Resistance training: cortical, spinal, and motor unit adaptations.

During the first few weeks of isometric resistance training there is an increase in maximal muscle force output that cannot be accounted for by muscle hypertrophy. Early on, researchers postulated the existence of neural adaptations to training primarily through the use of surface electromyographic recordings. More recent evidence also suggests that increased excitation may occur at the cortical levels following short-term resistance training. Alterations in synergistic activation and reductions in antagonist activation are neural factors that have been identified as changing during the early stages of resistance training which could contribute to maximal force generation. Neural adaptations that occur during the ramp-up phase of isometric contraction include decreases in motor unit recruitment thresholds, increased motor unit discharge rates, and increases in double discharges. An increase in the maximal rate of force development also occurs during the early stages of resistance training, but whether the neural mechanisms associated with the increase in the rate of rise are also associated with the increase in maximal force has not been elucidated. More work is needed to examine the integration of changes in cortical and spinal excitability with single motor unit firing patterns during this simple form of exercise before we can extend our understanding to different types of training.