Tendon Compliance Research Articles

Quality Of Semitendinosus Tendon Regeneration As A Function Of Time Post-ACL Reconstruction

Evidence exists of the semitendinosus tendon (ST) physically regenerating following harvest for ACL reconstruction. However, the quality of regenerated tissue, indicated by elastic modulus, is not well understood. The time-dependency of this regeneration is also important post-ACL reconstruction as the hamstring muscle group is inherently protective of the ACL. PURPOSE: Assess the quality of ST regeneration, as measured with shear modulus, as a function of time post-ACL reconstruction with comparisons to healthy controls. METHODS: Ultrasound Shear Wave Elastography determined shear modulus of the ST tendon on 10 ACL reconstructed individuals (age: 21.6±1.6 years, height: 171.6±8.5cm, mass: 71.4±7.1kg, Tegner scale: 5.9±1.0) and 10 healthy individuals (age: 20.6±2.0 years old, height: 173.4±9.3cm, mass: 71.6±13.0kg, Tegner scale: 5.6±1.1). Time since ST harvest averaged 4.3 years (range: 0.75-12.6 years) and all individuals were since cleared to return to play by their physician. While prone with the knee at full extension and relaxed, three ultrasound images (AIXPLORER, Supersonic Imagine S.A., France) were acquired of the distal ST tendon. Linear regression analysis determined the relationship between ST tendon shear modulus vs. time since tendon harvest. 99% confidence interval (CI) determined the range of tendon compliance in healthy subjects and served as comparison to ACL reconstructed subjects. RESULTS: The relationship between the shear modulus vs. time since ST harvest was R2=0.57 (p=0.012). 3 subjects under 2 years post-reconstruction had low shear modulus of ~100kPa or less. The 99% CI of the healthy tendons was 374-590kPa, and only 1 of the 10 reconstructed subjects reached this range. CONCLUSIONS: Despite these preliminary data supporting long-term time-dependent regeneration, ST quality is highly variable indicating other factors influence this regeneration post-ACL reconstruction. Low shear modulus has an effect on muscle function, which may alter the knee load leading to the early development of knee OA. Future research is needed to assess the clinical importance of regeneration quality post-harvest in order to establish interventions facilitating early material property recovery.

Enhancement of muscle and locomotor performance by a series compliance: A mechanistic simulation study.

The objective was to better understand how a series compliance alters contraction kinetics and power output of muscle to enhance the work done on a load. A mathematical model was created in which a gravitational point load was connected via a linear spring to a muscle (based on the contractile properties of the sartorius of leopard frogs, Rana pipiens). The model explored the effects of load mass, tendon compliance, and delay between onset of contraction and release of the load (catch) on lift height and power output as measures of performance. Series compliance resulted in increased lift height over a relatively narrow range of compliances, and the effect was quite modest without an imposed catch mechanism unless the load was unrealistically small. Peak power of the muscle-tendon complex could be augmented up to four times that produced with a muscle alone, however, lift height was not predicted by peak power. Rather, lift height was improved as a result of the compliance synchronizing the time courses of muscle force and shortening velocity, in particular by stabilizing shortening velocity such that muscle power was sustained rather than rising and immediately falling. With a catch mechanism, enhanced performance resulted largely from energy storage in the compliance during the period of catch, rather than increased time for muscle activation before movement commenced. However, series compliance introduced a trade-off between work done before versus after release of the catch. Thus, the ability of tendons to enhance locomotor performance (i.e. increase the work done by muscle) appears dependent not only on their established role in storing energy and increasing power, but also on their ability to modulate the kinetics of muscle contraction such that power is sustained over more of the contraction, and maximizing the balance of work done before versus after release of a catch.

Subtalar Joint Pronation and Energy Absorption Requirements During Walking are Related to Tibialis Posterior Tendinous Tissue Strain

During human walking, the tibialis posterior (TP) tendon absorbs energy in early stance as the subtalar joint (STJ) pronates. However, it remains unclear whether an increase in energy absorption between individuals, possibly a result of larger STJ pronation displacement, is fulfilled by greater magnitudes of TP tendon or muscle fascicle strain. By collecting direct measurements of muscle fascicle length (ultrasound), MTU length (3D motion capture and musculoskeletal modelling), and TP muscle activation (intramuscular electromyography) we endeavoured to illustrate that the TP tendinous tissue fulfils the requirements for energy absorption at the STJ as a result of an increase in muscle force production. While a significant relationship between TP tendon strain, energy absorption at the STJ (R2 = 0.53, P = < 0.01) and STJ pronation (R2 = 0.53, P = < 0.01) was evident, we failed to find any significant associations between tendon strain and surrogate measure of TP muscle force (TP muscle activation together with ankle and subtalar joint moments). These results suggest that TP tendon compliance may explain the variance in pronation and energy absorption at the STJ. Therefore, as the tendinous tissue of the TP is accountable for the absorption of energy at the STJ it may be predisposed to strain-induced injury.

Applying physical science principles to mid-substance Achilles tendinopathy and the relationship to eccentric lengthening exercises.

Mid-substance Achilles tendinopathy is common in the active population. Eccentric (lengthening) exercises are known to be effective in alleviating the clinical condition. To better understand mid-substance Achilles tendinopathy and the response to lengthening exercises physical science principles of elasticity are applied. We apply elastic motion laws to the spring-like tendon as well as the normal and pathological adaptation seen with this common injury. We will validate important assumptions of the spring-like behavior of the tendon and then apply these findings to the injured and rehabilitating states. Our analysis considers that the tendon can be viewed as being spring-like with elasticity principles being applicable and the force exerted on the tendon during lengthening is primarily in a uniaxial direction. This applied lengthening force results in tendon mechanical and structural adaptation. Injury, and ultimately the clinical condition, occurs when the applied force exceeds the ability of the tendon to normally adapt. Morphological changes of the injured tendon are an attempt by the body to make the tendon more compliant. Lengthening exercises can be assessed as achieving this task with an improvement of tendon compliance. Physical science analysis proposes that the preferred rehabilitation tendon pathway is to try and decrease tendon stiffness to allow for more tendon lengthening. The body's morphological alterations of the pathological tendon are also consistent with this approach. For mid-substance Achilles tendinopathy, this adaptation of decreased stiffness ultimately increases the tendons ability to withstand applied force during lengthening.

Prolonged stretching of the ankle plantarflexors elicits muscle-tendon adaptations relevant to ankle gait kinetics in children with spastic cerebral palsy

Tissue related ankle hyper-resistance has been reported to contribute to equinus gait in children with spastic cerebral palsy. Hence, ankle plantarflexor stretching programs have been developed in order to restore passive ankle dorsiflexion. Despite high quality evidence on the limited effects of stretching on passive joint mobility, further muscle-tendon adaptations have been reported which may impact gait performance. As such, children with spastic cerebral palsy subject to long-term manual static stretching achieved dorsiflexion gains through the reduction of muscle and fascicle strain whilst preserving tendon strain, and prolonged use of ankle-foot orthoses achieved similar dorsiflexion gains through increased tendon strain whilst preserving muscle and fascicle strain. The latter concurred with normalization of early stance plantarflexor moment yet reductions in push-off plantarflexor moment given the increase in tendon compliance. Therefore, similar limited gains in passive ankle joint mobility in response to stretching may be achieved either by preserving/restoring optimal muscle-tendon function, or at the expense of muscle-tendon function and thus contributing gait impairments. The largest increase in ankle passive joint mobility in children with SCP has been obtained through prolonged plantarflexor stretching through ankle casting combined with botulinum neurotoxin type A. However, to our knowledge, there are no published studies on muscle-tendinous adaptations to ankle casting combined with botulinum toxin type A and its effect on ankle joint gait kinetics. Therefore, we hypothesized that ankle casting elicits muscle-tendon adaptations which concur with altered ankle joint kinetics during the stance phase of gait in children with SCP. More information is needed about the relationships between muscle structure and function, and the effect of specific interventions designed to alter muscle properties and associated functional outcomes in children with spastic cerebral palsy.

The relative compliance of energy-storing tendons may be due to the helical fibril arrangement of their fascicles.

A nonlinear elastic microstructural model is used to investigate the relationship between structure and function in energy-storing and positional tendons. The model is used to fit mechanical tension test data from the equine common digital extensor tendon (CDET) and superficial digital flexor tendon (SDFT), which are used as archetypes of positional and energy-storing tendons, respectively. The fibril crimp and fascicle helix angles of the two tendon types are used as fitting parameters in the mathematical model to predict their values. The outer fibril crimp angles were predicted to be 15.1° ± 2.3° in the CDET and 15.8° ± 4.1° in the SDFT, and the average crimp angles were predicted to be 10.0° ± 1.5° in the CDET and 10.5° ± 2.7° in the SDFT. The crimp angles were not found to be statistically significantly different between the two tendon types (p = 0.572). By contrast, the fascicle helix angles were predicted to be 7.9° ± 9.3° in the CDET and 29.1° ± 10.3° in the SDFT and were found to be statistically highly significantly different between the two tendon types (p < 0.001). This supports previous qualitative observations that helical substructures are more likely to be found in energy-storing tendons than in positional tendons and suggests that the relative compliance of energy-storing tendons may be directly caused by these helical substructures.

The effects of Achilles tendon compliance on triceps surae mechanics and energetics in walking

Achilles tendon (AT) compliance can affect the generation and transmission of triceps surae muscle forces, and thus has important biomechanical consequences for walking performance. However, the uniarticular soleus (SOL) and the biarticular (GAS) function differently during walking, with in vivo evidence suggesting that their associated fascicles and tendinous structures exhibit unique kinematics during walking. Given the strong association between muscle fiber length, velocity and force production, we conjectured that SOL and GAS mechanics and energetic behavior would respond differently to altered AT compliance. To test this, we characterized GAS and SOL muscle and tendon mechanics and energetics due to systematic changes in tendon compliance using musculoskeletal simulations of walking. Increased tendon compliance enlarged GAS and SOL tendon excursions, shortened fiber operation lengths and affected muscle excitation patterns. For both muscles, an optimal tendon compliance (tendon strains of approximately 5% with maximum isometric force) existed that minimized metabolic energy consumption. However, GAS muscle-tendon mechanics and energetics were significantly more sensitive to changes in tendon compliance than were those for SOL. In addition, GAS was not able to return stored tendon energy during push-off as effectively as SOL, particularly for larger values of tendon compliance. These fundamental differences between GAS and SOL sensitivity to altered tendon compliance seem to arise from the biarticular nature of GAS. These insights are potentially important for understanding the functional consequences of altered Achilles tendon compliance due to aging, injury, or disease.

Can Weakness in End-Range Plantarflexion after Achilles Repair Be Prevented?

PURPOSE: Disproportionate end-range plantar flexion weakness, decreased passive stiffness, and inability to perform a heel rise on a decline after Achilles tendon repair, are thought to reflect increased tendon compliance or anatomical tendon lengthening. Since this was first noted, we have performed stronger repairs and avoided stretching into dorsiflexion for the first 12 weeks after surgery. It was hypothesized that these treatment changes would eliminate end-range plantar flexion weakness and normalize passive stiffness. METHODS: Achilles repairs with epitendinous augmentation were performed on 18 patients. Plantar flexion torque, dorsiflexion ROM, passive joint stiffness, and standing single-leg heel rise on a decline were assessed 43±24 months after surgery (range, 9 months-8 years). Maximum isometric plantar flexion torque was measured at 20° and 10° of dorsiflexion, neutral, and 10° and 20° of plantar flexion. Passive dorsiflexion ROM was measured goniometrically. Passive joint stiffness was computed from the increase in passive torque from 10° to 20° of dorsiflexion measured before isometric contractions. Tendon thickness was measured by digital calipers. Plantarflexor EMG was recorded during strength and functional tests. ANOVA and Wilcoxon tests were used to assess weakness and function. RESULTS: Marked weakness was evident on the involved side at 20° plantar flexion (deficit 26±18%; P<0.01) with no weakness at 20° dorsiflexion (deficit 6±17%; P=0.39). Compared to the noninvolved side, dorsiflexion range of motion was decreased 6±8° and tendon thickness was 7±3 mm greater (P<0.001) on the involved side. Passive joint stiffness was similar between the involved and noninvolved sides. Only 3 of 18 patients could perform a decline heel rise on the involved side versus 18 of 18 on the noninvolved side (P=0.01). There was no difference in EMG amplitude between the involved and noninvolved sides during all tests. CONCLUSION: Normalized passive joint stiffness and reduced dorsiflexion ROM were likely due to a stronger, protected repair. EMG data confirmed that end-range weakness was not due to neural inhibition. Weakness with the plantar flexors in a shortened position may be due to inefficient transmission of contractile forces through the thickened tendon when the muscles are in a shortened position.

Micro-mechanical properties of the tendon-to-bone attachment.

The tendon-to-bone attachment (enthesis) is a complex hierarchical tissue with features at a numerous scales that dissipate stress concentrations between compliant tendon and stiff bone. At the micrometer scale, the enthesis exhibits gradients in collagen and mineral composition and organization. However, the physiological mechanics of the enthesis at this scale remained unknown due to difficulty in preparing and testing micrometer scale samples. This study is the first to measure the tensile mechanical properties of the enthesis at the micrometer scale. Results demonstrated considerably enhanced mechanical performance at the micrometer length scale compared to the millimeter tissue length scale and identified a high-compliance zone near the mineralized gradient of the attachment. This understanding of tendon-to-bone micromechanics demonstrates the critical role of micrometer scale features in the mechanics of the tissue.

Effects of medial gastrocnemius relative fascicle excursion on stance phase plantarflexion work in children with cerebral palsy

A structural model was developed to explain sarcomere shortening at the expense of tendon lengthening in the frog semitendinosis (ST) muscle-tendon system. The model was based on the data of Lieber et al. [Am. J. Physiol.261, C86–C92 (1991)], who determined the relationship between the sarcomere length, tendon load (as a fraction of maximum isometric tension) and tendon, bone-tendon junction (BTJ), and aponeurosis strain. The model was generated assuming a finite time-course of cross-bridge attachment [Huxley, Prog. Biophys.7, 255–318 (1957)], an ideal sarcomere length-tension relationship [Gordon et al., J. Physiol.184, 170–192 (1966)] and an ideal force-velocity relationship [Katz, J. Physiol.96, 45–64 (1939); Edman, J. Physiol.291, 143–159 (1979)]. Functionally, sarcomeres operated on three distinct regions of the length-tension curve: (1) regions where the muscle force decreased as sarcomeres shortened (the shallow and steep ascending limbs); (2) regions where the muscle force increased as sarcomeres shortened and there was little passive tension (descending limb, where sarcomere length ≤3.0 μm); and (3) regions where the muscle force increased as sarcomeres shortened and there was a significant passive tension (descending limb where sarcomere length >3.0 μm). Using such a physiological model, it was found that the effect of tendon compliance was to ‘skew’ the sarcomere length-tension curve to the right and to increase the operating range of the muscle-tendon unit. Thus, maximum tension in the muscle occurred at an active sarcomere length of 2.0–2.2 μm, whereas in the muscle-tendon system, the maximum tension occurred at a longer resting sarcomere length of about 2.5 μm. The degree to which the tendon affected the muscle system depended on its material properties and dimensions. These data suggest that tendons are not merely rigid links connecting muscles to bones, but impart distinct properties to the muscular system.

Protection from Muscle Damage in the Absence of Changes in Muscle Mechanical Behavior.

The repeated bout effect characterizes the protective adaptation after a single bout of unaccustomed eccentric exercise that induces muscle damage. Sarcomerogenesis and increased tendon compliance have been suggested as potential mechanisms for the repeated bout effect by preventing muscle fascicles from being stretched onto the descending limb of the length-tension curve (the region where sarcomere damage is thought to occur). In this study, evidence was sought for three possible mechanical changes that would support either the sarcomerogenesis or the increased tendon compliance hypotheses: a sustained rightward shift in the fascicle length-tension relationship, reduced fascicle strain amplitude, and reduced starting fascicle length. Subjects (n = 10) walked backward downhill (5 km·h, 20% incline) on a treadmill for 30 min on two occasions separated by 7 d. Kinematic data and medial gastrocnemius fascicle lengths (ultrasonography) were recorded at 10-min intervals to compare fascicle strains between bouts. Fascicle length-torque curves from supramaximal tibial nerve stimulation were constructed before, 2 h after, and 2 d after each exercise bout. Maximum torque decrement and elevated muscle soreness were present after the first, but not the second, backward downhill walking bout signifying a protective repeated bout effect. There was no sustained rightward shift in the length-torque relationship between exercise bouts, nor decreases in fascicle strain amplitude or shortening of the starting fascicle length. Protection from a repeated bout of eccentric exercise was conferred without changes in muscle fascicle strain behavior, indicating that sarcomerogenesis and increased tendon compliance were unlikely to be responsible. As fascicle strains are relatively small in humans, we suggest that changes to connective tissue structures, such as extracellular matrix remodeling, are better able to explain the repeated bout effect observed here.

Stretching Your Energetic Budget: How Tendon Compliance Affects the Metabolic Cost of Running.

Muscles attach to bones via tendons that stretch and recoil, affecting muscle force generation and metabolic energy consumption. In this study, we investigated the effect of tendon compliance on the metabolic cost of running using a full-body musculoskeletal model with a detailed model of muscle energetics. We performed muscle-driven simulations of running at 2–5 m/s with tendon force–strain curves that produced between 1 and 10% strain when the muscles were developing maximum isometric force. We computed the average metabolic power consumed by each muscle when running at each speed and with each tendon compliance. Average whole-body metabolic power consumption increased as running speed increased, regardless of tendon compliance, and was lowest at each speed when tendon strain reached 2–3% as muscles were developing maximum isometric force. When running at 2 m/s, the soleus muscle consumed less metabolic power at high tendon compliance because the strain of the tendon allowed the muscle fibers to operate nearly isometrically during stance. In contrast, the medial and lateral gastrocnemii consumed less metabolic power at low tendon compliance because less compliant tendons allowed the muscle fibers to operate closer to their optimal lengths during stance. The software and simulations used in this study are freely available at simtk.org and enable examination of muscle energetics with unprecedented detail.

Muscle force loss and soreness subsequent to maximal eccentric contractions depend on the amount of fascicle strain invivo.

Defining the origins of muscle injury has important rehabilitation and exercise applications. However, current knowledge of muscle damage mechanics in human remains unclear invivo. This study aimed to determine the relationships between muscle-tendon unit mechanics during maximal eccentric contractions and the extent of subsequent functional impairments induced by muscle damage. The length of the muscle-tendon unit, fascicles and tendinous tissues was continuously measured on the gastrocnemius medialis using ultrasonography, in time with torque, during 10 sets of 30 maximal eccentric contractions of plantar flexors at 45°s(-1) , in seventeen participants. Muscle-tendon unit, fascicles and tendinous tissues were stretched up to 4.44±0.33cm, 2.31±0.64cm and 1.92±0.61cm respectively. Fascicle stretch length, lengthening amplitude and negative fascicle work beyond slack length were significantly correlated with the force decrease 48h post-exercise (r=0.51, 0.47 and 0.68, respectively; P<0.05). This study demonstrates that the strain applied to human muscle fibres during eccentric contractions strongly influences the magnitude of muscle damage invivo. Achilles tendon compliance decreases the amount of strain, while architectural gear ratio may moderately contribute to attenuating muscle fascicle lengthening and hence muscle damage. Further studies are necessary to explore the impact of various types of task to fully understand the contribution of muscle-tendon interactions during active lengthening to muscle damage.

Additional in-series compliance reduces muscle force summation and alters the time course of force relaxation during fixed-end contractions.

There are high mechanical demands placed on skeletal muscles in movements requiring rapid acceleration of the body or its limbs. Tendons are responsible for transmitting muscle forces, but, because of their elasticity, can manipulate the mechanics of the internal contractile apparatus. Shortening of the contractile apparatus against the stretch of tendon affects force generation according to known mechanical properties; however, the extent to which differences in tendon compliance alter force development in response to a burst of electrical impulses is unclear. To establish the influence of series compliance on force summation, we studied electrically evoked doublet contractions in the cane toad peroneus muscle in the presence and absence of a compliant artificial tendon. Additional series compliance reduced tetanic force by two-thirds, a finding predicted based on the force-length property of skeletal muscle. Doublet force and force-time integral expressed relative to the twitch were also reduced by additional series compliance. Active shortening over a larger range of the ascending limb of the force-length curve and at a higher velocity, leading to a progressive reduction in force-generating potential, could be responsible. Muscle-tendon interaction may also explain the accelerated time course of force relaxation in the presence of additional compliance. Our findings suggest that a compliant tendon limits force summation under constant-length conditions. However, high series compliance can be mechanically advantageous when a muscle-tendon unit is actively stretched, permitting muscle fibres to generate force almost isometrically, as shown during stretch-shorten cycles in locomotor activities. Restricting active shortening would likely favour rapid force development.

Immediate effects of whole body vibration on patellar tendon properties and knee extension torque

Reports about the immediate effects of whole body vibration (WBV) exposure upon torque production capacity are inconsistent. However, the changes in the torque-angle relationship observed by some authors after WBV may hinder the measurement of torque changes at a given angle. Acute changes in tendon mechanical properties do occur after certain types of exercise but this hypothesis has never been tested after a bout of WBV. The purpose of the present study was to investigate whether tendon compliance is altered immediately after WBV, effectively shifting the optimal angle of peak torque towards longer muscle length. Twenty-eight subjects were randomly assigned to either a WBV (n = 14) or a squatting control group (n = 14). Patellar tendon CSA, stiffness and Young's modulus and knee extension torque-angle relationship were measured using ultrasonography and dynamometry 1 day before and directly after the intervention. Tendon CSA was additionally measured 24 h after the intervention to check for possible delayed onset of swelling. The vibration intervention had no effects on patellar tendon CSA, stiffness and Young's modulus or the torque-angle relationship. Peak torque was produced at ~70° knee angle in both groups at pre- and post-test. Additionally, the knee extension torque globally remained unaffected with the exception of a small (-6%) reduction in isometric torque at a joint angle of 60°. The present results indicate that a single bout of vibration exposure does not substantially alter patellar tendon properties or the torque-angle relationship of knee extensors.

Doublet potentiation in the triceps surae is limited by series compliance and dynamic fascicle behavior.

Activation of skeletal muscle twice in quick succession results in nonlinear force summation (i.e., doublet potentiation). The force contributed by a second activation is typically of augmented amplitude, longer in duration, and generated at a greater rate. The purpose of this study was to examine force summation in a muscle attached to a compliant tendon, where considerable internal shortening occurs during a fixed-end contraction. The triceps surae of 21 (Experiment 1) and 9 (Experiment 2) young adults were maximally activated with doublet stimulation of different interstimulus intervals (ISIs) (5-100 ms) at several muscle lengths. Ultrasound images acquired from lateral gastrocnemius and soleus muscles allowed quantification of dynamic fascicle behavior. Force summation was muscle length dependent. Force augmentation was limited to a short muscle length. Lateral gastrocnemius and soleus fascicles underwent large amounts of active shortening and achieved high velocities in response to doublet stimulation, dynamics unfavorable for force production. Summation amplitude and the sensitivity of summation to ISI were dramatically depressed in the triceps surae after comparison to muscles with less fixed-end compliance. We propose that the internal shortening permitted by high series compliance limited force augmentation by offsetting and/or interfering with activation and cross-bridge processes driving augmentation. High series compliance may also reduce the sensitivity of the summated response to ISI, an assertion supported by predictions from a Hill-type muscle model. These muscles may exhibit greater force augmentation during more accustomed stretch-shorten tasks (i.e., hopping), where the compliance of the Achilles tendon actually enables near-isometric fascicle behavior.

Middle-aged adults exhibit altered spatial variations in Achilles tendon wave speed

The purpose of this study was to investigate spatial variations in measured wave speed in the relaxed and stretched Achilles tendons of young and middle-aged adults. Wave speed was measured from the distal Achilles tendon, soleus aponeurosis, medial gastrocnemius aponeurosis and medial gastrocnemius muscle in healthy young (n = 15, aged 25 ± 4 years) and middle-aged (n = 10, aged 49 ± 4 years) adults in resting, dorsiflexed and plantarflexed postures. In both age groups, Achilles tendon wave speed decreased proximally, with the lowest wave speed measured in the gastrocnemius aponeurosis. Measured wave speed increased with passive dorsiflexion, reflecting the strain-stiffening behavior of tendons. There were no significant aging effects on wave speed in the free tendon or soleus aponeurosis. However, a significant, inverse relationship between gastrocnemius aponeurosis wave speed and age was observed in the dorsiflexed posture. We also observed significantly lower wave speeds in the gastrocnemius muscles of middle-aged adults when compared with young adults. These results suggest that Achilles tendon compliance increases in a distal-to-proximal pattern, with middle-aged adults exhibiting greater compliance in the distal gastrocnemius muscle and tendinous structures. An age-related change in the spatial variation in Achilles tendon compliance could affect localized tissue deformation patterns and injury potential within the triceps surae muscle–tendon units.

Achilles tendon displacement patterns during passive stretch and eccentric loading are altered in middle-aged adults

The purpose of this study was to investigate middle-age effects on Achilles displacement patterns under passive stretch and eccentric loading. Healthy young (24.1 ± 1.4 years, n = 9) and middle-aged (49.0 ± 3.1 years, n = 9) adults were positioned prone and the ankle was cyclically dorsiflexed (0.5 Hz, 25° range) during passive stretch and active lengthening. Achilles displacements were tracked in cine ultrasound using 2D speckle tracking. Displacements were found to be non-uniform, with mid and deep portions of the tendon displacing more than superficial portions. However, the degree of non-uniformity was significantly reduced in middle-aged adults, suggesting a potential age-related reduction in inter-fascicle sliding or a shift in loading sharing between plantarflexors. Eccentric loading reduced displacement magnitudes, likely reflecting distal tendon stretch induced via active muscle contractions. Changes in tendon displacement with active loading were greater in middle-aged adults, which could reflect greater tendon compliance. The observed age-related changes in Achilles tendon behavior may have implications for both plantarflexor performance and injury risk.

The series elastic shock absorber: tendon elasticity modulates energy dissipation by muscle during burst deceleration.

During downhill running, manoeuvring, negotiation of obstacles and landings from a jump, mechanical energy is dissipated via active lengthening of limb muscles. Tendon compliance provides a 'shock-absorber' mechanism that rapidly absorbs mechanical energy and releases it more slowly as the recoil of the tendon does work to stretch muscle fascicles. By lowering the rate of muscular energy dissipation, tendon compliance likely reduces the risk of muscle injury that can result from rapid and forceful muscle lengthening. Here, we examine how muscle-tendon mechanics are modulated in response to changes in demand for energy dissipation. We measured lateral gastrocnemius (LG) muscle activity, force and fascicle length, as well as leg joint kinematics and ground-reaction force, as turkeys performed drop-landings from three heights (0.5-1.5 m centre-of-mass elevation). Negative work by the LG muscle-tendon unit during landing increased with drop height, mainly owing to greater muscle recruitment and force as drop height increased. Although muscle strain did not increase with landing height, ankle flexion increased owing to increased tendon strain at higher muscle forces. Measurements of the length-tension relationship of the muscle indicated that the muscle reached peak force at shorter and likely safer operating lengths as drop height increased. Our results indicate that tendon compliance is important to the modulation of energy dissipation by active muscle with changes in demand and may provide a mechanism for rapid adjustment of function during deceleration tasks of unpredictable intensity.

The role of tendon compliance in maximising muscle performance and minimising injury risk during running

The role of tendon compliance in maximising muscle performance and minimising injury risk during running