Acute Strength Training Increases Responses to Stimulation of Corticospinal Axons

AB Purpose: Acute strength training of forearm muscles increases resting twitch forces from motor cortex stimulation. It is unclear if such effects are spinal in origin and if they also occur with training of larger muscles. With the use of subcortical stimulation of corticospinal axons, the current study examined if one session of strength training of the elbow flexor muscles leads to spinal cord changes and if the type of training is important. Methods: In experiment 1, 10 subjects completed ballistic isometric training, ballistic concentric training, and no training (control) on separate days. In experiment 2, 13 subjects completed ballistic isometric training and slow-ramp isometric training. Before and after training, transcranial magnetic stimulation over the contralateral motor cortex elicited motor-evoked potentials (MEPs) in the resting biceps brachii, and electrical stimulation of corticospinal tract axons at the cervicomedullary junction elicited cervicomedullary motor-evoked potentials (CMEPs). Motor-evoked potential and CMEP twitch forces were also measured. Results: In experiment 1, CMEPs and CMEP twitch forces were significantly facilitated after ballistic isometric training compared to control. In experiment 2, MEPs, MEP twitch forces, CMEPs, and CMEP twitch forces increased for 15 to 25 min after ballistic and slow-ramp isometric training. Conclusion: Via processes within the spinal cord, one session of strength training of the elbow flexors increases net output from motoneurons projecting to the trained muscles. Likely mechanisms include increased efficacy of corticospinal-motoneuronal synapses or increased motoneuron excitability. However, the rate of force generation during training is not important for inducing these changes. A concomitant increase in motor cortical excitability is likely. These short-term changes may represent initial neural adaptations to strength training.