Corticospinal excitability scales with global arm cycling intensity but not instantaneous force.

Abstract

Corticospinal excitability (CSE) is modulated during arm cycling, but its relationship with force production remains unclear. This study examined (1) how force output changes across arm cycling workloads and (2) whether CSE scales with force during rhythmic movement. Eleven healthy adults performed bilateral arm cycling at 25, 50, and 100 W at 60 rpm. Vector force was measured continuously across crank positions using instrumented handles. Motor evoked potentials (MEPs) elicited with transcranial magnetic stimulation and cervicomedullary motor evoked potentials (CMEPs) elicited with transmastoid electrical stimulation were recorded from the biceps and triceps brachii at the 6 o'clock position to assess supraspinal and spinal excitability, respectively. Vector force increased with workload (p < 0.001) and peaked between 4 and 6 o'clock. MEP and CMEP amplitudes increased with workload in both the biceps (p < 0.001 and p = 0.002, respectively) and triceps (p = 0.003 and p = 0.041, respectively). Across pooled workloads, vector force was correlated with biceps MEP (r = 0.53, p = 0.002) and CMEP amplitudes (r = 0.37, p = 0.034), and with triceps MEP (r = 0.39, p = 0.031) and CMEP amplitudes (r = 0.50, p = 0.005). Within individual workloads, correlations between force and neural measures were consistently weak and non-significant for both muscles. These findings demonstrate that corticospinal excitability during arm cycling scales with overall workload but not with instantaneous force production, supporting the concept that descending neural drive primarily reflects task-level regulation rather than moment-to-moment force encoding during rhythmic movement.