Modulation of the soleus H-reflex during knee rotations is not consistent with muscle fascicle length changes

The purpose of this study was to examine whether passively rotating the knee would result in parallel or differential changes to the medial gastrocnemius (MG) and soleus (SOL) H-reflex amplitudes. Since passive knee rotation alters the muscle length of the MG, but not the SOL, it was hypothesized that the MG H-reflex would reflect the lengthening or shortening actions that occur during knee rotation, whereas the SOL H-reflex would remain unaltered. MG and SOL Hoffman reflexes (H-reflexes) were evoked with the knee joint held static at 10° or as the joint was passively flexed or extended past 10°. Ultrasound recordings were used to confirm whether the knee rotations altered MG but not SOL muscle fascicle lengths. In contrast to our hypothesis, results indicated that the MG and SOL H-reflexes were similarly affected during knee rotations, with both MG and SOL H(max):M(max) smaller during the knee extension than the knee flexion (33-43% reduction) and static (22-28% reduction) conditions. Parallel changes to the MG and SOL H-reflexes occurred despite a differential effect of knee rotation on muscle fascicle lengths. Whereas, MG muscle fascicles lengthened and shortened during knee extension and flexion, respectively, SOL fascicles length remained unchanged. Given the strong neural coupling between the MG and SOL motoneuron pools, the results highlight the difficulty in isolating specific variables (e.g., muscle length) when determining the modulatory influences on the triceps surae H-reflex amplitude.