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

Abstract

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.

Effect of Training-Induced Changes in Achilles Tendon Stiffness on Muscle–Tendon Behavior During Landing

“…Here again, region-specific changes in tendon strain patterns or mechanical properties may have limited the effect of training on the free tendon and therefore the soleus contractile behavior. In support of this possibility, recently published modeling data demonstrate the reduced sensitivity of soleus muscle–tendon mechanics to changes in tendon compliance compared to gastrocnemius ( Orselli et al, 2017 ). Consistent with the data on fascicle lengthening during landing, gastrocnemius fascicle lengthening velocity was also reduced after training.…”

Plantarflexor fiber length and tendon slack length are the strongest determinates of simulated single-leg heel raise function

“…[14][15][16][17] Similarly, tendon stiffness and slack length dictates the shortening demands of the plantarflexor muscles and impacts movement efficiency. 16,18,19 Resting ankle angle is often measured in clinical settings as surrogate measure of Achilles tendon slack length, 11,20 which is difficult to measure in vivo. While the effects of MTU parameters on walking biomechanics have been studied in great detail, 16,17,21,22 the multi-factorial implications of clinically-relevant MTU parameters on single-leg heel raise performance is poorly understood.…”

Ultrasound imaging links soleus muscle neuromechanics and energetics during human walking with elastic ankle exoskeletons

“…Given the non-linear curvature of muscle's F-L relationship, a given shift in muscle length could induce a larger improvement in economy when starting from even shorter lengths. For example, Achilles tendon compliance increases with advanced aging 73 , stroke 74 or paralysis 75 and this likely shifts the plantar flexor muscles to shorter operating lengths and faster shortening velocities, conditions that increase metabolic rate 76,77 . Furthermore, in cases where the foot-ankle complex becomes stiffer (e.g., diabetic neuropathy 78 ), intervening by adding stiffness to the foot instead of the ankle joint may improve plantar flexor capacity by yielding slower, more economical shortening velocities.…”