Rhythmic arm cycling modulates Hoffmann reflex excitability differentially in the ankle flexor and extensor muscles.

Rhythmic arm movement significantly suppresses H-reflex amplitude in the soleus (SOL) muscle. This is evidence of neural linkages between the arms and legs which can be exploited during locomotion and have been ascribed to the descending effects of CPGs for arm cycling. However, the generalizability of the effects of arm movement on reflex excitability within the lower leg musculature has not been confirmed, as findings have been limited solely to the ankle extensor group. Here we tested the hypothesis that rhythmic arm movement similarly modulates H-reflex amplitude in both the ankle flexors and extensors by observing responses in the SOL and tibialis anterior (TA) muscles. SOL and TA H-reflex recruitment curves were recorded bilaterally during control and 1Hz arm cycling conditions. Our results showed significant suppression in H-reflex amplitude (H(max)) in the SOL muscle in both the dominant and non-dominant legs during arm movement. However, results also revealed an unpredicted bidirectional (i.e. either suppression or facilitation) modulation of TA reflex amplitude that was not present in the SOL muscle. These findings suggest a differential regulation of ankle flexor and extensor H-reflex responses during rhythmic arm movement. This may be the result of differences in CPG output to the flexors and extensors during rhythmic movement, as well as increased involvement of cortical drive to the flexors relative to the extensors during rhythmic movement. These findings may be pertinent to future investigation of rehabilitative therapies that involve facilitative modulation of ankle flexor motor responses.