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Existing approaches for automated tracking of fascicle length (FL) and pennation angle (PA) rely on the presence of a single, user-defined fascicle (feature tracking) or on the presence of a specific intensity pattern (feature detection) across all the recorded ultrasound images. These prerequisites are seldom met during large dynamic muscle movements or for deeper muscles that are difficult to image. Deep-learning approaches are not affected by these issues, but their applicability is restricted by their need for large, manually analyzed training data sets. To address these limitations, the present study proposes a novel approach that tracks changes in FL and PA based on the distortion pattern within the fascicle band. The results indicated a satisfactory level of agreement between manual and automated measurements made with the proposed method. When compared against feature tracking and feature detection methods, the proposed method achieved the lowest average root mean squared error for FL and the second lowest for PA. The strength of the proposed approach is that the quantification process does not require a training data set and it can take place even when it is not possible to track a single fascicle or observe a specific intensity pattern on the ultrasound recording.

The COVID-19 pandemic is an unprecedented health crisis as entire populations have been asked to self-isolate and live in home-confinement for several weeks to months, which in itself represents a physiological challenge with significant health risks. This paper describes the impact of sedentarism on the human body at the level of the muscular, cardiovascular, metabolic, endocrine and nervous systems and is based on evidence from several models of inactivity, including bed rest, unilateral limb suspension, and step-reduction. Data form these studies show that muscle wasting occurs rapidly, being detectable within two days of inactivity. This loss of muscle mass is associated with fibre denervation, neuromuscular junction damage and upregulation of protein breakdown, but is mostly explained by the suppression of muscle protein synthesis. Inactivity also affects glucose homeostasis as just few days of step reduction or bed rest, reduce insulin sensitivity, principally in muscle. Additionally, aerobic capacity is impaired at all levels of the O 2 cascade, from the cardiovascular system, including peripheral circulation, to skeletal muscle oxidative function. Positive energy balance during physical inactivity is associated with fat deposition, associated with systemic inflammation and activation of antioxidant defences, exacerbating muscle loss. Importantly, these deleterious effects of inactivity can be diminished by routine exercise practice, but the exercise dose-response relationship is currently unknown. Nevertheless, low to medium-intensity high volume resistive exercise, easily implementable in home-settings, will have positive effects, particularly if combined with a 15-25% reduction in daily energy intake. This combined regimen seems ideal for preserving neuromuscular, metabolic and cardiovascular health.

The relationship between muscle structure and function has been a matter of investigation since the Renaissance period. Extensive use of anatomical dissections and the introduction of the scientific method enabled early scholars to lay the foundations of muscle physiology and biomechanics. Progression of knowledge in these disciplines led to the current understanding that muscle architecture, together with muscle fibre contractile properties, has a major influence on muscle mechanical properties. Recently, advances in laser diffraction, optical microendoscopy and ultrasonography have enabled in vivo investigations into the behaviour of human muscle fascicles and sarcomeres with varying joint angle and muscle contraction intensity. With these technologies it has become possible to identify the length region over which fascicles and sarcomeres develop maximum isometric force in vivo as well as the operating ranges of fascicles and sarcomeres during real-life activities such as walking. Also, greater insights into the remodelling of muscle architecture in response to overloading and unloading, and in ageing, have been obtained by the use of ultrasonography; these have led to the identification of clinical biomarkers of disuse atrophy and sarcopenia. Recent evidence also shows that the pattern of muscle hypertrophy in response to chronic loading is contraction-mode dependent (eccentric versus concentric), as similar gains in muscle mass, but through differing addition of sarcomeres in series and in parallel (as indirectly inferred from changes in fascicle length and pennation angle), have been found. These innovative observations prompted a new set of investigations into the molecular mechanisms regulating this contraction-specific muscle growth., (© 2016. Published by The Company of Biologists Ltd.)

Aim: To examine the effects of a 16-week resistance exercise training intervention on the speed of ankle and knee strength generation during stair ascent and descent, in people with neuropathy., Methods: A total of 43 people: nine with diabetic peripheral neuropathy, 13 with diabetes but no neuropathy and 21 healthy control subjects ascended and descended a custom-built staircase. The speed at which ankle and knee strength were generated, and muscle activation patterns of the ankle and knee extensor muscles were analysed before and after a 16-week intervention period., Results: Ankle and knee strength generation during both stair ascent and descent were significantly higher after the intervention than before the intervention in the people with diabetes who undertook the resistance exercise intervention (P < 0.05). Although muscle activations were altered by the intervention, there were no observable patterns that underpinned the observed changes., Conclusions: The increased speed of ankle and knee strength generation observed after the intervention would be expected to improve stability during the crucial weight acceptance phase of stair ascent and descent, and ultimately contribute towards reducing the risk of falling. Improvements in muscle strength as a result of the resistance exercise training intervention are likely to be the most influential factor for increasing the speed of strength generation. It is recommended that these exercises could be incorporated into a multi-faceted exercise programme to improve safety in people with diabetes and neuropathy., (© 2015 The Authors. Diabetic Medicine © 2015 Diabetes UK.)

Older people may operate much closer to their maximum capabilities than young adults when ascending stairs due to their lower maximum musculoskeletal capabilities. The purpose of this study was to establish the joint moment and range of motion demands of stair ascent relative to maximum capabilities in elderly and young adults. Fifteen elderly (mean age 75 years) and 17 young adult (mean age 25 years) participants ascended a purpose-built 4-step staircase with force platforms embedded into the steps and kinematic data was acquired using motion capture. Maximum musculoskeletal capabilities were assessed using a dynamometer. This study showed for the first time that stair ascent approaches the joint moment limits at the ankle in both young and older participants ( approximately 90%). One of the most important and novel findings of this study was that elderly people were only capable of meeting the high demands by adopting a number of alternative strategies not observed in young adults: (i) applying the joint moments differently than young adults across the knee and ankle, (ii) translocating energy from the knee to the ankle, thereby enhancing the ankle joint moment upon maximum demand and (iii) by enabling the plantarflexors to act over a more favourable portion of the moment-angle relation upon maximum ankle joint moment demand. The elderly displayed a more cautious strategy to optimize positional stability during stair ascent, by maintaining a smaller separation between the centre of mass and centre of pressure in the frontal plane. It seems that elderly people may meet the demands of unaided stair ascent by adopting a number of alternative strategies to compensate for their reduced musculoskeletal capabilities.

The gastrocnemius medialis (GM) muscle plays an important role in stair negotiation. The aim of the study was to investigate the influence of cadence on GM muscle fascicle behaviour during stair ascent and descent. Ten male subjects (young adults) walked up and down a four-step staircase (with forceplates embedded in the steps) at three velocities (63, 88 and 116 steps/min). GM muscle fascicle length was measured using ultrasonography. In addition, kinematic and kinetic data of the lower legs, and GM electromyography (EMG) were measured. For both ascent and descent, the amount of fascicular shortening, shortening velocity, knee moment, ground reaction force and EMG activity increased monotonically with gait velocity. The ankle moment increased up to 88 steps/min where it reached a plateau. The lack of increase in ankle moment coinciding with further shortening of the fascicles can be explained by an increased shortening of the GM musculotendon complex (MTC), as calculated from the knee and ankle angle changes, between 88 and 116 steps/min only. For descent, the relative instant of maximum shortening, which occurred during touch down, was delayed at higher gait velocities, even to the extent that this event shifted from the double support to the single support phase.

Aim: To investigate the influence of tendinous and synaptic changes induced by unilateral lower limb suspension (ULLS) on the tendon tap reflex., Methods: Eight young men underwent a 23-day period of ULLS. Muscle cross-sectional area (CSA), torque and electromyographic (EMG) activity of the plantar flexor muscles (normalized to the M wave), Achilles tendon-aponeurosis mechanical properties, soleus (SOL) H and T reflexes and associated peak twitch torques were measured at baseline, after 14 and 23 days of ULLS, and 1 week after resuming ambulatory activity., Results: Significant decreases in muscle CSA (-9%), in maximal voluntary torque (-10%) and in the associated SOL EMG activity (-16%) were found after ULLS (P < 0.05). In addition to a 36% (P < 0.01) decrease in tendon-aponeurosis stiffness, normalized H reflex increased by 35% (P < 0.05). An increase in the slope (28%, P < 0.05) and intercept (85%, P < 0.05) of the T reflex recruitment curve pointed to an increase in the gain and to a decrease in the sensitivity of this reflex, possibly resulting from the decrease in the tendon-aponeurosis stiffness at low forces. Following ULLS, changes in tendinous stiffness correlated with changes in neuromuscular efficiency (peak twitch torque to reflex ratio) at higher tendon tap forces., Conclusion: These findings point out the dual and antagonistic influences of spinal and tendinous adaptations upon the tendon tap reflex in humans under conditions of chronic unloading. These observations have potential implications for the sensitivity of the short-latency Ia stretch response involved in rapid compensatory contractions to unexpected postural perturbations.

Aim: The present study investigated the influence of muscle architectural changes on muscle torque during 3-week unilateral lower limb suspension (ULLS)., Methods: Plantarflexion maximal voluntary contraction (MVC), soleus (SOL), gastrocnemius medialis (GM) and lateralis (GL) muscle volume (VOL), GL fascicle length (L(f)) and pennation angle (theta), physiological cross-sectional area (PCSA), and electromyographic (EMG) activity were assessed in eight healthy men (aged 19 +/- 0 years) after days 14 and 23 of ULLS., Results: After 14 day of ULLS, MVC and SOL EMG decreased (P < 0.05) by 10% and 29%, respectively, but did not further decline between days 14 and 23. SOL, GM and GL muscle VOL decreased by 5%, 6% and 5%, respectively (P < 0.05), on day 14, and by 7% (SOL), 10% (GM) and 6% (GL) on day 23. In GL, theta and L(f) were reduced by 3% (P < 0.05) and 2% (NS), respectively, on day 14, and by 5% (P < 0.05) and 4% (P < 0.05), respectively, on day 23. Consequently, GL PCSA declined by 3% (P < 0.05) on day 14, but did not further decrease on day 23. Similarly, the 7% (P < 0.05) loss in GL force/PCSA observed on day 14 persisted until the end of the unloading period., Conclusion: These findings suggest that rapid muscle architecture remodelling occurs with lower limb unloading in humans, with changes occurring within 14 days of weight bearing removal. These adaptations, mitigating the decline in muscle PCSA, might protect from a larger loss of muscle force.

The effect of spastic cerebral palsy on in vivo gastrocnemius muscle fascicle length is not clear. Similarity of fascicle lengths in children with diplegia and typically developing children, but shortening of fascicle lengths in the paretic legs of children with hemiplegia compared with the non-paretic legs, are both reported. In the former case, comparisons were made between fascicle lengths normalized to leg length, whereas in the latter case, absolute fascicle lengths were compared. The inherent assumptions when normalizing fascicle length (measured via ultrasonography) were not validated, raising the possibility that inappropriate normalization contributed to the controversy. We used statistical methods to control the potential confounding effect of leg length on fascicle length, and tested the feasibility of the normalization method for a group of 18 children with diplegia (nine males, nine females; mean age 8y 7mo [SD 3y 11mo], range 2-15y; Gross Motor Function Classification System levels II and III) and 50 typically developing children (20 males, 30 females; mean age 9y 1mo [SD 2y 4mo], range 4-14y). Children with diplegia, as a group, had shorter absolute and normalized fascicle lengths (p<0.05) but we could not refute the appropriateness of the normalization method. Other methodological issues (such as sample characteristics) might have contributed to the apparent controversy between the studies.

To better understand the role of the ankle plantar flexor muscles in stair negotiation, we examined the effects of manipulation of kinematic and kinetic constraints on the behavior of the gastrocnemius medialis (GM) muscle during stair ascent. Ten subjects ascended a four-step staircase at four different step-heights (changing the kinematic constraints): standard (17 cm), 50% decreased, 50% increased and 75% increased. At the standard height, subjects also ascended the stairs wearing a weighted jacket, adding 20% of their body mass (changing the kinetic constraints). During stair ascent, kinematics and kinetics of the lower legs were determined using motion capture and ground reaction force measurements. The GM muscle fascicle length was measured during the task with ultrasonography. The amount of GM muscle fascicle shortening increased with step-height, coinciding with an increase in ankle joint moment. The increase in body mass resulted in an increased ankle joint moment, but the amount of GM muscle fascicle shortening during the lift-off phase did not increase, instead, the fascicles were shorter over the whole stride cycle. Increasing demands of stair ascent, by increasing step-height or body mass, requires higher joint moments. The increased ankle joint moment with increasing demands is, at least in part, produced by the increase in GM muscle fascicle shortening.

Passive stretching is commonly used to increase limb range of movement prior to athletic performance but it is unclear which component of the muscle-tendon unit (MTU) is affected by this procedure. Movement of the myotendinous junction (MTJ) of the gastrocnemius medialis muscle was measured by ultrasonography in eight male participants (20.5 +/- 0.9 years) during a standard stretch in which the ankle was passively dorsiflexed at 1 deg s(-1) from 0 deg (the foot at right angles to the tibia) to the participants' volitional end range of motion (ROM). Passive torque, muscle fascicle length and pennation angle were also measured. Standard stretch measurements were made before (pre-) and after (post-) five passive conditioning stretches. During each conditioning stretch the MTU was taken to the end ROM and held for 1 min. Pre-conditioning the extension of the MTU during stretch was taken up almost equally by muscle and tendon. Following conditioning, ROM increased by 4.6 +/- 1.5 deg (17%) and the passive stiffness of the MTU was reduced (between 20 and 25 deg) by 47% from 16.0 +/- 3.6 to 10.2 +/- 2.0 Nm deg(-1). Distal MTJ displacement (between 0 and 25 deg) increased from 0.92 +/- 0.06 to 1.16 +/- 0.05 cm, accounting for all the additional MTU elongation and indicating that there was no change in tendon properties. Muscle extension pre-conditioning was explicable by change in length and pennation angle of the fascicles but post-conditioning this was not the case suggesting that at least part of the change in muscle with conditioning stretches was due to altered properties of connective tissue.

Muscle atrophy and neuromuscular de-conditioning occur in response to space flight and bed-rest. In this study, we investigated the efficacy of flywheel training to conserve jumping power and height during 90 days bed rest. Twenty-four young healthy men underwent strict bed-rest (-6 degrees head down tilt) for 90 days. Eight participants were assigned to a flywheel group (FW) and 16 to a control group (Ctrl). The ground reaction force was measured during vertical jump tests twice during baseline data collection, and on day 4, 7, 14, 90 and 180 of recovery. In half of the participants, jump tests were also performed within minutes after re-ambulation and on four more occasions during the first 2 days of recovery. Jump height was reduced from 40.6 cm (SD 6.1 cm) during the first baseline measurement to 27.6 cm (SD 5.6 cm) on day 4 of recovery in Ctrl, but only from 38.6 cm (SD 3.9 cm) to 34.4 cm (SD 6.5 cm) in FW (P < 0.001). At the same time, peak power was reduced from 47.4 W/kg (SD 8.0 W/kg) to 34.5 W/kg in Ctrl, but only from 46.2 W/kg (6.0 W/kg) to 42.2 W/kg SD 4.6 W/kg) in FW (P < 0.001). Jump height and peak power were completely recovered after 163 and 140 days in Ctrl, respectively, and after 72 and 18 days in FW (regression analysis). In conclusion, flywheel exercise could effectively offset neuromuscular de-conditioning during bed-rest, and led to full recovery at an earlier stage. These findings nourish the hope that adequate training paradigms can fully sustain neuromuscular function under microgravity conditions.

The aim of the present study was to establish the behavior of human medial gastrocnemius (GM) muscle fascicles during stair negotiation. Ten healthy male subjects performed normal stair ascent and descent at their own comfortable speed on a standard-dimension four-step staircase with embedded force platforms in each step. Kinematic, kinetic, and electromyographic data of the lower limbs were collected. Real-time ultrasound scanning was used to determine GM muscle fascicle length changes. Musculotendon complex (MTC) length changes were estimated from ankle and knee joint kinematics. The GM muscle was mainly active during the push-off phase in stair ascent, and the muscle fascicles contracted nearly isometrically. The GM muscle was mainly active during the touch-down phase of stair descent where the MTC was lengthened; however, the GM muscle fascicles shortened by approximately 7 mm. These findings show that the behavior and function of GM muscle fascicles in stair negotiation is different from that expected on the basis of length changes of the MTC as derived from joint kinematics.

Bone loss from the paralysed limbs after spinal cord injury (SCI) is well documented. Under physiological conditions, bones are adapted to forces which mainly emerge from muscle pull. After spinal cord injury (SCI), muscles can no longer contract voluntarily and are merely activated during spasms. Based on the Ashworth scale, previous research has suggested that these spasms may mitigate bone losses. We therefore wished to assess muscle forces after SCI with a more direct measure and compare it to measures of bone strength. We hypothesized that the bones in SCI patients would be in relation to the loss of muscle forces. Six male patients with SCI 6.4 (SD 4.3) years earlier and 6 age-matched, able-bodied control subjects were investigated. Bone scans from the right knee were obtained by pQCT. The knee extensor muscles were electrically stimulated via the femoral nerve, isometric knee extension torque was measured and patellar tendon force was estimated. Tendon force upon electrical stimulation in the SCI group was 75% lower than in the control subjects (p<0.01). Volumetric bone mineral density of the patella and of the proximal tibia epiphysis were 50% lower in the SCI group than in the control subjects (p<0.01). Cortical area was lower by 43% in the SCI patients at the proximal tibia metaphysis, and by 33% at the distal femur metaphysis. No group differences were found in volumetric cortical density. Close curvilinear relationships were found between stress and volumetric density for the tibia epiphysis (r(2)=0.90) and for the patella (r(2)=0.91). A weaker correlation with the tendon force was found for the cortical area of the proximal tibia metaphysis (r(2)=0.63), and none for the distal femur metaphysis. These data suggest that, under steady state conditions after SCI, epiphyseal bones are well adapted to the muscular forces. For the metaphysis of the long bones, such an adaptation appears to be less evident. The reason for this remains unclear.

The age-related loss of muscle mass known as senile sarcopenia is one of the main determinants of frailty in old age. Molecular, cellular, nutritional and hormonal mechanisms are at the basis of sarcopenia and are responsible for a progressive deterioration in skeletal muscle size and function. Both at single-fibre and at whole-muscle level, the loss of force exceeds that predicted by the decrease in muscle size. For single fibres, the loss of intrinsic force is mostly due to a loss in myofibrillar protein content. For whole muscle, in addition to changes in neural drive, alterations in muscle architecture and in tendon mechanical properties, exemplified by a reduction in tendon stiffness, have recently been shown to contribute to this phenomenon. Resistance training can, however, cause substantial gains in muscle mass and strength and provides a protective effect against several of the cellular and molecular changes associated with muscle wasting and weakness. In old age, not only muscles but also tendons are highly responsive to training, since an increase in tendon stiffness has been observed after a period of increased loading. Many of the myotendinous factors characterizing ageing can be at least partly reversed by resistance training.

The purpose of the present study was to investigate the effects of a soccer training session on the balance ability of the players and assess whether the effectiveness of a balance program is affected by its performance before or after the regular soccer training. Thirty-nine soccer players were randomly divided into three subject groups (n=13 each), one control group (C group), one training group that followed a balance program (12 weeks, 3 times per week, 20 min per session) before the regular soccer training (TxB group), and one training group that performed the same balance program after the soccer training (TxA group). Standard testing balance boards and the Biodex Stability System were used to assess balance ability in the C, TxB, and TxA groups at baseline (T0) and after completing the balance program (T12). The same tests and additional isokinetic knee joint moment measurements were carried out in the TxB and TxA groups pre- and post-soccer training. Two main results were obtained: (1) No differences (p>0.05) were found in balance ability and knee joint moment production between pre- and post-soccer training. (2) The balance program increased (p<0.01) the balance ability in the TxB and TxA groups, and the improvement in the TxA group was greater (p<0.05) than that in the TxB group post-soccer training. Result (1) is in contrast to the notion of a link between fatigue induced by a soccer training session or game and injury caused by impaired balance, and result (2) has implications for athletic training and rehabilitation.

This study investigated changes in elderly muscle joint angle-torque relation induced by resistance training. Older adults were assigned to either training (n = 9, age 74.3 +/- 3.5 years; mean +/-s.d.) or to control groups (n = 9, age 67.1 +/- 2 years). Leg-extension and leg-press exercises were performed three times per week for 14 weeks. Maximal isometric knee extension torque was measured across the knee joint angle range of movement. Vastus lateralis muscle architecture was examined in vivo using ultrasonography. The vastus lateralis muscle fascicle force was estimated from the measured joint torque, enabling construction of the fascicle length-force relation. Electromyographic (EMG) activity was measured from representative agonist and antagonist muscles. Training altered the angle-torque relation: (a) displacing it by 9-31% towards higher torque values (P < 0.05); and (b) shifting the optimal angle from 70 deg (corresponding torque: 121.4 +/- 61 N m) before to 60 deg (134.2 +/- 57.2 N m; P < 0.05) after training. Training also altered the fascicle length-force relation: (a) displacing it by 11-35% towards higher force values; and (b) shifting the optimal fascicle length from 83.7 +/- 8 mm (corresponding force: 847.9 +/- 365.3 N) before to 93.2 +/- 12.5 mm (939.3 +/- 347.8 N; P < 0.01) after training. The upward displacement of the angle-torque relation was mainly due to a training-induced increase in agonist activation, whilst the shift in the optimal angle was associated with changes in muscle-tendon properties.

Neuropathic, metabolic, hormonal, nutritional and immunological factors contribute to the development of sarcopenia. This loss of muscle mass associated with ageing, is a main cause of muscle weakness, but the loss of muscle strength typically exceeds that of muscle size, with a resulting decrease in force per unit of muscle cross-sectional area. Recent evidence suggests that, in addition to a reduction in neural drive and in fibre specific tension, changes in muscle architecture contribute significantly to the loss of muscle force through alterations in muscle mechanical properties. Older muscle, however, maintains a high degree of plasticity in response to increased loading since considerable hypertrophy and a reversal of the alterations in muscle architecture associated with ageing are observed with resistive training.

This study assessed muscle-specific force in vivo following strength training in old age. Subjects were assigned to training (n = 9, age 74.3 +/- 3.5 yr; mean +/- SD) and control (n = 9, age 67.1 +/- 2 yr) groups. Leg-extension and leg-press exercises (2 sets of 10 repetitions at 80% of the 5 repetition maximum) were performed three times/wk for 14 wk. Vastus lateralis (VL) muscle fascicle force was calculated from maximal isometric voluntary knee extensor torque with superimposed stimuli, accounting for the patella tendon moment arm length, ultrasound-based measurements of muscle architecture, and antagonist cocontraction estimated from electromyographic activity. Physiological cross-sectional area (PCSA) was calculated from the ratio of muscle volume to fascicle length. Specific force was calculated by dividing fascicle force by PCSA. Fascicle force increased by 11%, from 847.9 +/- 365.3 N before to 939.3 +/- 347.8 N after training (P < 0.05). Due to a relatively greater increase in fascicle length (11%) than muscle volume (6%), PCSA remained unchanged (pretraining: 30.4 +/- 8.9 cm(2); posttraining: 29.1 +/- 8.4 cm(2); P > 0.05). Activation capacity and VL muscle root mean square electromyographic activity increased by 5 and 40%, respectively, after training (P < 0.05), indicating increased agonist neural drive, whereas antagonist cocontraction remained unchanged (P > 0.05). The VL muscle-specific force increased by 19%, from 27 +/- 6.3 N/cm(2) before to 32.1 +/- 7.4 N/cm(2) after training (P < 0.01), highlighting the effectiveness of strength training for increasing the intrinsic force-producing capacity of skeletal muscle in old age.

The effect of aging on human gastrocnemius medialis (GM) muscle architecture was evaluated by comparing morphometric measurements on 14 young (aged 27-42 yr) and on 16 older (aged 70-81 yr) physically active men, matched for height, body mass, and physical activity. GM muscle anatomic cross-sectional area (ACSA) and volume (Vol) were measured by computerized tomography, and GM fascicle length (Lf) and pennation angle (theta) were assessed by ultrasonography. GM physiological cross-sectional area (PCSA) was calculated as the ratio of Vol/Lf. In the elderly, ACSA and Vol were, respectively, 19.1% (P < 0.005) and 25.4% (P < 0.001) smaller than in the young adults. Also, Lf and were found to be smaller in the elderly group by 10.2% (P < 0.01) and 13.2% (P < 0.01), respectively. When the data for the young and elderly adults were pooled together, significantly correlated with ACSA (P < 0.05). Because of the reduced Vol and Lf in the elderly group, the resulting PCSA was found to be 15.2% (P < 0.05) smaller. In conclusion, this study demonstrates that aging significantly affects human skeletal muscle architecture. These structural alterations are expected to have implications for muscle function in old age.

The effect of hamstring fatigue on knee extension torque was examined at different knee angles for seven male subjects. Before and after a dynamic flexion fatigue protocol (180 degrees s(-1), until dynamic torque had declined by 50%), maximal voluntary contraction extension torque was measured at four knee flexion angles (90 degrees, 70 degrees, 50 degrees and 30 degrees ). Maximal torque generating capacity and voluntary activation of the quadriceps muscle were determined using electrical stimulation. Average rectified EMG of the biceps femoris was determined. Mean dynamic flexion torque declined by 48+/-11%. Extensor maximal voluntary contraction torque, maximal torque generating capacity, voluntary activation and average rectified EMG at the four knee angles were unaffected by the hamstring fatigue protocol. Only at 50 degrees knee angle was voluntary activation significantly lower (15.7%) after fatigue ( P<0.05). In addition, average rectified EMG before fatigue was not significantly influenced by knee angle. It was concluded that a fatigued hamstring muscle did not increase the maximal voluntary contraction extension torque and knee angle did not change coactivation. Three possible mechanisms may explain the results: a potential difference in recruited fibre populations in antagonist activity compared with the fibres which were fatigued in the protocol, a smaller loss in isometric torque generating capacity of the hamstring muscle than was expected from the dynamic measurements and/or a reduction in voluntary activation.

Aponeurotic deformation measurements have traditionally been taken by loading dissected muscles; thus the values obtained may not reflect in vivo function. In the present study, we estimated dimensional changes in the central aponeurosis of the intact human tibialis anterior muscle upon loading induced by muscle contraction. Measurements were taken in seven males, and involved real-time ultrasound scanning of the tibialis anterior muscle at 30 degrees of plantarflexion at rest and during isometric dorsiflexion maximum voluntary contraction (MVC). At each contraction state, the length of the aponeurosis, the width along its length, and its area were estimated from sagittal-plane and axial-plane sonographs. In the transition from rest to MVC, the length of the aponeurosis increased by 7% (P < 0.05), its width increased by up to 21 % (P < 0.05), and its area increased by 17% (P < 0.05). These results indicate that the in vivo tibialis anterior aponeurosis behaves as a compliant material upon active muscle shortening. The methodology employed allows cross-sectional and longitudinal design investigations, circumventing the problems associated with epimysial removal under in vitro experimental conditions.

In the present study, the in vivo force-length relations of the human soleus (SOL) and tibialis anterior (TA) muscles were estimated. Measurements were taken in six men at ankle angles from 30 degrees of dorsiflexion to 45 degrees of plantarflexion in steps of 15 degrees, and involved dynamometry, electrical stimulation, ultrasonography and magnetic resonance imaging (MRI). For each muscle and ankle angle studied the following three measurements were carried out: (1) dynamometry-based measurement of maximal voltage tetanic moment, (2) ultrasound-based measurement of pennation angle and fibre length and (3) MRI-based measurement of tendon moment arm length. Tendon forces were calculated dividing moments by moment arm lengths, and muscle forces were calculated dividing tendon forces by the cosine of pennation angles. In the transition from 30 degrees of dorsiflexion to 45 degrees of plantarflexion the SOL muscle fibre length decreased from 3.8 to 2.4 cm and its force decreased from 3330 to 290 N. Over the same range of ankle angles the TA muscle fibre length increased from 3.7 to 6 cm and its force increased from 157 to 644 N. Over the longest muscle fibre lengths reached the force of both muscles remained approximately constant. These results indicate that the intact human SOL and TA muscles operate in the ascending limb and plateau region of the force-length relationship. Similar conclusions were reached when calculating the theoretical operating range of the two muscle sarcomeres in the study.

In this study, we estimated the specific tensions of soleus (Sol) and tibialis anterior (TA) muscles in six men. Joint moments were measured during maximum voluntary contraction (MVC) and during electrical stimulation. Moment arm lengths and muscle volumes were measured using magnetic resonance imaging, and pennation angles and fascicular lengths were measured using ultrasonography. Tendon and muscle forces were modeled. Two approaches were followed to estimate specific tension. First, muscle moments during electrical stimulation and moment arm lengths, fascicular lengths, and pennation angles during MVC were used (data set A). Then, MVC moments, moment arm lengths at rest, and cadaveric fascicular lengths and pennation angles were used (data set B). The use of data set B yielded the unrealistic specific tension estimates of 104 kN/m(2) in Sol and 658 kN/m(2) in TA. The use of data set A, however, yielded values of 150 and 155 kN/m(2) in Sol and TA, respectively, which agree with in vitro results from fiber type I-predominant muscles. In fact, both Sol and TA are such muscles. Our study demonstrates the feasibility of accurate in vivo estimates of human muscle intrinsic strength.

The aim of this study was to estimate the moment arm of human tibialis anterior (TA) muscle-tendon unit at rest and during isometric dorsiflexion maximum voluntary contraction (MVC) from in vivo sagittal-plane magnetic resonance (MR) and ultrasound scans. Two methods were employed, both of them based on the assumption that the ankle joint complex and TA muscle-tendon unit operate in the sagittal plane. Using method A, moment arms were obtained from MR scans of the foot by measuring the perpendicular distance between a moving centre of rotation in the talo-crural joint and the TA tendon action line. Using method B, moment arms were calculated from the ratio of TA tendon displacement, which was estimated from a planimetric muscle model using pennation angles and muscle thickness measured by ultrasonography, to the tibial rotation around the talus, which was measured from the foot MR scans. Using either of the two methods at rest, the estimated TA moment arm decreased from approximately 4.5 to approximately 2.9 cm in the transition from dorsiflexion to plantarflexion. Using method A, moment arms during MVC were larger by 0.9-1.5 cm (33-44%, P < 0.01) than the respective resting estimations. In contrast, no difference (P > 0.05) was found between the resting and MVC moment arm estimations of method B. Limitations in the oversimplified musculoskeletal model used raise questions for the validity of both method estimations.

The aim of this study was to assess the predictability of in vivo, ultrasound-based changes in human tibialis anterior (TA) pennation angle from rest to maximum isometric dorsiflexion (MVC) using a planimetric model assuming constant thickness between aponeuroses and straight muscle fibres. Sagittal sonographs of TA were taken in six males at ankle angles of -15 degrees (dorsiflexion direction), 0 degrees (neutral position), + 15 (plantarflexion direction) and + 30 degrees both at rest and during dorsiflexor MVC trials performed on an isokinetic dynamometer. At all four ankle angles scans were taken from the TA proximal, central and distal regions. TA architecture did not differ (P > 0.05) neither between its two unipennate parts nor along the scanned regions over its length at a given ankle angle and state of contraction. Comparing MVC with rest at any given ankle angle, pennation angle was larger (62-71%, P < 0.01), fibre length smaller (37-40%, P < 0.01) and muscle thickness unchanged (P > 0.05). The model used estimated accurately (P > 0.05) changes in TA pennation angle occurring in the transition from rest to MVC and therefore its use is encouraged for estimating the isometric TA ankle moment and force generating capacity using musculoskeletal modelling.

The present study examined the hypothesis that the antagonistic ankle dorsiflexor coactivation level during maximum isometric voluntary plantarflexion (MVC) is a function of ankle angle. Six male subjects generated plantarflexion and dorsiflexion MVC trials at ankle angles of -15 deg (dorsiflexed direction), 0 deg (neutral position), +15 deg (plantarflexed direction) and +30 deg having the knee flexed at an angle of 90 deg. In all contractions surface EMG measurements were taken from tibialis anterior and soleus which were considered representative muscles of all dorsiflexors and plantarflexors, respectively. Antagonistic dorsiflexor coactivation was expressed as normalized EMG and moment. Calculations of the antagonistic dorsiflexor moment were based on the tibialis anterior EMG-dorsiflexor moment relationship from contractions at 50, 40, 30, 20 and 10 % of the dorsiflexion MVC moment. In both legs dorsiflexor coactivation level followed an open U-shaped pattern as a function of ankle angle. Differences of 9 and 14 % (P < 0.05) were found in the measured net plantarflexion MVC moment between legs at ankle angles of -15 and +30 deg, respectively. No difference (P > 0.05) was found in the calf circumference between legs. Differences were found in the antagonistic dorsiflexor coactivation between legs at ankle angles of -15 and +30 deg. In the weaker leg the antagonistic EMG measurements were higher by 100 and 45 % (P < 0.01) and the estimated antagonistic moments were higher by 70 and 43 % (P < 0.01) compared with the weaker leg at -15 and +30 deg, respectively. This finding was associated with a decreased range of motion (ROM) in the weaker leg (14 %, P < 0.01), such that no difference (P > 0.05) was found in dorsiflexor antagonistic coactivation between legs at end-range ankle angles. The findings of the study (i) have to be taken into consideration when estimating musculoskeletal loads in the lower extremity, (ii) imply that stretching training can result in a stronger plantarflexion at end-range ankle angles through inhibition of the dorsiflexors, and (iii) imply a neural drive inadequacy during a plantarflexion MVC at end-range angles.

1. The objectives of this study were to (1) quantify experimentally in vivo changes in pennation angle, fibre length and muscle thickness in the triceps surae complex in man in response to changes in ankle position and isometric plantarflexion moment and (2) compare changes in the above muscle architectural characteristics occurring in the transition from rest to a given isometric plantarflexion intensity with the estimations of a planimetric muscle model assuming constant thickness and straight muscle fibres. 2. The gastrocnemius medialis (GM), gastrocnemius lateralis (GL) and soleus (SOL) muscles of six males were scanned with ultrasonography at different sites along and across the muscle belly at rest and during maximum voluntary contraction (MVC) trials at ankle angles of -15 deg (dorsiflexed direction), 0 deg (neutral position), +15 deg (plantarflexed direction) and +30 deg. Additional images were taken at 80, 60, 40 and 20% of MVC at an ankle angle of 0 deg. 3. In all three muscles and all scanned sites, as ankle angle increased from -15 to +30 deg, pennation increased (by 6-12 deg, 39-67%, P < 0.01 at rest and 9-16 deg, 29-43%, P < 0.01 during MVC) and fibre length decreased (by 15-28 mm, 32-34%, P < 0.01 at rest and 8-10 mm, 27-30%, P < 0.05 during MVC). Thickness in GL and SOL increased during MVC compared with rest (by 5-7 mm, 36-47%, P < 0.01 in GL and 6-7 mm, 38-47%, P < 0.01 in SOL) while thickness of GM did not differ (P > 0.05) between rest and MVC. 4. At any given ankle angle the model underestimated changes in GL and SOL occurring in the transition from rest to MVC in pennation angle (by 9-12 deg, 24-38%, P < 0.01 in GL and 9-14 deg, 25-28%, P < 0.01 in SOL) and fibre length (by 6-15 mm, 22-39%, P < 0.01 in GL and 6-8 mm, 23-24%, P < 0.01 in SOL). 5. The findings of the study indicate that the mechanical output of muscle as estimated by the model used may be unrealistic due to errors in estimating the changes in muscle architecture during contraction compared with rest.