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

Area of Science:

• Biomechanics
• Musculoskeletal Physiology

Background:

• The triceps surae muscle-tendon unit is crucial for ankle function during walking.
• Ankle joint quasi-stiffness (k_A) quantifies ankle mechanical behavior but its modulation by muscle activation is not fully understood.

Purpose of the Study:

• To investigate how soleus muscle activation influences ankle joint quasi-stiffness (k_A).
• To examine the role of soleus muscle length-tension behavior in modulating k_A.

Main Methods:

• 10 subjects performed eccentric isokinetic contractions at rest and at 25% and 75% soleus activation levels.
• Electromyographic biofeedback controlled soleus activation.
• Ultrasound imaging assessed soleus muscle length-tension behavior.

Main Results:

• Both soleus muscle stiffness (k_M) and k_A showed nonlinear relationships with muscle activation.
• k_M and k_A were more sensitive to the initial increase in activation than to further increases.
• Soleus muscle activation modulates k_A through changes in muscle length-tension behavior.

Conclusions:

• Ankle joint quasi-stiffness modulation by activation is complex, involving interactions between active muscle and passive tissues.
• Findings inform understanding of normal and pathological ankle function.
• Results have implications for designing impedance-based prostheses.