Muscle-tendon Interaction Research Articles

Lower Extremity Muscle Tendon Interaction Around Knee During Gait Among Adolescent Children with Cerebral Palsy with Varying Crouch Angle.

Optimal management and surgical planning of severe bony deformities and muscle tendon unit contractures demands comprehensive evaluation of all structures including the dynamic muscle tendon length of all muscles around the joint during gait. Present study aimed to explore dynamic muscle-tendon length for all muscles around the knee joint along with pelvis, hip, and ankle joint kinematics among adolescent children with varying crouch angle. Muscle-tendon length of 29 adolescent children with cerebral palsy with varying crouch angles was computed using a full-body musculo-skeletal model and expressed as a percentage of muscle-tendon length during walking compared to resting condition. Children with knee flexion angle greater than 25° demonstrated lower anterior pelvic tilt and 11% greater muscle-tendon length of semimembranosus and biceps femoris during stance phase of gait compared to children with knee flexion angle less than 25° and typically developing children (p < 0.01). The findings of present study reported that routine bedside clinical evaluation in adolescent children with knee flexion angle greater than 25° revealed moderate shortening of hamstring muscle in supine position. Whereas instrumented objective evaluation of gait demonstrated lengthened hamstring muscle and reduced hip extension and relatively lower anterior pelvic tilt. Therefore, it may be valuable to add objective assessment of dynamic muscle-tendon length to kinematics of all lower-extremity joint motion during gait, in order to understand the muscle-joint interactions; particularly in children with severe crouch and plan specific, tailor-made surgical and non-surgical interventions.

In-vivo and in-vitro environments affect the storage and release of energy in tendons.

Understanding tendon mechanical properties, such as stiffness and hysteresis, can provide insights into injury mechanisms. This research addresses the inconsistency of previously reported in-vivo and in-vitro tendon hysteresis properties. Although limited, our preliminary findings suggest that in-vivo hystereses (Mean ± SD; 55% ± 9%) are greater than in-vitro hystereses (14% ± 1%) when directly comparing the same tendon for the same loading conditions in a sheep model in-vivo versus within 24h post-mortem. Overall, it therefore appears that the tendon mechanical properties are affected by the testing environment, possibly related to differences in muscle-tendon interactions and fluid flow experienced in-vivo versus in-vitro. This communication advocates for more detailed investigations into the mechanisms resulting in the reported differences in tendon behaviour. Overall, such knowledge contributes to our understanding of tendon function towards improving modelling and clinical interventions, bridging the gap between in-vivo and in-vitro observations and enhancing the translational relevance of biomechanical studies.

Energy transfer in reactive movements as a function of individual stretch load

Background: By directly recording electromyographic activity profiles and muscle-tendon interaction, this study aimed to elucidate the mechanisms why well-trained track and field athletes (experts) are able to outperform untrained individuals without former systematic experience in reactive jump training (novices). In particular, reactive power output and the elastic recoil properties of the muscle-tendon unit (MTU) were of special interest. For this purpose, stiffness regulation on muscle and joint level, energy management in terms of storing or dissipating elastic energy were compared between experts and novices during various stretch loads.Methods: Experts were compared with novices during reactive drop jumps (DJs) from drop heights ranging between 25 and 61 cm. Delta kinetic energy (Ekin) was calculated as the difference between the Ekin at take-off and ground contact (GC) to determine energy management. By recording electromyography of the lower limb muscles, in vivo fascicle dynamics (gastrocnemius medialis) and by combining kinematics and kinetics in a 3D inverse dynamics approach to compute ankle and knee joint kinetics, this study aimed to compare reactive jump performance, the neuromuscular activity and muscle-tendon interaction between experts and novices among the tested stretch loads.Results: Experts demonstrated significantly higher power output during DJs. Among all drop heights experts realized higher delta Ekin compared to novices. Consequently, higher reactive jump performance shown for experts was characterized by shorter GC time (GCT), higher jump heights and higher neuromuscular activity before and during the GC phase compared to novices. Concomitantly, experts were able to realize highest leg stiffness and delta Ekin in the lowest stretch load; however, both groups compensated the highest stretch load by prolonged GCT and greater joint flexion. On muscle level, experts work quasi-isometrically in the highest stretch load, while in novices GM fascicles were forcefully stretched.Conclusion: Group-specific stiffness regulation and elastic recoil properties are primarily influenced by the neuromuscular system. Due to their higher neuromuscular activity prior and during the GC phase, experts demonstrate higher force generating capacity. A functionally stiffer myotendinous system through enhanced neuromuscular input enables the experts loading their elastic recoil system more efficiently, thus realizing higher reactive power output and allowing a higher amount of energy storage and return. This mechanism is regulated in a stretch load dependent manner.

Vertical Jumping Performance: Recording the Effects of Proprioceptive Neuromuscular Facilitation Stretching at Different Plantar Flexor Lengths.

Flexibility seems to be an essential part of both the training and rehabilitation processes. Several stretching techniques have been used to improve the range of motion (ROM) of the joints with the proprioceptive neuromuscular facilitation (PNF) method being the most effective one. Although plantar flexors are ideal to compare the acute effects of synergistic muscle groups on performance, it is not clear whether the PNF stretch at different muscle lengths could result in different alterations. Sixteen male students randomly performed 2 levels of stretching (PNF with bended knees, or PNFshort, and with extended knee, or PNFlong) and 3 types of jumps, separated by 48 hours (7 sessions in total). Jumping parameters were recorded by a force plate, and the final jumping height (H) and ground reaction forces (Fz) were analyzed. Furthermore, the ROM of the ankle joint was recorded before, right after, and 15 minutes after the stretches. Results: The ankle joint's ROM joint was increased after both interventions. No significant changes were found in the jumping height of all jumps. The Fz, during the squat jump (SJ) and countermovement jump (CMJ), were increased after PNFshort. Similarly, a significant increase was found in Fz in drop jumps (DJ) right after the PNFshort. Our findings demonstrated that PNF stretches of different lengths could potentially alter the stretch-shortening cycle's performance, possibly leading to a non-optimal muscle-tendon interaction.

Measurements of tendon length changes during stretch–shortening cycles in rat soleus

The muscle force attained during concentric contractions is augmented by a preceding eccentric contraction (the stretch–shortening cycle (SSC) effect). At present, tendon elongation is considered the primary mechanism. However, we recently found that the magnitude of the SSC effect was not different, even after removing the Achilles tendon. To resolve these discrepant results, direct measurement of changes in Achille tendon length is required. Therefore, this study aimed to elucidate the influence of tendon elongation on the SSC effect by directly measuring the changes in Achilles tendon length. The rat soleus was subjected to pure concentric contractions (pure shortening trials) and concentric contractions with a preceding eccentric contraction (SSC trials). During these contractions, the Achilles tendon length was visualized using a video camera. The muscle force attained during the concentric contraction phase in the SSC trial was significantly larger than that in the pure shortening trial (p = 0.022), indicating the existence of the SSC effect. However, the changes in Achilles tendon length were not different between trials (i.e., the magnitude of tendon shortening attained during the shortening phase was 0.20 ± 0.14 mm for the SSC trial vs. 0.17 ± 0.09 mm for the pure shortening trial), indicating that the observed SSC effect is difficult to be explained by the elastic energy stored in tendons or muscle–tendon interaction. In conclusion, the effect of tendon elongation on the SSC effect should be reconsidered, and other factors may contribute to the SSC effect.

Joint Coordination and Muscle-Tendon Interaction Differ Depending on The Level of Jumping Performance.

The countermovement jump is a popular measurement modality to evaluate muscle power in sports and exercise. Muscle power is essential to achieve a high jump, yet the well-coordinated movement of the body segments, which optimizes the stretch-shortening cycle (SSC) effects, is also required. Among the proposed explanations of SSC effects, this study investigated whether the ankle joint kinematics, kinetics, and muscle-tendon interaction depend on the level of jump skill and the jump task. Sixteen healthy males were grouped as a function of their jump height (High jumpers; greater than 50 cm, Low jumpers; less than 50 cm). They were instructed to jump with two intensities; light effort (20 % of their height) and maximal effort. Joint kinematics and kinetics of the lower limbs were analyzed using a 3-dimensional motion analysis system. The muscle-tendon interaction was investigated using B-mode real-time ultrasonography. As the jump intensity increased, all participants jumped with increased joint velocity and power. However, the high jumper shows less fascicle shortening velocity (-0.2 ± 0.1 m/s) than the low jumper group (-0.3 ± 0.1 m/s) and greater tendon velocity, which indicated the capability of elastic energy recoil. In addition, the delayed onset time of ankle extension in the high jumper implies better use of the catapult mechanism. The findings of this study showed that the muscle-tendon interaction differs depending on the jump skill level, suggesting a more efficient neuromuscular control in skilled jumpers.

Does different activation between the medial and the lateral gastrocnemius during walking translate into different fascicle behavior?

The functional difference between the medial gastrocnemius (MG) and lateral gastrocnemius (LG) during walking in humans has not yet been fully established. Although evidence highlights that the MG is activated more than the LG, the link with potential differences in mechanical behavior between these muscles remains unknown. In this study, we aimed to determine whether differences in activation between the MG and LG translate into different fascicle behavior during walking. Fifteen participants walked at their preferred speed under two conditions: 0% and 10% incline treadmill grade. We used surface electromyography and B-mode ultrasound to estimate muscle activation and fascicle dynamics in the MG and LG. We observed a higher normalized activation in the MG than in the LG during stance, which did not translate into greater MG normalized fascicle shortening. However, we observed significantly less normalized fascicle lengthening in the MG than in the LG during early stance, which matched with the timing of differences in activation between muscles. This resulted in more isometric behavior of the MG, which likely influences the muscle-tendon interaction and enhances the catapult-like mechanism in the MG compared with the LG. Nevertheless, this interplay between muscle activation and fascicle behavior, evident at the group level, was not observed at the individual level, as revealed by the lack of correlation between the MG-LG differences in activation and MG-LG differences in fascicle behavior. The MG and LG are often considered as equivalent muscles but the neuromechanical differences between them suggest that they may have distinct functional roles during locomotion.

Automated analysis of medial gastrocnemius muscle-tendon junction displacements in heathy young adults during isolated contractions and walking using deep neural networks

Background and objectiveDirect measurement of muscle-tendon junction (MTJ) position is important for understanding dynamic tendon behavior and muscle-tendon interaction in healthy and pathological populations. Traditionally, obtaining MTJ position during functional activities is accomplished by manually tracking the position of the MTJ in cine B-mode ultrasound images – a laborious and time-consuming process. Recent advances in deep learning have facilitated the availability of user-friendly open-source software packages for automated tracking. However, these software packages were originally intended for animal pose estimation and have not been widely tested on ultrasound images. Therefore, the purpose of this paper was to evaluate the efficacy of deep neural networks to accurately track medial gastrocnemius MTJ positions in cine B-mode ultrasound images across tasks spanning controlled loading during isolated contractions to physiological loading during treadmill walking. MethodsCine B-mode ultrasound images of the medial gastrocnemius MTJ were collected from 15 subjects (6M/9F, 23 yr, 71.9 kg, 1.8 m) during treadmill walking at 1.25 m/s and during maximal voluntary isometric plantarflexor contractions (MVICs). Five deep neural networks were trained using 480 manually-labeled images collected during walking, defined as the ground truth, and were then used to predict MTJ position in images from novel subjects: 1) during walking (novel-subject) and 2) during MVICs (novel-condition). ResultsWe found an average mean absolute error of 1.26±1.30 mm and 2.61±3.31 mm between the ground truth and predicted MTJ positions in the novel-subject and novel-condition evaluations, respectively. ConclusionsOur results provide support for the use of open-source software for creating deep neural networks to reliably track MTJ positions in B-mode ultrasound images. We believe this approach to MTJ position tracking is an accessible and time-saving solution, with broad applications for many fields, such as rehabilitation or clinical diagnostics.

Series elasticity facilitates safe plantar flexor muscle-tendon shock absorption during perturbed human hopping.

In our everyday lives, we negotiate complex and unpredictable environments. Yet, much of our knowledge regarding locomotion has come from studies conducted under steady-state conditions. We have previously shown that humans rely on the ankle joint to absorb energy and recover from perturbations; however, the muscle-tendon unit (MTU) behaviour and motor control strategies that accompany these joint-level responses are not yet understood. In this study, we determined how neuromuscular control and plantar flexor MTU dynamics are modulated to maintain stability during unexpected vertical perturbations. Participants performed steady-state hopping and, at an unknown time, we elicited an unexpected perturbation via rapid removal of a platform. In addition to kinematics and kinetics, we measured gastrocnemius and soleus muscle activations using electromyography and in vivo fascicle dynamics using B-mode ultrasound. Here, we show that an unexpected drop in ground height introduces an automatic phase shift in the timing of plantar flexor muscle activity relative to MTU length changes. This altered timing initiates a cascade of responses including increased MTU and fascicle length changes and increased muscle forces which, when taken together, enables the plantar flexors to effectively dissipate energy. Our results also show another mechanism, whereby increased co-activation of the plantar- and dorsiflexors enables shortening of the plantar flexor fascicles prior to ground contact. This co-activation improves the capacity of the plantar flexors to rapidly absorb energy upon ground contact, and may also aid in the avoidance of potentially damaging muscle strains. Our study provides novel insight into how humans alter their neural control to modulate in vivo muscle-tendon interaction dynamics in response to unexpected perturbations. These data provide essential insight to help guide design of lower-limb assistive devices that can perform within varied and unpredictable environments.

Effects Of An 8-week Nordic Hamstring Vs. Isokinetic Eccentric Training Intervention On Biceps Femoris Muscle-tendon Interactions

Resistance eccentric training triggers adaptations in both active and passive elements of the muscle tendon unit (MTU). Previous research highlight the buffering role that tendinous tissues may play to mitigate muscle strain and to optimize operating fascicle lengths. However, the effects of eccentric training on the muscle-tendon interactions of the biceps femoris remain unexplored. PURPOSE: To evaluate the effects of eccentric training on torque and muscle-tendon interactions of the biceps femoris muscle during lengthening contractions. METHODS: Eighteen participants completed an 8-wk standardized eccentric training intervention comprising 15 sessions of Nordic Hamstring (NH) or isokinetic leg curl (IK; n=10) exercise. Pre and post training, torque and fascicle, tendinous tissue and MTU length of the biceps femoris muscle were measured during maximal 70° lengthening knee extensions at 60°.s-1. One sample t-test statistical parametric mapping (SPM) analyses were performed to evaluate fascicle and tendon behaviour during the contractions. Training effects were evaluated with two-way repeated measures ANOVA and paired t-test SPM. RESULTS: Both groups increased torque from 38% to 100% of the contraction duration, with greater improvements in IK from 78.5 to 100% (p < 0.001). The contribution of tendinous tissue to MTU length changes slightly increased only in IK at the last 2% of the contraction (p = 0.04). Despite opposite trend changes in NH, no training effects on fascicle or tendon behavior occurred. The tendinous tissues contribution to MTU lengthening was greater than fascicles up to 75% of the contraction. The fascicles contribution progressively augmented at the end of the contraction (p < 0.05). CONCLUSION: We provide novel findings on the muscle-tendon interactions of the biceps femoris muscle during lengthening contractions. A 15-session eccentric training program comprising NH or IK exercise does not seem to affect muscle-tendon interactions.

Progressive resistance training for adolescents with cerebral palsy: the STAR randomized controlled trial.

To evaluate the effect of progressive resistance training of the ankle plantarflexors on gait efficiency, activity, and participation in adolescents with cerebral palsy (CP). Sixty-four adolescents (10-19y; 27 females, 37 males; Gross Motor Function Classification System [GMFCS] levels I-III) were randomized to 30 sessions of resistance training (10 supervised and 20 unsupervised home sessions) over 10 weeks or usual care. The primary outcome was gait efficiency indicated by net nondimensional oxygen cost (NNcost). Secondary outcomes included physical activity, gross motor function, participation, muscle strength, muscle and tendon size, and muscle and tendon stiffness. Analysis was intention-to-treat. Median attendance at the 10 supervised sessions was 80% (range 40-100%). There was no between-group difference in NNcost at 10 (mean difference: 0.02, 95% confidence interval [CI] -0.07 to 0.11, p=0.696) or 22 weeks (mean difference: -0.08, 95% CI -0.18 to 0.03, p=0.158). There was also no evidence of between-group differences in secondary outcomes at 10 or 22 weeks. There were 123 adverse events reported by 27 participants in the resistance training group. We found that 10 supervised sessions and 20 home sessions of progressive resistance training of the ankle plantarflexors did not improve gait efficiency, muscle strength, activity, participation, or any biomechanical outcome among adolescents with CP. Thirty sessions of progressive resistance training of the ankle plantarflexors over 10 weeks did not improve gait efficiency among ambulatory adolescents with cerebral palsy. Resistance training did not improve muscle strength, activity, or participation. Ninety percent of participants experienced an adverse event. Most adverse events were expected and no serious adverse events were reported.

Do Stretch-Shortening Cycles Really Occur in the Medial Gastrocnemius? A Detailed Bilateral Analysis of the Muscle-Tendon Interaction During Jumping.

The effect of stretch-shortening cycles (SSCs) is often studied in laboratory settings, yet it remains unclear whether highly active muscle SSCs actually occur during in vivo movement. Nine highly trained jumping athletes performed single-leg pre-hop forward jumps at maximal effort. We hypothesized that these jumps would induce a SSC at the level of the muscle in the medial gastrocnemius. Kinematic and kinetic data were collected together with electromyography signals (EMG) and muscle fascicle length and pennation angle changes of the medial gastrocnemius of both legs and combined with a musculoskeletal model to calculate the stretch-shortening behavior of the muscle (fascicles) and tendon (series-elastic element). The length changes of the fascicles, longitudinal muscle displacement, series-elastic element, and whole muscle-tendon unit further allowed for a detailed analysis of the architectural gearing ratio between different phases of the SSC within a single movement. We found a SSC at the level of the joint, muscle-tendon unit and tendon but not at the muscle. We further found that the average architectural gearing ratio was higher during the stretching of the series-elastic element as compared to when the series-elastic element was shortening, yet this was not statistically tested because of low sample size for this parameter. However, we found no correlation when plotting the architectural gearing ratio as a function of the fascicle velocities at each instance in time. Despite the athletes having a clear preferred leg for jumping, we found no differences in any kinematic or kinetic parameter between the preferred and non-preferred leg or any parameter from the muscle-tendon interaction analysis other than a reduced longitudinal muscle shortening in the non-preferred leg (p = 0.008). We conclude that, although common at the level of the joints, MTUs, and tendon (series-elastic element), highly active SSCs very rarely occur in the medial gastrocnemius, even in movements that induce high loading. This has important implications for the translation of ex vivo findings on SSC effects, such as residual force enhancement, in this muscle. We further conclude that there is no precise tuning of the architectural gearing ratio in the medial gastrocnemius throughout the whole movement.

Surface properties affect the interplay between fascicles and tendinous tissues during landing.

Muscle-tendon units are forcefully stretched during rapid deceleration events such as landing. Consequently, tendons act as shock absorbers by buffering the negative work produced by muscle fascicles likely to prevent muscle damage. Landing surface properties can also modulate the amount of energy dissipated by the body, potentially effecting injury risk. This study aimed to evaluate the influence of three different surfaces on the muscle-tendon interactions of gastrocnemius medialis (GM), and vastus lateralis (VL) during single- and double-leg landings from 50cm. Ultrasound images, muscle activity and joint kinematics were collected for 12 participants. Surface testing was also performed, revealing large differences in mechanical behavior. During single-leg landing, stiffer surfaces increased VL fascicle lengthening and velocity, and muscle activity independent of joint kinematics while GM length changes showed no difference between surfaces. Double-leg landing resulted in similar fascicle and tendon behavior despite greater knee flexion angles on stiffer surfaces. This demonstrates that VL fascicle lengthening is greater when the surface stiffness increases, when performing single-leg landing. This is due to the combination of limited knee joint flexion and lower surface absorption ability which resulted in greater mechanical demand mainly withstood by fascicles. GM muscle-tendon interactions remain similar between landing surfaces and types. Together, this suggests that surface damping properties primarily affect the VL muscle-tendon unit with a potentially higher risk of injury as a result of increased surface stiffness when performing single-leg landing tasks.

Muscle-Tendon Interaction During Human Dolphin-Kick Swimming.

Without high impact forces, it is not clear how humans can utilize tendon elasticity during low-impact activities. The purpose of the present study was to examine the muscle-tendon behavior together with the electromyographic (EMG) activities of the vastus lateralis (VL) muscle during the human dolphin-kicking. In a swimming pool, each subject (n = 11) swam the 25 m dolphin-kicking at two different speeds (NORMAL and FAST). Surface EMGs were recorded from the VL and biceps femoris (BF) muscles. Simultaneous recordings of the knee joint angle by electro-goniometer and of the VL fascicle length by ultrasonography were used to calculate the muscle-tendon unit and tendinous length of VL (LMTU and LTT, respectively). In the dolphin-kicking, the stretching and shortening amplitudes of VL LMTU did not differ significantly between the two kicking speed conditions. However, both stretching and shortening amplitudes of the VL fascicle length were lower at FAST than at NORMAL speed whereas the opposite was found for the VL LTT values. At FAST, the contribution of the VL tendinous length to the entire VLMTU length changes increased. The EMG analysis revealed at FAST higher agonist VL activation from the late up-beat (MTU stretching) to the early down-beat phases as well as increased muscle co-activation of VL and BF muscles from the late down-beat to early up-beat phases of dolphin-kicking. These results suggest that at increasing kicking speeds, the VL fascicles and tendinous tissues during aquatic movements can utilize tendon elasticity in a similar way than in terrestrial forms of locomotion. However, these activation profiles of VL and BF muscles may differ from their activation pattern in terrestrial locomotion.

Effects of physical training on sarcomere lengths and muscle-tendon interface of the cervical region in an experimental model of menopause.

The aim of this study was to describe the structural and ultrastructural aspects of the myotendinous junction (MTJ) and the proximal and distal sarcomeres of the sternomastoid of aged Wistar rats subjected to an experimental model of menopause and swimming training. A total of 20 female elderly rats were divided into the following four groups (n=5 in each group): sedentary/no-menopausal (SNM), trained/nomenopausal (TNM), sedentary/menopausal (SM), and trained/menopausal (TM). The MTJ samples were dissected and analyzed using transmission electron microscopy. We showed that the TNM Group rats exhibited changes in morphological characteristics as a consequence of physical exercise, which included an increase of 36.60% (P<0.001) in the evagination length of the MTJ and a reduction in the length of the distal (77.38%) (P<0.0001) and proximal (68.15%) (P<0.0001) sarcomeres. The SM Group exhibited a reduction of about 275.93% (P<0.001) in the muscle-tendon interface and in the lengths of distal sarcomeres (55.87%) (P<0.0001) compared with SNM Group. Our results suggest that the swimming training under experimental model of menopause promoted tissue reorganization and increased muscle-tendon interaction with a drastic development in the length and thickness of the sarcoplasmatic invaginations and evaginations. In addition, the sarcomeres exhibited different lengths and a reduction in both groups subjected to swimming training.

System identification of ankle joint dynamics based on plane-wave ultrasound muscle imaging.

Effective treatment of movement disorders requires thorough understanding of human limb control. Joint dynamics can be assessed using robotic manipulators and system identification. Due to tendon compliance, joint angle and muscle length are not proportional. This study uses plane-wave ultrasound imaging to investigate the dynamic relation between ankle joint angle and muscle fiber stretch. The first goal is to determine the feasibility of using ultrasound imaging with system identification; the second goal is to assess the relation between ankle angle, muscle stretch, and reflex size. Soleus and gastrocnemius muscle stretches were assessed with ultrasound imaging and image tracking. For small (1° SD) continuous motions, muscle stretch was proportional to ankle angle during a relax task, but images were too noisy to make that assessment during an active position task. For transient perturbations with high velocity (> 90°/s) the muscle length showed oscillations that were not present in the ankle angle, demonstrating a non-proportional relationship and muscle-tendon interaction. The gastrocnemius velocity predicted the size of the short-latency reflex better than the ankle angle velocity. Concluding, plane-wave ultrasound muscle imaging is feasible for system identification experiments and shows that muscle length and ankle angle are proportional during a relax task with small continuous perturbations.

Triceps surae muscle–subtendon interaction differs between young and older adults

ABSTRACTBackground: Mechanical power generated via triceps surae muscle-tendon interaction during walking is important for walking performance. This interaction is made complex by distinct “subtendons” arising from the lateral and medial gastrocnemius (GAS) and soleus (SOL) muscles. Comparative data and our own in vivo evidence allude to a reduced capacity for sliding between adjacent subtendons compromising the Achilles tendon in old age. However, its unclear if and how these changes affect muscle contractile behavior.Objective: We investigated aging effects on triceps surae muscle-subtendon interaction using dual-probe ultrasound imaging during isolated muscle contractions. We hypothesized that, compared to young adults, older adults would have more uniform subtendon tissue displacements that are accompanied by anatomically consistent differences in GAS versus SOL muscle length change behavior.Materials and Methods: 9 younger subjects (age: 25.1 ± 5.6 years) and 10 older adult subjects (age: 74.3 ± 3.4 years) completed a series of ramped maximum isometric voluntary contractions at ankle angles spanning 0° (neutral) to 30° plantarflexion. Two linear array ultrasound transducers simultaneously recorded GAS and SOL fascicle kinematics and tissue displacements in their associated tendinous structures.Results: We revealed that older adults have more uniform subtendon tissue displacements that extend to anatomically consistent and potentially unfavorable changes in muscle contractile behavior – evidenced by smaller differences between gastrocnemius and soleus peak shortening during isometric force generation.Conclusions: These findings provide an important biomechanical basis for previously reported correlations between more uniform Achilles subtendon behavior and reduced ankle moment generation during waking in older adults.

Distinct muscle-tendon interaction during running at different speeds and in different loading conditions.

The interaction between the Achilles tendon and the triceps surae muscles seems to be modulated differently with various task configurations. Here we tested the hypothesis that the increased forces and ankle joint work during running under contrasting conditions (altered speed or load) would be met by different, time-dependent adjustments at the muscle-tendon level. Ultrasonography, electromyography, kinematics, and ground reaction force measurements were used to examine Achilles tendon, gastrocnemius, and soleus muscle mechanics in 16 runners in four different running conditions, consisting of a combination of two different speeds (preferred and +20% of preferred speed) and two loading conditions (unloaded and +20% of body mass). Positive ankle joint work increased similarly (+13%) with speed and load. Gastrocnemius and soleus muscle fascicle length and peak velocity were not altered by either condition, suggesting that contractile conditions are mostly preserved despite the constraints imposed in this experimental design. However, at higher running speed, tendon length changes were unaltered but mean muscle electromyographic activity increased in gastrocnemius (+10%, P < 0.01) and soleus (+14%, P < 0.01). Conversely, when loading was increased, mean muscle activity remained similar to unloaded conditions but the mean velocity of gastrocnemius fascicles was reduced and tendon recoil increased (+29%, P < 0.01). Collectively, these results suggest that the neuromuscular system meets increased mechanical demands by favoring economical force production when enough time is available. NEW & NOTEWORTHY We demonstrate that muscle-tendon mechanics are adjusted differently when running under constraints imposed by speed or load, despite comparable increases in work. The neuromuscular system likely modulates the way force is produced as a function of availability of time and potential energy.

Morphological and mechanical properties of the human triceps surae aponeuroses taken from elderly cadavers: Implications for muscle-tendon interactions.

The human triceps surae (two gastrocnemii and soleus) has aponeuroses in the proximal and distal aspects, the latter of which insert into the calcaneus by sharing the common Achilles tendon. These tendinous tissues are known to have elasticity and upon muscle contraction the aponeurosis is stretched both longitudinally (along the muscle’s line of action) and transversely. Higher aponeurosis transverse deformability has been documented, but there is a paucity of information on the morphology and mechanical properties of human aponeurosis. This study aimed to identify morphological and mechanical characteristics of the human triceps surae aponeuroses. Twenty-five triceps surae muscle-tendon units were procured from 13 human donors (formalin fixed, 6 males, 7 females) aged 67–91 years. Specimens of aponeuroses were excised from the eight regions (posterior and anterior regions of the gastrocnemius medialis and lateralis, medial and lateral parts of soleus; proximal, middle, and distal sites each, 2–4 cm × 2–4 cm). Aponeurosis thickness was measured using a digital caliper. Uniaxial tensile tests were implemented to determine the mechanical properties of specimens loaded longitudinally (along the muscle’s line of action) and transversely. The aponeurosis thickness showed significant differences between muscles and sites, while Young’s modulus showed direction-dependent (longitudinal vs. transverse) differences within sites. Results show different morphology and mechanical properties of aponeuroses between synergist muscles. The reason for site-dependent differences in stiffness is due to a reduced aponeurosis thickness rather than a reduction in the material property. The anisotropic elastic feature (differences between longitudinal and transverse directions) of the aponeuroses was more pronounced than previous in vivo findings, suggesting inherent material design of the aponeurosis that matches three-dimensional contractile behavior of muscle fibers.

Effect of habitual foot-strike pattern on the gastrocnemius medialis muscle-tendon interaction and muscle force production during running.

The interaction between gastrocnemius medialis (GM) muscle and Achilles tendon, i.e., muscle-tendon unit (MTU) interaction, plays an important role in minimizing the metabolic cost of running. Foot-strike pattern (FSP) has been suggested to alter MTU interaction and subsequently the metabolic cost of running. However, metabolic data from experimental studies on FSP are inconsistent, and a comparison of MTU interaction between FSP is still lacking. We, therefore, investigated the effect of habitual rearfoot and mid-/forefoot striking on MTU interaction, ankle joint work, and plantar flexor muscle force production while running at 10 and 14 km/h. GM muscle fascicles of 9 rearfoot and 10 mid-/forefoot strikers were tracked using dynamic ultrasonography during treadmill running. We collected kinetic and kinematic data and used musculoskeletal models to determine joint angles and calculate MTU lengths. In addition, we used dynamic optimization to assess plantar flexor muscle forces. During ground contact, GM fascicle shortening ( P = 0.02) and average contraction velocity ( P = 0.01) were 40-45% greater in rearfoot strikers than mid-/forefoot strikers. Differences in contraction velocity were especially prominent during early ground contact. Moreover, GM ( P = 0.02) muscle force was greater during early ground contact in mid-/forefoot strikers than rearfoot strikers. Interestingly, we did not find differences in stretch or recoil of the series elastic element between FSP. Our results suggest that, for the GM, the reduced muscle energy cost associated with lower fascicle contraction velocity in mid-/forefoot strikers may be counteracted by greater muscle forces during early ground contact. NEW & NOTEWORTHY Kinetic and kinematic differences between foot-strike patterns during running imply (not previously reported) altered muscle-tendon interaction. Here, we studied muscle-tendon interaction using ultrasonography. We found greater fascicle contraction velocities and lower muscle forces in rearfoot compared with mid-/forefoot strikers. Our results suggest that the higher metabolic energy demand due to greater fascicle contraction velocities might offset the lower metabolic energy demand due to lower muscle forces in rearfoot compared with mid-/forefoot strikers.