Modulation of intracortical inhibition and excitation in agonist and antagonist muscles following acute strength training

Purpose: Transcranial magnetic stimulation (TMS) usually investigates the corticospinal responses of
the agonist muscle to strength training, despite the role of the antagonist muscle in strength
development. We examined the intracortical responses from an agonist and antagonist muscle following
a single session of heavy-loaded strength training (dominant-arm only) to identify the early antagonistic
responses to a single session that may accompany improvements in strength.
Methods: Corticospinal and motor-cortical excitability and inhibition was collected from agonist and
antagonist muscles prior to and following a single session of heavy-loaded wrist flexor training in 18
individuals. Training consisted of 4 sets 6-8 repetitions at 80% of 1-repetition maximum (1-RM).
Recruitment curves for corticospinal excitability and inhibition of the right wrist flexor and wrist
extensor muscles were constructed and assessed by examining the area under the recruitment curve
(AURC). Intracortical measures were obtained using paired-pulse TMS.
Results: Following a single training session, increases in corticospinal excitability (CSE) were observed
in both the agonist and antagonist muscles. This was accompanied by decreases in corticospinal
inhibition (CSP) in both muscles. Intracortical inhibition was reduced and intracortical facilitation was
increased for the agonist muscle only. Intracortical measures in the antagonist muscle remained
unchanged after training.
Conclusions: These findings indicate that the corticospinal responses to a single session of strength
training are similar between agonist and antagonist muscles, but the intrinsic cortico-cortical circuitry
of the antagonist remains unchanged. The corticospinal responses are likely due to increased
involvement/co-activation of the antagonist muscle during training as the agonist muscle fatigues.