Effect of afferent feedback and central motor commands on soleus H-reflex suppression during arm cycling.

Suppression of soleus H-reflex amplitude in stationary legs is seen during rhythmic arm cycling. We examined the influence of various arm-cycling parameters on this interlimb reflex modulation to determine the origin of the effect. We previously showed the suppression to be graded with the frequency of arm cycling but not largely influenced by changes in peripheral input associated with crank length. Here, we more explicitly explored the contribution of afferent feedback related to arm movement on the soleus H-reflex suppression. We explored the influence of load and rate of muscle stretch by manipulating crank-load and arm-muscle vibration during arm cycling. Furthermore, internally driven ("Active") and externally driven ("Passive") arm cycling was compared. Soleus H-reflexes were evoked with tibial nerve stimulation during stationary control and rhythmic arm-cycling conditions, including: 1) six different loads; 2) with and without vibration to arm muscles; and 3) Active and Passive conditions. No significant differences were seen in the level of suppression between the different crank loads or between conditions with and without arm-muscle vibration. Furthermore, in contrast to the clear effect seen during active cycling, passive arm cycling did not significantly suppress the soleus H-reflex amplitude. Current results, in conjunction with previous findings, suggest that the afferent feedback examined in these studies is not the primary source responsible for soleus H-reflex suppression. Instead, it appears that central motor commands (supraspinal or spinal in origin) associated with frequency of arm cycling are relatively more dominant sources.

Phase-dependent modulation of soleus H-reflex amplitude induced by rhythmic arm cycling.

Rhythmic arm cycling is known to suppress the Hoffmann (H-) reflex amplitudes in the soleus (Sol) muscles of stationary legs. However, it has remained unclear if this suppression is modulated according to the phase of movement in the cycle path or is rather a general setting of excitability level related to rhythmic movement. In the present study we investigated the phase-dependent modulation of the Sol H-reflex induced by rhythmic arm cycling by examining reflex amplitudes at 12 phases of the arm cycle movement. Arm cycling tasks consisted of bilateral, ipsilateral and contralateral movement. Additionally, data were also sampled at 12 static arm positions mimicking those occurring during movement. H-reflexes were evoked and recorded at constant motor wave amplitudes across all conditions. Suppression of Sol H-reflex amplitude was dependent upon the phase of movement (main effect p<0.0001) during arm cycling, but not during static positioning. Results suggest that locomotor central pattern generators may contribute to the phasic reflex modulation observed in this study. The phasic modulation was more pronounced during bilateral movement, however aspects of the neural control driving this modulation were also present during ipsilateral and contralateral movement.