Suppression of soleus H-reflex amplitude is graded with frequency of rhythmic arm cycling

In humans, rhythmic arm cycling has been shown to significantly suppress the soleus H-reflex amplitude in stationary legs. The specific nature of the relationship between frequency of arm cycling and H-reflex modulation in the legs has not been explored. We speculated that the effect of arm cycling on reflexes in leg muscles is related to the neural control of arm movement; therefore, we hypothesized that a graded increase in arm cycling frequency would produce a graded suppression of the soleus H-reflex amplitude. We also hypothesized that a threshold frequency of arm cycling would be identified at which the H-reflex amplitude significantly differed from static control trials (i.e., the arms were stationary). Soleus H-reflexes were evoked in stationary legs with tibial nerve stimulation during both control and rhythmic arm cycling (0.03-2.0 Hz) trials. The results show a significant inverse linear relation between arm cycling frequency and soleus H-reflex amplitude (P<0.05). Soleus H-reflex amplitude significantly differed from control at an average threshold cycling frequency of 0.8 Hz. The results demonstrate that increased frequency of upper limb movement increases the intensity of interlimb influences on the neural activity in stationary legs. Further, a minimum threshold frequency of arm cycling is required to produce a significant effect. This suggests that achieving a threshold frequency of rhythmic arm movement may be important to incorporate in rehabilitation strategies to engage the appropriate interlimb neural pathways.