Corticospinal excitability to the biceps brachii and its relationship to postactivation potentiation of the elbow flexors

We examined the effects of a submaximal voluntary elbow flexor contraction protocol on measures of corticospinal excitability and postactivation potentiation of evoked muscle forces and if these measures were state-dependent (rest vs. voluntary muscle contraction). Participants completed four experimental sessions where they rested or performed a 5% maximum voluntary contraction (MVC) of the elbow flexors prior to, immediately, and 5 min following a submaximal contraction protocol. During rest or 5% MVC, transcranial magnetic stimulation, transmastoid electrical stimulation, electrical stimulation of biceps brachii motor point and Erb's point were elicited to induce motor-evoked potentials (MEPs), cervicomedullary MEPs (CMEPs), potentiated twitch (PT) force, and maximal muscle compound action potential (Mmax), respectively prior to, immediately, and 5 min postcontraction protocol. MEP amplitudes increased (215 and 165%Mmax, P ≤ 0.03) only at 1 and 6s postcontraction protocol, respectively during rest but not 5% MVC CMEP amplitudes decreased during rest and 5% MVC (range:21-58%Mmax, P ≤ 0.04) for up to 81 sec postcontraction protocol. Peak twitch force increased immediately postcontraction protocol and remained elevated for 90 sec (range:122-147% increase, P < 0.05). There was a significant positive correlation between MEP and PT force during rest (r = 0.88, P = 0.01) and a negative correlation between CMEP and PT force during rest (r = -0.85, P < 0.02 and 5% MVC (r = -0.96, P < 0.01) immediately postcontraction protocol. In conclusion, the change in corticospinal and spinal excitability was state- and time-dependent whereas spinal excitability and postactivation potentiation were time-dependent following the contraction protocol. Changes in corticospinal excitability and postactivation potentiation correlated and were also state-dependent.