Activation-Dependent Changes in Soleus Length–Tension Behavior Augment Ankle Joint Quasi-Stiffness

Abstract

The triceps surae muscle-tendon units are important in governing walking performance, acting to regulate mechanical behavior of the ankle through interaction between active muscle and passive elastic structures. Ankle joint quasi-stiffness (the slope of the relation between ankle moment and ankle rotation, k A), is a useful aggregate measure of this mechanical behavior. However, the role of muscle activation and length-tension behavior in augmenting k A remains unclear. Here, 10 subjects completed eccentric isokinetic contractions at rest and at two soleus activation levels (25% and 75% isometric voluntary contraction-IVC) prescribed using electromyographic biofeedback. Ultrasound imaging quantified activation-dependent modulation of soleus muscle length-tension behavior and its role in augmenting k A. We found that soleus muscle stiffness (k M) and k A exhibit non-linear relations with muscle activation and were both more sensitive to the onset of activation than to subsequent increases in activation. Our findings also suggest that k A can be modulated via activation through changes in soleus muscle length-tension behavior. However, this modulation is more complex than previously appreciated-reflecting interaction between active muscle and passive elastic tissues. Our findings may have implications for understanding normal and pathological ankle joint function and the design of impedance-based prostheses.

Reduced Achilles Tendon Stiffness Disrupts Calf Muscle Neuromechanics in Elderly Gait

“…b Average GAS fascicle length (adapted from Browne and Franz[29]) and peak SOL activation reported as a percent of maximum voluntary isometric contraction (i.e., %MVC) (adapted from Franz and Kram[31]) during walking in young (gray) and older (black) adults. c SOL muscle stiffness, given by the relationship between SOL muscle force and SOL muscle length, in young adults during isokinetic eccentric contractions of the triceps surae muscles as a function of muscle activation (α -indicated with the large arrow from α = 0 to 75%) prescribed via biofeedback, adapted from Clark and Franz[34]. d Model-predicted effects of peak Achilles tendon strain (ε o ) on GAS and SOL mass-normalized metabolic energy consumption.…”

The nervous system leverages the non-linear properties of the Achilles tendon to regulate ankle impedance during postural control

“…This is likely due to the small size of our perturbations compared to earlier studies. All previous estimates of human triceps surae muscle stiffness used perturbations at least twice as large as those we applied (20° or larger) [12, 38]. Previously, Hauraix et al [12] reported a triceps surae muscle stiffness value of 218 N/mm at 40% MVC, while Clark et al [38] reported a muscle stiffness of 118 N/mm at 25% MVC.…”

“…All previous estimates of human triceps surae muscle stiffness used perturbations at least twice as large as those we applied (20° or larger) [12, 38]. Previously, Hauraix et al [12] reported a triceps surae muscle stiffness value of 218 N/mm at 40% MVC, while Clark et al [38] reported a muscle stiffness of 118 N/mm at 25% MVC. For comparison, we estimate muscle stiffness to be 261 N/mm at 25% MVC for an average participant in our study.…”

Different Lower-Limb Setup Positions Do Not Consistently Change Backstroke Start Time to 10 m

“…We estimated the rate of force development for the gluteus maximus and vastus lateralis muscles based on EMG data, which is appropriate since linear envelope EMG closely resembles muscle force [34,41]. While this approach accounts for the muscle activation-dependent active components muscle force, it does not account for force produced by passive properties of the muscles and tendons [42]. Since force plate data only describe net forces, we estimated muscle force from EMG to provide a muscle-based metric of rate of force development despite the limitations.…”