Effects of contraction intensity on muscle fascicle and stretch reflex behavior in the human triceps surae

The aims of this study were to examine changes in the distribution of a stretch to the muscle fascicles with changes in contraction intensity in the human triceps surae and to relate fascicle stretch responses to short-latency stretch reflex behavior. Thirteen healthy subjects were seated in an ankle ergometer, and dorsiflexion stretches (8 degrees; 250 degrees/s) were applied to the triceps surae at different moment levels (0-100% of maximal voluntary contraction). Surface EMG was recorded in the medial gastrocnemius, soleus, and tibialis anterior muscles, and ultrasound was used to measure medial gastrocnemius and soleus fascicle lengths. At low forces, reflex amplitudes increased despite a lack of change or even a decrease in fascicle stretch velocities. At high forces, lower fascicle stretch velocities coincided with smaller stretch reflexes. The results revealed a decline in fascicle stretch velocity of over 50% between passive conditions and maximal force levels in the major muscles of the triceps surae. This is likely to be an important factor related to the decline in stretch reflex amplitudes at high forces. Because short-latency stretch reflexes contribute to force production and stiffness regulation of human muscle fibers, a reduction in afferent feedback from muscle spindles could decrease the efficacy of human movements involving the triceps surae, particularly where high force production is required.