Mechanical and neural stretch responses of the human soleus muscle at different walking speeds

During humanwalking, a sudden tripmay elicit a Ia afferent fibremediated short latency stretch reflex. The aim of this study was to investigate soleus (SOL) muscle mechanical behaviour in response to dorsiflexionperturbations, and to relate this behaviour to short latency stretch reflex responses. Twelve healthy subjects walked on a treadmill with the left leg attached to an actuator capable of rapidly dorsiflexing the ankle joint.Ultrasoundwasused tomeasure fascicle lengths in SOLduringwalking, and surface electromyography (EMG)wasused to recordmuscle activation. Dorsiflexion perturbations of 6 degwere applied duringmid-stance at walking speeds of 3, 4 and 5 km h−1. At each walking speed, perturbations were delivered at three different velocities (slow: ∼170 deg s–1, mid:∼230 deg s–1, fast:∼280 deg s–1). At 5 km h−1, fascicle stretch amplitudewas 34–40 % smaller and fascicle stretch velocity 22–28 % slower than at 3 km h−1 in response to a constant amplitude perturbation, whilst stretch reflex amplitudes were unchanged. Changes in fascicle stretch parameters can be attributed to an increase in muscle stiffness at faster walking speeds. As stretch velocity is a potent stimulus to muscle spindles, a decrease in the velocity of fascicle stretch at faster walking speeds would be expected to decrease spindle afferent feedback and thus stretch reflex amplitudes, which did not occur. It is therefore postulated that othe