Elucidation of Strain Mechanism Focusing on Spinal Reciprocal Inhibition

Abstract

Excessive co-contraction interferes with smooth joint movement via mechanisms including failed spinal reciprocal inhibition against antagonists. However, the function of spinal reciprocal inhibition during co-contraction remains unclear. To investigate the influence of changes in contraction intensity during co-contraction on spinal reciprocal inhibition, 20 healthy adults were subjected to four stimulation conditions: a conditioning stimulus?test stimulation interval （CTI） of −2, 2, or 20 ms or a test stimulus without a conditioning stimulus （single）．Co-contraction （change in soleus muscle [Sol] vs. tibialis anterior [TA] activity） was examined at task A 0% vs. 0% maximal voluntary contraction （MVC）， task B 5% vs. 5% MVC, task C 15% vs. 15% MVC, task D 5% vs. 15% MVC, and task E 15% vs. 5% MVC.At CTI of 2 ms, the H-reflex amplitude value was significantly lower in tasks A, B, C, and D than in the single condition. Among the tasks, the H-reflex amplitude values were lower for A, B, C, and D than for E. At CTI of 20 ms, the H-reflex amplitude was significantly lower in tasks A, B, C, D, and E. Among the tasks, the H-reflex amplitude was significantly lower from task A and B to task E. For co-contraction <15% MVC, reciprocal Ia inhibition may be modulated depending on the Sol vs. TA muscle activity ratio. D1 inhibition was equivalent when the Sol/TA ratio was equal or TA muscle activity was high. During co-contraction with high Sol muscle activity, D1 inhibition decreased from rest but D1 inhibition remained.