Arm cycling modulates corticospinal transmission in the biceps brachii: potential involvement of premotoneuronal networks

Rhythmic arm movements have been reported to improve motor function in individuals with stroke and spinal cord injury. However, the underlying mechanism remains unclear, particularly whether this benefit involves modulation of corticospinal transmission via premotoneuronal systems, which have been implicated in motor recovery in previous animal studies. Therefore, we investigated how arm cycling modulates the activities of presumed premotoneuronal networks receiving inputs from the corticospinal tract (CST) and the ulnar nerve (NERVE). Nineteen healthy volunteers participated in the study. Electromyographic activity was collected from elbow and shoulder muscles. Arm cycling was performed clockwise at 60 rpm. The CST was stimulated using transcranial magnetic stimulation (TMS) applied over the contralateral motor cortex. During arm cycling, single TMS-induced motor-evoked potentials (MEPs) in the biceps brachii were significantly facilitated during the elbow flexion phase (2-7 o'clock the crank position) compared with static condition. At the 2 o'clock crank position, combined stimulation (CS) of NERVE and TMS (inter stimulus intervals of 7.5 to 12 ms, NERVE ahead) produced significantly greater MEP facilitation (CS- induced MEP facilitation) than the algebraic sum of responses elicited by TMS alone and NERVE alone, in both static and arm cycling tasks. Importantly, CS- induced MEP facilitation during the arm cycling was significantly greater than during static task at 2 o'clock. Furthermore, enhancement of CS-induced MEP facilitation was observed exclusively during the flexion phase but was suppressed during extension. These findings suggest that arm cycling modulates indirect CST outputs via presumed premotoneuronal networks in a task- and phase-dependent manner.