Interlimb coupling from the arms to legs is differentially specified for populations of motor units comprising the compound H-reflex during "reduced" human locomotion.

Recent experiments have identified neuromechanical interactions between the arms and legs during human locomotor movement. Previous work reported that during the rhythmic movement of all four limbs, the influence of the arms on reflex expression in the legs was superimposed on the dominant effect of the legs. This evidence was based upon studies using cutaneous and H-reflex modulation as indices of neuronal activity related to locomotion. The earlier H-reflex study was restricted to one phase of movement and to only a fixed H-reflex amplitude. Also, all four limbs were actively engaged in locomotor movement, and this led to the speculation that the effect from the arms could be underestimated by "swamping" of the conditioning during movement of the test limb. Work from the cat suggests that descending locomotor drive may be differentially specified for different motor unit populations in the hindlimb. Accordingly, details of interlimb coordination between the arms and legs in humans require further characterization and an examination of different populations of motor units as can be obtained from H-reflex recruitment curve (RC) parameters. Using modulation of H-reflex amplitudes across the entire ascending limb as neural probes for interlimb coupling, the present study evaluated the separated influences of rhythmic activity of the arms and leg on neuronal excitability of a stationary "test leg". This three-limb "reduced" locomotion approach was applied using a stepping ergometer during the performance of three rhythmic movement tasks: arms (A); contralateral leg (L); and arms and contralateral leg (AL). Data were sampled at four different phases of the stepping cycle (using the moving leg as reference): start power (SP); end power (EP); start recovery (SR); and end recovery (ER). The main result was a large and significant influence of rhythmic AL activity on RC parameters of the H-reflex at EP and SP phases. However, the parameters (and thus motor unit populations) were differentially affected at each phase and task. For instance, a significant contribution of arms movement was noticed for H (max) (largest motor units) at EP phase (P < 0.05), but no changes was observed for other parameters related to lower reflex amplitude (e.g., H-reflex evoked with an input that elicited 50% of maximum reflex response during static condition; H@50%). On the other hand, at SR phase, the parameter H@50% was significantly affected during AL compared to L. It is suggested that the remote effect from arms rhythmic activity has been differentially manifested across motor unit populations for each phase of movement. These findings provide definitive evidence for interlimb coupling between cervical and lumbar oscillators in gating the excitability of reflex pathways to a leg muscle for different populations of motorneurons within the pool. This further supports the contention of similar functional organization for locomotor networks in the human when compared to other animals. Additionally, these data provide additional confirmation of the significant role of the output of neural control for rhythmic arm movement in modulating reflex excitability of the legs that is specifically adjusted according to the phase and task.