATA Accurate assessment of load weight is important for lifters. Unfortunately, this information is not always available to individuals handling containers of liquids or shifting loads, or those encountering impact loading or a sudden unexpected load. METHODS In this study, 12 subjects were subjected to time-variant loads, in which a box weighing 18 kg was used to generate a sudden 12 (kg. m)/second load impact internally during the lifting process. Responses were investigated as a function of different load-shift conditions (anterior-to-posterior and posterior-to-anterior rolling loads and stable load), two foot placements (wide and narrow stance), and two lifting techniques (stoop and squat). Normalized electromyography signals, joint angles, and total linear length for center of pressure (COP-length) were investigated. RESULTS Lifting an unstable load significantly increases total lifting time, COP-length, and muscular contraction levels. When responding to impact momentum, subjects attempted to lift with greater elbow extension and shoulder and trunk flexion. A more flexed knee joint (approximately an additional 5 degrees ) was observed during preparation for load impact in the anterior-to-posterior rolling task, allowing the formation of a more stable platform to compensate for the load variation. The largest erector spinae contraction for unstable lifting was approximately 12% to 25% greater maximal voluntary contraction than for the stable analog. CONCLUSIONS The normalized electromyography data demonstrate that the central nervous system detects and responds to the need to stabilize the joints closest to the location of load-shift perturbation.

Study design An analysis of the geometric data of the lumbar spine in Chinese men. Lateral radiographs were obtained of 16 men in the upright position to a trunk flexion of 90 degrees in 30 degrees increments. Objectives To establish reference data concerning the geometry of the lumbar spine for various degrees of trunk flexion. Summary of background data In previous studies, the morphometry and alignment of the lumbar spine have been investigated. Reports of systematic analyses of the lumbar spine in trunk flexion have lacked adequate radiographic data. Moreover, few studies have focused on measuring lumbar spine geometric data in Chinese individuals. Methods A total of 67 landmarks on each radiograph were identified. They were marked by investigators and digitized using a HyperSpace digitizing system (V.17: Mira Imaging Inc., Salt Lake City, UT). The geometric configuration of the vertebrae and discs then was derived from the digitized points. Results The dimensions of the vertebrae and disc on the lumbar spine obtained in this study were similar to those of previous studies. The motion of the lumbosacral spine had the greatest contribution in trunk flexion, approximately 90% (i.e., rotating at the rate of 9 degrees for each 10 degrees of trunk flexion) when the trunk was flexed from 30 degrees to 60 degrees. After that, the hips were dominant in accomplishing the trunk flexion from 60 degrees to horizontal. This motion patterns may be useful for making clinical diagnoses of lumbar function in Chinese men. Conclusions No obvious interracial differences were found in the geometric data found in this study, which suggests that morphometric data obtained from Caucasian individuals may be applied to Chinese patients for clinical purposes.

The purpose of this study was to determine normal values for torque of trunk flexion and trunk flexion with axial rotation. Eighty-two healthy subjects (39 men, 43 women) were tested for isometric torque of trunk flexion at four angles, and eccentric and concentric torque at 30 degrees/sec. All tests were performed in three positions of axial rotation: neutral rotation, trunk rotated 30 degrees to the right, and trunk rotated 30 degrees to the left. The results of two separate three-way analyses of variance (gender vs. position vs. angle/contraction) indicated that the torque of men was greater than that of women, eccentric torque was greater than concentric, and the torque in neutral rotation was greater than in right or left rotation. Isometric torque decreased with increased trunk flexion. Regression and correlation analyses demonstrated that gender was generally the best predictor of torque, and that all the torque measures were significantly correlated.

Sudden and unexpected loading to the torso has been reported in the literature as a potential cause of low-back disorders. When such loadings occur, it is hypothesized that the body's response is designed to minimize the destabilizing postural disturbance, and to minimize the mechanical loading of the musculoskeletal system. This study tested hypotheses regarding the role of task experience in the development of preparatory strategies that potentially minimize the postural disturbance to the body and minimize the mechanical loading of the spine. These strategies were hypothesized to consist of muscle pretensioning, postural changes, and intra-abdominal pressure (IAP). Four subjects participated in five to six experimental sessions in which a sudden load was applied by dropping a weight once a minute for 30 minutes. Electromyographic (EMG) data from 10 trunk muscles, IAP data, and postural data were collected during the initial session and final sessions for each subject. The results indicate where each subject developed a unique preparatory strategy. The preparation always involved the pretensioning of the erector spinae muscles, although the coactivation of the other trunk muscles was quite variable across subjects. During the sudden loading the overall postural disturbance was not consistently reduced; however, the trunk flexion was significantly reduced in most subjects. Furthermore, the estimated spinal compression due to muscle loading was significantly reduced in all subjects.

The purpose of this study was 1) to provide normative data for isokinetic trunk strength as measured by peak torque, best work repetition, and a new composite variable, "muscle performance index"; and 2) to examine the effects of protocol velocity, subject age, gender, height, and weight on these measurements. A significant decrease in peak torque of extension for females (P less than 0.0001) was seen as protocol velocity increased, but no differences were seen in males. A statistically significant amount (0.5-0.7) of the variance in isokinetic measurements was explained by subject age, height, and weight. Multivariant analysis of covariance showed the effects of these variances to be significant on all measurements except for the muscle performance index, which was unaffected by protocol velocity and remained stable without adjustments for subject age, weight, or height.

A clinical trial, aimed at secondary prevention of low-back pain, was performed in 142 hospital employees reporting at least three annual episodes of this condition. Participants were randomly assigned to one of three groups: a calisthenics program (CAL) for 3 months with biweekly sessions of flexion exercises, a back school program (5 sessions), and a control group. The effectiveness of the two intervention programs was evaluated over a 1-year period. Baseline preintervention data and evaluation at the end of 3 months of intervention and after an additional 6 months were collected. A monthly surveillance for the whole year showed a mean of 4.5 "painful months" in the CAL group versus 7.3 and 7.4 months in the back school and control groups, respectively (P less than 0.0001). The superiority of the CAL group was achieved partly because of the significant increase in trunk forward flexion and to initial increment in abdominal muscle strength. The increased trunk flexion was associated with the rate of participation in the CAL sessions. Further research is needed to answer the question of "intensity versus type of exercise" by comparing different intervention programs, with similar intensity.

Study design A quasi-experimental study was conducted. Objective To compare intraindividual differences between trunk flexion isometric force, fatigue during isometric contraction (isometric fatigue), and fatigue during sit-ups (dynamic fatigue). Summary of background data Trunk flexion force and trunk flexion fatigue commonly are used to assess trunk flexion fitness. The use of one test is appropriate only if the tests are highly correlated. Methods This study measured force and either isometric fatigue (time to task failure, n = 79) or dynamic fatigue (number of sit-ups in 2 minutes, n = 73) in self-selected, healthy secondary school (I-III) subjects. In 15 subjects, both isometric and dynamic fatigue were measured. Results After control was used for upper body mass, the R2(adj) was 0.34 between isometric force and isometric fatigue, 0.31 between isometric force and dynamic fatigue was, and 0.27 between dynamic fatigue and isometric fatigue. Including gender in the model did not affect the results. Conclusions The lack of a high correlation between trunk flexion isometric fatigue, dynamic fatigue, and isometric force suggests that health care professionals should be careful to select the test that answers their particular needs or question.

Timing of activation of the hamstrings and erector spinae was assessed using surface electromyography.To investigate the influence of posture and movement speed during trunk flexion-extension on the flexion-relaxation response and trunk muscle activation patterns.The literature contains numerous reports on coactivity and synergistic behavior of major muscle groups during trunk flexion-extension. There are few reports on the timing of muscle activation.Six subjects were recruited for a training session and six biweekly test sessions. Ten surface electromyogram electrodes and a lordosimeter were used to record timing of lumbar motion and muscle recruitment in the hamstrings and at four sites in the thoracolumbar region. A 3 x 2 within-subject factorial design was used to test the effects of posture and speed on activation patterns.Patterns of muscle activation were found to be dependent on posture and the direction of movement. The flexion-relaxation response was pervasive in the lumbar region but was less consistent at the T9 and hamstring sites. Significant differences in the delay between electromyogram activation and lumbar motion were found for the standing postures at initiation of extension, in which activation progressed in the caudad-to-cephalad direction.The flexion-relaxation response is ubiquitous in the lumbar erector spinae and is present in the hamstrings and lower thoracic erector spinae, although not consistently in all subjects. In standing, timing of activation differed significantly by site in extension but not in flexion. Muscle activation patterns and flexion-relaxation were consistent over six biweekly test sessions.

Study design Isometric and isokinetic trunk flexion-extension strengths were studied among 73 subjects (41 males and 32 females) and 10 patients (9 males and 1 female). The isometric strengths were measured in four postures of trunk flexion. The isokinetic trunk strengths were measured through the range of these postures. Objectives The study had two objectives: to develop a database for isometric flexion-extension strength at different levels of trunk flexion and isokinetic strength at corresponding angles and to determine the decrement in strength characteristics among patients of idiopathic low back pain. Summary of background data Though flexion-extension strength among normal subjects and patients has been studied, its variation due to standardized postural change has not been reported extensively. The influence of postural deviation on flexion-extension ratios are also largely unknown. Methods To achieve the objectives of the study, a device flexion-extension-lateral flexion tester (FELT) was designed and fabricated. The male and female subjects performed isometric flexion and extension at 0 degree, 20 degrees, 40 degrees and 60 degrees of trunk flexion; and isokinetic flexion starting at neutral posture, and isokinetic extension starting at 60 degrees of trunk flexion. The patients performed isometric flexion and extension in neutral posture only; and isokinetic flexion-extension similar to normal sample. Results The average strength in each activity was between 70 and 80% of the corresponding peak strength. The isokinetic activity strengths ranged between 60 and 70% of the corresponding isometric activities in each group. The strength of females ranged between 60 and 70% of males. The ANOVA revealed that posture had a significant effect on strength. The multiple regression explained 73% of variance in isometric and 34% in isokinetic modes. Conclusions A patient's strength profile should be determined in a posture that simulates activities of daily living to determine impairment and plan rehabilitation.

Thirty-one healthy men were tested for the effects of trunk-flexion positions (0 degrees, 15 degrees, and 35 degrees) in standing on triaxial torques and electromyogram of 10 trunk muscles during voluntary maximal and submaximal isometric trunk extension. At a more flexed position, both erector spinae and latissimus dorsi showed significantly higher RMS-EMG. The abdominal obliques were coactivated only during 100% maximum voluntary exertion at each posture. In all tests, the rectus abdomini were quiet. Mean maximum extension torque increased significantly at 15 degrees and 35 degrees of trunk flexion. The ratio of extension torque over RMS-EMG of the trunk extensor muscles, called the neuromuscular efficiency ratio (NMER), also increased in the more flexed posture. However, NMER has to be interpreted with caution because it is affected both by posture and exertion levels. The effects of posture on the torque generation capability of the trunk question the validity of the current lifting recommendations.

Electromyographic activity of erectores spinae exhibits points of abrupt change during trunk flexion from the erect position and return extension. This study examined the positions at which the myoelectric activity suddenly disappeared and later reappeared. Forty adults were investigated to define accurately the inclinations of the trunk, pelvis, and vertebral column at these positions. The positions at the commencement and cessation of the period of electrical silence both occurred at two-thirds of maximum trunk flexion (mean = 80 degrees +/- 13 degrees SD). At these positions, all flexion measurements were significantly less than their maxima (P less than 0.001). Hip flexion at the commencement of electrical silence was slightly above one-half its maximum range, and similar to the position at the recommencement of electrical activity (mean = 40 degrees +/- 12 degrees SD). The most reproducible measurement (r = 0.88) in both positions was vertebral flexion (89% Max.; mean = 48 degrees +/- 6 degrees SD). Eleven of the male subjects repeated the experimental task holding 10.1 kg in their hands. The effect of this was to produce inhibition and reactivation of erectores spinae at a greater degree of vertebral flexion.

Maximum voluntary isometric strengths of the trunk were measured in 93 girls who had mild idiopathic scoliosis and 109 girls who had structurally normal spines. Subject ages ranged from 10 to 16 years, and the mean Cobb measurement in the girls with scoliosis was approximately 22 degrees. Strengths were measured in attempted trunk flexion, attempted extension, ad attempted left and right lateral bending. No consistent differences appeared in performance between the girls with scoliosis and the structurally normal girls.

Using a modeling approach the conventional sit-up, the double straight-leg lowering, and prone trunk extension, the authors have shown these to have limitations as tests of maximum trunk flexion and extension strength and as modes of resistive exercise. These maneuvers are poor discriminators of trunk muscle strength, and they lack the range of resistance necessary to cover the spectrum of human trunk muscle strength capability. Use of manually handled weights can improve the above clinical tests and resistive exercise methods. As a result of selective positioning of the weights, variable resistance above and below that provided by the body segments alone can be achieved. The partial sit-up may be appropriate only for patients with significant trunk flexion weakness, and the straight head-trunk sit-up may be useful as a more strenuous test and resistive exercise. Rationale and derivations of the rigid-body equilibrium equations, as well as some of the limitations and assumptions associated with the modeling approach, are presented.