There Are No Differences in Startle Conditioned Cervicomedullary Motor Evoked Potentials Across Isometric, Concentric, and Eccentric Muscle Actions at the Same Absolute Force Output

Human movement involves a dynamic interplay of isometric, concentric, and eccentric muscle actions. There is a need to understand the contribution of the reticulospinal tract (RST) to human movement control during different muscle actions. This research aimed to determine the excitability of the RST during isometric, concentric, and eccentric muscle actions. Fourteen neurologically intact participants (age: 26 ± 7 years; sex: 3 female, 11 male; stature: 176 ± 8 cm; mass: 78.5 ± 10.9 kg) performed isometric, concentric, and eccentric muscle actions with the right biceps brachii. Participants performed a submaximal contraction at 25% of their isometric maximum voluntary contraction (MVC) during all muscle actions. Neurophysiological electrical stimulations to indirectly measure RST excitability consisted of conditioned (startling auditory stimulus of ≥ 110 dB) and unconditioned (no auditory stimulus) cervicomedullary motor evoked potentials (CMEPs). Larger conditioned CMEP responses compared with unconditioned CMEPs were observed for all muscle actions (p = 0.008). However, no differences in RST excitability, inferred from the difference between conditioned and unconditioned CMEP responses, were observed across the three muscle actions (p = 0.319). These results suggest that across isometric, concentric, and eccentric muscle actions, there are no differences in RST excitability while performing a submaximal contraction at 25% of their isometric MVC. It could therefore be inferred from this that RST input to motoneurons is not different between isometric, concentric, and eccentric muscle actions of the biceps brachii at a relatively low fixed absolute contraction intensity.