Yukiko Makihara

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

Introduction: The soleus H‐reflex is dynamically modulated during walking. However, modulation of the gastrocnemii H‐reflexes has not been studied systematically. Methods: The medial and lateral gastrocnemii (MG and LG) and soleus H‐reflexes were measured during standing and walking in humans. Results: Maximum H‐reflex amplitude was significantly smaller in MG (mean 1.1 mV) or LG (1.1 mV) than in soleus (3.3 mV). Despite these size differences, the reflex amplitudes of the three muscles were positively correlated. The MG and LG H‐reflexes were phase‐ and task‐dependently modulated in ways similar to the soleus H‐reflex. Conclusions: Although there are anatomical and physiological differences between the soleus and gastrocnemii muscles, the reflexes of the three muscles are similarly modulated during walking and between standing and walking. Our findings support the hypothesis that these reflexes are synergistically modulated during walking to facilitate ongoing movement. Muscle Nerve 45: 116–125, 2012

Operant conditioning of the soleus H-reflex does not induce long-term changes in the gastrocnemius H-reflexes and does not disturb normal locomotion in humans

In normal animals, operant conditioning of the spinal stretch reflex or the H-reflex has lesser effects on synergist muscle reflexes. In rats and people with incomplete spinal cord injury (SCI), soleus H-reflex operant conditioning can improve locomotion. We studied in normal humans the impact of soleus H-reflex down-conditioning on medial (MG) and lateral gastrocnemius (LG) H-reflexes and on locomotion. Subjects completed 6 baseline and 30 conditioning sessions. During conditioning trials, the subject was encouraged to decrease soleus H-reflex size with the aid of visual feedback. Every sixth session, MG and LG H-reflexes were measured. Locomotion was assessed before and after conditioning. In successfully conditioned subjects, the soleus H-reflex decreased 27.2%. This was the sum of within-session (task dependent) adaptation (13.2%) and across-session (long term) change (14%). The MG H-reflex decreased 14.5%, due mainly to task-dependent adaptation (13.4%). The LG H-reflex showed no task-dependent adaptation or long-term change. No consistent changes were detected across subjects in locomotor H-reflexes, EMG activity, joint angles, or step symmetry. Thus, in normal humans, soleus H-reflex down-conditioning does not induce long-term changes in MG/LG H-reflexes and does not change locomotion. In these subjects, task-dependent adaptation of the soleus H-reflex is greater than it is in people with SCI, whereas long-term change is less. This difference from results in people with SCI is consistent with the fact that long-term change is beneficial in people with SCI, since it improves locomotion. In contrast, in normal subjects, long-term change is not beneficial and may necessitate compensatory plasticity to preserve satisfactory locomotion.

Stretch Reflex Conditioning in Humans – Implications for Function

Based on evidence from animal experiments and work on spinal cord injured patients, we have been developing a protocol to train healthy subjects to alter the size of their stretch reflex. As reflexes to a sudden stretch of a muscle mark an automated response it is conceivable that by this an increased contribution of afferent feedback to joint stiffness may occur. This may be particularly advantageous in recreational as well as elite athletes where joint stiffness and in particular its reduction induced by fatigue has been suggested as a possible risk factor leading to injury. We have to date trained eight subjects to up-regulate their soleus reflex response of which six were successful. Data show that indeed afferent feedback is enhanced leading to increased stiffness around the ankle joint. Further, regulation of center of pressure (CoP) excursions when landing on one leg following a drop jump from a 30 cm height were substantially reduced. These results imply that ankle safety in injury prone situations may be improved and may help to reduce injury rates in sports.