Determining the early corticospinal-motoneuronal responses to strength training: A systematic review and meta-analysis

Several studies have used Transcranial Magnetic Stimulation (TMS) to probe the corticospinal-motoneuronal responses to a single session of strength-training; however, the findings are inconsistent. This systematic review and meta-analysis examined whether a single bout of strength-training affects the excitability and inhibition of intracortical circuits of the primary motor cortex (M1) and corticospinal-motoneuronal pathway. A systematic review was completed tracking studies between January 1990 and May 2018. Methodological quality of studies was determined using the Downs and Black quality index. Data were synthesised and interpreted from meta-analysis. Nine studies (n=107) investigating the acute corticospinal-motoneuronal responses to strength training met the inclusion criteria. Meta-analyses detected that following strength training compared to control, corticospinal excitability (SMD 1.26, 95% CI 0.88, 1.63, P < 0.0001), and intracortical facilitation; ICF (SMD 1.60, 95% CI 0.18, 3.02; P = 0.003) were increased. The duration of the corticospinal silent period was reduced (SMD -17.57, 95% CI -21.12, -14.01; P = 0.00001), but strength training had no effect on the excitability of the intracortical inhibitory circuits (SICI; SMD 1.01, 95% CI -1.67, 3.69; P = 0.46; LICI; SMD 0.50, 95% CI -1.13, 2.13; P = 0.55). Strength-training increased the excitability of corticospinal axons (SMD 4.47, 95% CI 3.45, 5.49; P < 0.0001). This systematic review and meta-analyses revealed that the acute neural changes to strength-training involve subtle changes along the entire neuroaxis from the M1 to the spinal cord. These findings suggest that strength-training is a clinically useful tool to modulate intracortical circuits involved in motor control