Corticospinal and intracortical excitability is modulated in the knee extensors after acute strength training

The corticospinal-responses to high-intensity and low-intensity strength-training of the upper-limb are modulated in an intensity-dependent manner. Whether an intensity-dependent threshold occurs following acute strength-training of the knee extensors (KE) remains unclear. We assessed the corticospinal-responses to an acute bout of either high-intensity (85% of maximal strength) or low-intensity (30% of maximal strength) KE strength-training with measures taken during an isometric KE task at baseline, post 5, 30 and 60 minutes. Twenty-eight healthy volunteers (23 ± 3 years) were randomized to high-intensity (n = 11), low-intensity (n = 10) or to a control group (n = 7). Corticospinal-responses were evoked with transcranial magnetic stimulation (TMS) at intracortical and corticospinal levels. An acute bout of high- or low- intensity KE strength-training had no effect on maximum voluntary contraction (MVC) force post-exercise (P > 0.05). High-intensity training increased corticospinal excitability (range 130% to 180%) from 5-60 minutes post-exercise compared to low-intensity training (17-30% increase). Large effect sizes (ES) showed that short-interval cortical inhibition (SICI) was reduced only for the high-intensity training group from 5-60 minutes post-exercise (24-44% decrease), compared to low-intensity (ES ranges 1-1.3). These findings show a training-intensity threshold is required to adjust CSE and SICI following strength training in the lower-limb.