Acute neural adaptations to resistance training performed with low and high rates of muscle activation

Training is associated with specific neural adaptations. Skill training has been associated with increases in corticospinal tract excitability, leading to long term adaptations within motor cortex. Neural adaptations associated with strength training are less well established. It is not known how they are affected by volume, intensity, rate of muscle activation, or rest period. We evaluated the acute neural adaptations to a single session of strength training with the goal of evaluating the influence of rate of muscle activation on neural adaptations at the cortical and spinal levels.

Thirty subjects participated in a single session of maximal, isometric knee extensions with the right leg. The training consisted of 4 sets of 5 contractions, with sets separated by approximately 5 minutes. Subjects were randomized into a high rate of muscle-activation group (Ballistic), a low rate of muscle-activation group (Ramp), and a Control group that did all testing but no training. Cortical spinal tract excitability was assessed using transcranial magnetic stimulation, spinal excitability was assessed using peripheral nerve stimulation, and inhibition of motor cortex was assessed using short-interval intracortical inhibition. In addition, we measured changes in motor performance. These same measures were assessed 24 hours later.

Results showed that an acute strength training session is associated with a depression in resting cortical spinal tract excitability, but no change in active excitability. This change was immediate, taking place after just 2 contractions. Training was also associated with an increase in the excitability of the monosynaptic reflex circuit within the spinal cord during muscle activation, but not at rest. After 24 hours, cortical motor tract excitability had returned to normal, but intracortical inhibition was decreased from the original measure. Subjects from all groups increased maximum rate of torque development from Day 1 to Day 2.

These results indicate that strength-training is associated with neural adaptations, though the adaptations were different than those for skill training. Understanding these adaptations will allow coaches and clinicians to better design programs to optimize the strength potential of the nervous system along with that of the muscles.