Mechanical loading and fatigue-induced changes in triceps surae muscle-tendon unit: sex-specific responses and recovery dynamics

This study investigates the effects of a repetitive plantar flexion fatigue protocol performed to volitional exhaustion on the contractility, structure, and passive mechanical properties of the triceps surae muscle-tendon unit (MTU) in young, physically active males and females. Twenty-two participants (10 females, 12 males) underwent a fatigue protocol consisting of repetitive bilateral standing calf raises, followed by assessments conducted pre-, post-, and 24 h after exercise. The study measured muscle contractility through peak torque, muscle and tendon structure using ultrasound echo intensity (EI), and mechanical properties via passive stiffness and hysteresis efficiency. Findings indicated a significant reduction in peak torque immediately after the fatigue protocol, alongside altered neuromuscular activation, with the gastrocnemius lateralis (GL) showing reduced EMG activity and the soleus (SOL) demonstrating compensatory recruitment. Muscle contractility exhibited partial recovery after 24 h, while tendon EI decreased progressively, suggesting prolonged structural changes. MTU passive stiffness increased immediately post-fatigue but decreased after 24 h, implying transient alterations in mechanical properties. Sex-related differences in MTU responses are present but limited, and recovery following fatigue occurs at different time scales across MTU tissues.