Functional implications of h-reflex modulation and modification in human soleus, medial gastrocnemius, and lateral gastrocnemius muscles

Sensorimotor integration of the central nervous system (CNS) plays an important role in motor control. In order to evaluate this highly complicated phenomenon, the H-reflex has been used as a window to assessing neural activity in the spinal cord. This dissertation investigated principles governing functional association among the H-reflexes in three calf muscles. First, the H-reflexes of the soleus, and medial and lateral gastrocnemii (MG and LG) were examined during walking, and between standing and walking. The H-reflexes of all three muscles showed similar phase-dependent and task-dependent modulations, and thus, the H-reflexes of three muscles are synergistically modulated to facilitate ongoing motor task. Second, long-term modifications of the H-reflexes in the three muscles, induced by the soleus H-reflex operant down-conditioning, were examined. Operant conditioning induced acute adaptation in the soleus and MG H-reflexes, however, long-term change occurred only in the soleus H-reflex. It appeared that compensatory plasticity may occur to prevent long-term change in the MG H-reflex, and in turn, to preserve the existing repertoire of motor skills. Third, effects of long-term change in the soleus H-reflex induced by operant conditioning were investigated during locomotion. Although decrease of the soleus H-reflex was retained across conditioning sessions, the soleus locomotor H-reflex did not change. Other EMG activity as well as joint kinematics were maintained the same after conditioning, and thus, the normal locomotion was preserved. Minimum adjustment of reflex gain could be the mechanism responsible for maintaining the appropriate reflex during walking. Based on findings from the three studies, the present dissertation demonstrated that the CNS controls the H-reflexes of the three synergist muscles to be synergistic and/or possibly compensatory, which are logical directions to facilitate or preserve the motor skills in the current behavioral repertoire. Furthermore, especially in the existing repertoire, normal locomotion is maintained due to minimum adjustment by the CNS. Thus, the CNS is shown to be capable of accommodating a new skill while preserving the current motor skills. Preserving the existing repertoire may not occur in patients with abnormal movement pattern, and thus, similar investigations with patients should occur in the future

H-reflex modulation in the human medial and lateral gastrocnemii during standing and walking

The soleus H-reflex is dynamically modulated during walking. However, modulation of the gastrocnemii H-reflexes has not been studied systematically