Fascicle Length Changes Research Articles

Passive Muscle-Tendon Unit Gearing Is Joint Dependent in Human Medial Gastrocnemius.

Skeletal muscles change length and develop force both passively and actively. Gearing allows muscle fiber length changes to be uncoupled from those of the whole muscle-tendon unit. During active contractions this process allows muscles to operate at mechanically favorable conditions for power or economical force production. Here we ask whether gearing is constant in passive muscle; determining the relationship between fascicle and muscle-tendon unit length change in the bi-articular medial gastrocnemius and investigating the influence of whether motion occurs at the knee or ankle joint. Specifically, the same muscle-tendon unit length changes were elicited by rotating either the ankle or knee joint whilst simultaneously measuring fascicle lengths in proximal and distal muscle regions using B-mode ultrasound. In both the proximal and distal muscle region, passive gearing values differed depending on whether ankle or knee motion occurred. Fascicle length changes were greater with ankle motion, likely reflecting anatomical differences in proximal and distal passive tendinous tissues, as well as shape changes of the adjacent mono-articular soleus. This suggests that there is joint-dependent dissociation between the mechanical behavior of muscle fibers and the muscle-tendon unit during passive joint motions that may be important to consider when developing accurate models of bi-articular muscles.

Difference in fascicle behaviors between superficial and deep quadriceps muscles during isometric contractions.

In this study we examined changes in fascicle length and pennation of vastus intermedius (VI) and vastus lateralis (VL) from rest to isometric contraction. Fourteen healthy men performed submaximal (30% and 50% of maximal voluntary contraction; MVC) isometric knee extensions at 60° and 110° knee flexion (0° = full extension). Panoramic images of VL and VI were obtained at rest and during contractions to measure fascicle length and pennation using extended field-of-view ultrasonography, after confirmation that the technique could be used reliably during sustained submaximal isometric contractions. The relative change in fascicle length from rest to 50% MVC was greater in VI than VL at 110° (P < 0.05). Pennation increased during contraction at 60° for VL (P < 0.05), but not for VI. Muscle-dependent fascicle behaviors were observed during contraction. These findings may provide a mechanistic underpinning for differences in sarcomere length changes, relative force contribution, and blood flow during contraction.

Architectural Changes of the Biceps Femoris Long Head after Concentric or Eccentric Training

To determine the architectural adaptations of the biceps femoris long head (BFlh) after concentric or eccentric strength training interventions and the time course of adaptation during training and detraining. Participants in this intervention (concentric training group [n = 14], eccentric training group [n = 14], male subjects) completed a 4-wk control period, followed by 6 wk of either concentric- or eccentric-only knee flexor training on an isokinetic dynamometer and finished with 28 d of detraining. Architectural characteristics of BFlh were assessed at rest and during graded isometric contractions using two-dimensional ultrasonography at 28 d prebaseline; baseline; and days 14, 21, and 42 of the intervention and then again after 28 d of detraining. BFlh fascicle length was significantly longer in the eccentric training group (P < 0.05; d range, 2.65-2.98) and shorter in the concentric training group (P < 0.05; d range, -1.62 to -0.96) after 42 d of training compared with baseline at all isometric contraction intensities. After the 28-d detraining period, BFlh fascicle length was significantly reduced in the eccentric training group at all contraction intensities compared with the end of the intervention (P < 0.05; d range, -1.73 to -1.55). There was no significant change in fascicle length of the concentric training group after the detraining period. These results provide evidence that short-term resistance training can lead to architectural alterations in the BFlh. In addition, the eccentric training-induced lengthening of BFlh fascicle length was reversed and returned to baseline values after 28 d of detraining. The contraction mode specific adaptations in this study may have implications for injury prevention and rehabilitation.

Muscle structural assembly and functional consequences.

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.

Effects of muscle activation on shear between human soleus and gastrocnemius muscles.

Lateral connections between muscles provide pathways for myofascial force transmission. To elucidate whether these pathways have functional roles invivo, we examined whether activation could alter the shear between the soleus (SOL) and lateral gastrocnemius (LG) muscles. We hypothesized that selective activation of LG would decrease the stretch-induced shear between LG and SOL. Eleven volunteers underwent a series of knee joint manipulations where plantar flexion force, LG, and SOL muscle fascicle lengths and relative displacement of aponeuroses between the muscles were obtained. Data during a passive full range of motion were recorded, followed by 20° knee extension stretches in both passive conditions and with selective electrical stimulation of LG. During active stretch, plantar flexion force was 22% greater (P<0.05) and relative displacement of aponeuroses was smaller than during passive stretch (P<0.05). Soleus fascicle length changes did not differ between passive and active stretches but LG fascicles stretched less in the active than passive condition when the stretch began at angles of 70° and 90° of knee flexion (P<0.05). The activity-induced decrease in the relative displacement of SOL and LG suggests stronger (stiffer) connectivity between the two muscles, at least at flexed knee joint angles, which may serve to facilitate myofascial force transmission.

Unexpected Fascicle Length Changes In Denervated Feline Soleus Muscle During Stance Phase Of Walking.

After surgical repair of traumatically severed peripheral nerves, associated muscles are paralyzed for weeks. Little is known about fascicle length changes in paralyzed muscles during locomotion. The aim of this study was to investigate to what extent, if any, muscle fascicles of denervated feline soleus (SO) change length during stance of walking when intact SO synergists are actively contracting. Hindlimb kinematics, SO fascicle and muscle-tendon unit (MTU) length, and EMG activity of SO, lateral gastrocnemius (LG) and medial gastrocnemius (MG) were measured during level and slope walking in adult cats. Measurements were taken before and 1–2 weeks following SO-LG denervation. Unexpectedly, SO fascicle lengthening and shortening during stance in all walking conditions were evident after denervation. The greatest SO fascicle shortening (17.3 ± 2.2% of a reference length) and least fascicle lengthening (1.5 ± 0.8%) after denervation were found during upslope walking, where MG EMG activity was greatest across slopes (P < 0.05) and greatest discrepancies between post denervation SO fascicle and MTU length changes occurred. These findings suggest that myofascial linkages between denervated SO and its active synergists might affect its fascicle length changes. Further studies are needed to directly test this suggestion.

Cross-education effect after unilateral eccentric-only isokinetic vs dynamic constant external resistance training

The aim of this study was to evaluate the effects of unilateral eccentric training using constant velocity or constant external load on untrained limb. Forty-nine participants were randomized in isokinetic (IK), dynamic constant external resistance (DCER) unilateral eccentric training or control groups. Knee-extension 1RM and isometric, eccentric and concentric knee extensors’ peak torques, as well as changes in vastus lateralis muscle thickness, fascicle length, pennation angle and quadriceps fat-free mass were measured. After training, both IK and DCER similarly increased over time 1RM (respectively, +3.6 kg, CI 95 % 0.6–6.5 and +4.3 kg, CI 95 % 1.6–6.9), concentric (respectively, +8.4 N/m, CI 95 % 0.0 to +16.4 and 9.8 CI 95 % 0.6–19.2), eccentric (respectively, +28.5 N/m, CI 95 % 11.0 to +46.0 and 21.1 CI 95 % 15.1–37.0), and isometric (respectively, +15.4 N/m, CI 95 % 0.7–30.0 and +13.9, CI 95 % 0.3–27.5) peak torques. No increase was found for vastus lateralis muscle thickness, fascicle length, pennation angle and quadriceps fat-free mass. Eccentric training was effective for inducing strength, but not structural, adaptations in untrained limb. Both in rehabilitation and training practice, use of easily available gym devices can be a good substitute for expensive and often unavailable isokinetic devices.

Unilateral eccentric resistance training: A direct comparison between isokinetic and dynamic constant external resistance modalities

Aim of the study was to compare the effects of unilateral eccentric-only training using constant velocity vs. constant external load. Forty-seven participants were randomized in isokinetic (IK), dynamic constant external resistance (DCER) unilateral eccentric training or control groups. Knee extension 1RM and isometric, eccentric and concentric knee extensors peak torque, as well as changes in vastus lateralis fascicle pennation angle, fascicle length, muscle thickness, and quadriceps fat-free mass were measured. Both IK and DCER training consisted in 5 × 8 eccentric-only repetitions, 2d/w, for 6 weeks. IK and DCER training sessions were matched for total volume. After training, both IK and DCER similarly increased 1RM (respectively, +4.4 kg, CI95% 1.8–7.0 and +5.5 kg, CI95% 3.3–7.9), isometric (respectively, +34.5 N/m, CI95% 23.0–45.9 and +15.8, CI95% 5.4–26.2) and concentric peak torque (respectively, +17.0 N/m, CI95% 6.6 to +27.4 and 12.2 CI95% 2.8–21.7). IK increased eccentric peak torque significantly more than DCER (respectively, +84.2 N/m, CI95% 66.3–102.1 and +38.2 N/m, CI95% 21.9–54.4). Both IK and DCER similarly increased fascicle length (respectively, +14.7 mm, CI95% 5.4–24.0 and +14.4 mm, CI95% 5.4–23.3) and muscle thickness (respectively, +3.3 mm, CI95% 1.5–5.1, and +4.1 mm, CI95% 2.5–5.7). Matching the training volume resulted in similar adaptations comparing eccentric-only IK or DCER resistance training. Both in rehabilitation and in training practice, the use of easily available gym devices can be a good substitute for expensive and often unavailable IK devices.

Triceps surae fascicle stretch is poorly correlated with short latency stretch reflex size

The short latency stretch reflex (SLR) is well described, but the stimulus that evokes the SLR remains elusive. One hypothesis states that reflex size is proportional to muscle fiber stretch, so in this study we examined the relationship between these 2 parameters in human triceps surae muscles. Achilles tendon taps and dorsiflexion stretches with different amplitudes and preactivation torques were applied to 6 participants while electromyography and muscle fascicle length changes were recorded in soleus and medial gastrocnemius (MG). In response to tendon taps, neither fascicle length nor velocity changes were correlated with SLR size in either muscle, but accelerometer peaks were observed immediately after hammer-tendon contact. Similar results were obtained after dorsiflexion stretches. Muscle fascicle stretch is poorly correlated with SLR size, regardless of perturbation parameters. We attribute the SLR trigger to the transmission of vibration through the lower limb, rather than muscle fiber stretch.

Effects of isometric quadriceps strength training at different muscle lengths on dynamic torque production

This study aims to (1) determine whether isometric training at a short vs. long quadriceps muscle length affects concentric torque production; (2) examine the relationship between muscle hypertrophy and concentric torque; and (3) determine whether changes in fascicle length are associated with changes in concentric torque.Sixteen men performed isometric training at a short (SL, n = 8) or a long muscle length (LL, n = 8). Changes in maximal concentric torque were measured at 30, 60, 90, 120, 180, 240 and 300 rad · s−1. The relationships between the changes in concentric torque, cross-sectional area, volume and fascicle length were tested.Concentric torque increased significantly after training only in LL and at angular velocities of 30 and 120 rad · s−1 by 12–13% (P < 0.05). Muscle size increased in LL only, the changes were correlated (r = 0.73–0.93, P < 0.05) with the changes in concentric torque. Vastus lateralis (VL) fascicle length increased in both groups (5.4 ± 4.9%, P = 0.001) but the change was not correlated with changes in concentric torque in either group.Isometric training-induced increases in muscle size and concentric torque were best elicited by training at long muscle lengths. These results highlight a clear muscle length dependence of isometric training on dynamic torque production.

Muscle fascicle behavior during eccentric cycling and its relation to muscle soreness.

A single bout of eccentric exercise confers a protective effect against muscle damage and soreness in subsequent eccentric exercise bouts, but the mechanisms underpinning this effect are unclear. This study compared vastus lateralis (VL) muscle-tendon behavior between two eccentric cycling bouts to test the hypothesis that muscle-tendon behavior would be different between bouts and would be associated with the protective effect. Eleven untrained men (27.1 ± 7.0 yr) performed two bouts of eccentric cycling (ECC1 and ECC2) separated by 2 wk for 10 min at 65% of maximal concentric workload (191.9 ± 44.2 W) each. Muscle soreness (by visual analog scale) and maximal voluntary isometric contraction (MVC) torque of the knee extensors were assessed before and 1-2 d after exercise. Using ultrasonography, VL fascicle length and angle changes during cycling were assessed, and tendinous tissue (TT) length changes were estimated. VL EMG amplitude, crank torque, and knee joint angles were measured during cycling. Soreness was greater (P < 0.0001) after ECC1 than ECC2, although MVC changes were not different between bouts (P = 0.47). No significant differences in peak EMG amplitude (normalized to EMG during MVC), crank peak torque, or knee angles were evident between bouts. However, fascicle elongation was 16% less during ECC2 than ECC1 (P < 0.01), indicating less fascicle strain in ECC2. Maximum TT length occurred at a smaller knee joint angle during ECC2 than ECC1 (P = 0.055). These results suggest that a lesser fascicle elongation and earlier TT elongation were associated with reduced muscle soreness after ECC2 than ECC1; thus, changes in muscle-tendon behavior may be an important mechanism underpinning the protective effect.

Concurrent deficits of soleus and gastrocnemius muscle fascicles and Achilles tendon post stroke.

Calf muscles and Achilles tendon play important roles in functional activities. However, it is not clear how biomechanical properties of the uniarticular soleus (SOL) and biarticular gastrocnemius muscle and Achilles tendon, including the fascicle length, pennation angle, and stiffness, change concurrently post stroke. Biomechanical properties of the medial gastrocnemius (GM) and soleus muscles were evaluated bilaterally in 10 hemiparetic stroke survivors using combined ultrasonography-biomechanical measurements. Biomechanical properties of the Achilles tendon including the length, cross-sectional area (CSA), stiffness, and Young's modulus were evaluated, together with calf muscle biomechanical properties. Gastrocnemius and SOL contributions were separated using flexed and extended knee positions. The impaired side showed decreased fascicle length (GM: 6%, P = 0.002 and SOL: 9%, P = 0.03, at full knee extension and 0° ankle dorsiflexion) and increased fascicular stiffness (GM: 64%, P = 0.005 and SOL: 19%, P = 0.012, at a common 50 N force level). In contrast, Achilles tendon on the impaired side showed changes in the opposite direction as the muscle fascicles with increased tendon length (5%, P < 0.001), decreased tendon CSA (5%, P = 0.04), decreased tendon stiffness (42%, P < 0.001) and Young's modulus (30%, P < 0.001) compared with the unimpaired side. The fascicle and tendon stiffness changes were correlated negatively to the corresponding fascicle and tendon length changes, and decrease in Achilles tendon stiffness was correlated to the increases of SOL and GM fascicular stiffness (P < 0.05). Characterizations of calf muscle fascicles and Achilles tendon biomechanical properties help us better understand concurrent changes of fascicles and tendon as part of the calf muscle-tendon unit and facilitate development of more effective treatments.

Is ankle contracture after stroke due to abnormal intermuscular force transmission?

Contracture after stroke could be due to abnormal mechanical interactions between muscles. This study examined if ankle plantarflexor muscle contracture after stroke is due to abnormal force transmission between the gastrocnemius and soleus muscles. Muscle fascicle lengths were measured from ultrasound images of soleus muscles in five subjects with stroke and ankle contracture and six able-bodied subjects. Changes in soleus fascicle length or pennation during passive knee extension at fixed ankle angle were assumed to indicate intermuscular force transmission. Changes in soleus fascicle length or pennation were adjusted for changes in ankle motion. Subjects with stroke had significant ankle contracture. After adjustment for ankle motion, 9 of 11 subjects demonstrated small changes in soleus fascicle length with knee extension, suggestive of intermuscular force transmission. However, the small changes in fascicle length may have been artifacts caused by movement of the ultrasound transducers. There were no systematic differences in change in fascicle length (median between-group difference adjusting for ankle motion = -0.01, 95% CI -0.26-0.08 mm/degree of knee extension) or pennation (-0.05, 95% CI -0.15-0.07 degree/ degree of knee extension). This suggests ankle contractures after stroke were not due to abnormal (systematically increased or decreased) intermuscular force transmission between the gastrocnemius and soleus.

Ultrasound reveals negligible cocontraction during isometric plantar flexion and dorsiflexion despite the presence of antagonist electromyographic activity.

Because of the approximate linear relationship between muscle force and muscle activity, muscle forces are often estimated during maximal voluntary isometric contractions (MVICs) from torque and surface electromyography (sEMG) measurements. However, sEMG recordings from a target muscle may contain cross-talk originating from nearby muscles, which could lead to erroneous force estimates. Here we used ultrasound imaging to measure in vivo muscle fascicle length (Lf) changes and sEMG to measure muscle activity of the tibialis anterior, medial gastrocnemius, lateral gastrocnemius, and soleus muscles during ramp MVICs in plantar and dorsiflexion directions (n = 8). After correcting longitudinal Lf changes for ankle rotation, the antagonist Lf at peak antagonist root-mean-square (RMS) amplitude were significantly longer than the agonist Lf at this sEMG-matched level. On average, Lf shortened from resting length by 1.29 to 2.90 mm when muscles acted as agonists and lengthened from resting length by 0.43 to 1.16 mm when muscles acted as antagonists (depending on the muscle of interest). The lack of fascicle shortening when muscles acted as antagonists indicates that cocontraction was likely to be negligible, despite cocontraction as determined by sEMG of between 7 and 23% MVIC across all muscles. Different interelectrode distances (IEDs) over the plantar flexors revealed significantly higher antagonist RMS amplitudes for the 4-cm IEDs compared with the 2-cm IEDs, which further indicates that cross-talk was present. Consequently, investigators should be wary about performing agonist torque corrections for isometric plantar flexion and dorsiflexion based on the antagonist sEMG trace and predicted antagonist moment.

Determination of the fascicle length of the gastrocnemius muscle during calf raise exercise using ultrasonography.

[Purpose] The purpose of this study was to find a strength training protocol which maintains isometric contraction of the triceps surae during dorsal flexion of the ankle. [Subjects] The left feet of 22 young normal volunteers who did not have orthopedic injuries or lower limb pain participated in this study. [Methods] All subjects performed four sets of five repetitions of four sets calf-raise (CR) exercise at were (1) 60 bpm without a pedestal, (2) 60 bpm with a pedestal, (3) 90 bpm without a pedestal, and (4) 90 bpm with a pedestal. The fascicle length of the lateral head of the gastrocnemius and ankle angle were measured using ultrasonography and a video camera. The CR exercise was divided into two or three phases using the kinematics of the ankle. The average change in fascicle length over the five repetitions of each phase were compared. [Results] The change of the fascicle length during the hyper-dorsiflexion phase was significantly smaller than during the other two phases. [Conclusion] It is possible that eccentric CR exercises have progressed to motor learning of the isometric contraction during counter movement, and improved the release of elastic energy of the Achilles tendon during running, jumping, and other athletic activities.

Effect of prolonged critical care admissions on upper and lower limb muscle architecture

Muscle wasting is a common consequence of long-term stay in the critical care environment, which may slow down the rehabilitation of survivors. Previous ultrasound studies have demonstrated a loss of cross-sectional area of lower limb muscles during a 10-day intensive care stay. In this study, we have looked at how markers of muscle architecture (muscle thickness, pennation angle and fascicle length) change in the lower limb, as well as looking at changes in muscle thickness in the upper limb.

Range of motion, neuromechanical, and architectural adaptations to plantar flexor stretch training in humans

The neuromuscular adaptations in response to muscle stretch training have not been clearly described. In the present study, changes in muscle (at fascicular and whole muscle levels) and tendon mechanics, muscle activity, and spinal motoneuron excitability were examined during standardized plantar flexor stretches after 3 wk of twice daily stretch training (4 × 30 s). No changes were observed in a nonexercising control group (n = 9), however stretch training elicited a 19.9% increase in dorsiflexion range of motion (ROM) and a 28% increase in passive joint moment at end ROM (n = 12). Only a trend toward a decrease in passive plantar flexor moment during stretch (-9.9%; P = 0.15) was observed, and no changes in electromyographic amplitudes during ROM or at end ROM were detected. Decreases in H(max):M(max) (tibial nerve stimulation) were observed at plantar flexed (gastrocnemius medialis and soleus) and neutral (soleus only) joint angles, but not with the ankle dorsiflexed. Muscle and fascicle strain increased (12 vs. 23%) along with a decrease in muscle stiffness (-18%) during stretch to a constant target joint angle. Muscle length at end ROM increased (13%) without a change in fascicle length, fascicle rotation, tendon elongation, or tendon stiffness following training. A lack of change in maximum voluntary contraction moment and rate of force development at any joint angle was taken to indicate a lack of change in series compliance of the muscle-tendon unit. Thus, increases in end ROM were underpinned by increases in maximum tolerable passive joint moment (stretch tolerance) and both muscle and fascicle elongation rather than changes in volitional muscle activation or motoneuron pool excitability.

Reciprocal activation of gastrocnemius and soleus motor units is associated with fascicle length change during knee flexion.

While medial gastrocnemius (MG) and soleus (SOL) are considered synergists, they are anatomically exclusive in that SOL crosses only the ankle, while MG crosses both the knee and ankle. Due to the force‐length properties of both active and passive structures, activation of SOL and MG must be constantly regulated to provide the required joint torques for any planned movement. As such, the aim of this study was to investigate the neural regulation of MG and SOL when independently changing their length by changing only the knee joint angle, thus exclusively altering the length of MG fibers. MG and SOL motor units (MU) were recorded intramuscularly along with ultrasound imaging of MG and SOL fascicle lengths, while moving the knee through 60° of rotation and maintaining a low level of voluntary plantar flexor torque. The results showed a reciprocal activation of MG and SOL as the knee was moved into flexion and extension. A clear reduction in MG MU firing rates occurred as the knee was flexed (MG fascicles shortening), with de‐recruitment of most MG MU occurring at close to full knee flexion. A concomitant increase in SOL MU activity was observed while no change in the length of its fascicles was found. The opposite effects were found when the knee was moved into extension. A strong correlation (ICC = 0.78) was found between the fascicle length at which MG MUs were de‐recruited and subsequently re‐recruited. This was stronger than the relationship of de‐recruitment and re‐recruitment with knee angle (ICC = 0.52), indicating that in this instance, muscle fascicle length rather than joint angle is more influential in regulating MG recruitment. Such a reciprocal arrangement like the one presented here for SOL and MG is essential for human voluntary movements such as walking or cycling.

Differences in Plantar Flexor Fascicle Length and Pennation Angle between Healthy and Poststroke Individuals and Implications for Poststroke Plantar Flexor Force Contributions.

Poststroke plantar flexor muscle weakness has been attributed to muscle atrophy and impaired activation, which cannot collectively explain the limitations in force-generating capability of the entire muscle group. It is of interest whether changes in poststroke plantar flexor muscle fascicle length and pennation angle influence the individual force-generating capability and whether plantar flexor weakness is due to uniform changes in individual muscle force contributions. Fascicle lengths and pennation angles for the soleus, medial, and lateral gastrocnemius were measured using ultrasound and compared between ten hemiparetic poststroke subjects and ten healthy controls. Physiological cross-sectional areas and force contributions to poststroke plantar flexor torque were estimated for each muscle. No statistical differences were observed for any muscle fascicle lengths or for the lateral gastrocnemius and soleus pennation angles between paretic, nonparetic, and healthy limbs. There was a significant decrease (P < 0.05) in the paretic medial gastrocnemius pennation angle compared to both nonparetic and healthy limbs. Physiological cross-sectional areas and force contributions were smaller on the paretic side. Additionally, bilateral muscle contributions to plantar flexor torque remained the same. While the architecture of each individual plantar flexor muscle is affected differently after stroke, the relative contribution of each muscle remains the same.

Interaction between gastrocnemius medialis fascicle and Achilles tendon compliance: a new insight on the quick-release method

The insufficient temporal resolution of imaging devices has made the analysis of very fast movements, such as those required to measure active muscle-tendon unit stiffness, difficult. Thus the relative contributions of tendon, aponeurosis, and fascicle to muscle-tendon unit compliance remain to be determined. The present study analyzed the dynamic interactions of fascicle, tendon, and aponeurosis in human gastrocnemius medialis during the first milliseconds of an ankle quick-release movement, using high-frame-rate ultrasonography (2,000 frames/s). Nine subjects performed the tests in random order at six levels of maximal voluntary contraction (MVC) (30% to 80% of MVC). These tests were carried out with the ultrasound probe placed on the muscle belly and on the myotendinous junction. Tendon, muscle fascicle, and aponeurosis length changes were quantified in relation to shortening of the muscle-tendon unit during the first few milliseconds following the release. The tendon was the main contributor (around 72%) to the shortening of the muscle-tendon unit, whereas the muscle fascicle and aponeurosis contributions were 18% and 10%, respectively. Because these structures can be considered in series, the quantified contributions can be regarded as relative contributions to muscle-tendon compliance. These contributions were not modified with the level of MVC or the time range used for the analysis between 10 and 25 ms. The constant contribution of tendon, muscle fascicle, and aponeurosis to muscle-tendon unit compliance may help to simplify the mechanism of compliance regulation and to maintain the important role of tendons in enhancing work output and movement efficiency.