Duchateau, J. (2009). Training effects on the characteristics and behavior of human motor units. A paper presented at the 14th Annual Congress of the European College of Sport Science, Oslo, Norway, June 24-27.

"The force generating capacity of an isolated muscle is strongly associated with its cross-sectional area. However, the strength produced during natural movements depends on the extent to which a muscle and its synergist and antagonist are activated by the nervous system. Although strength training is known to induce hypertrophy of the muscle fibers, some studies have suggested that increase in voluntary activation can also contribute to the improvement in performance. A greater activation can be the result of a more complete recruitment of motor units (ensemble comprising a motor neuron, its axon and the muscle fibers innervated by this axon) and/or an increase in discharge rate. In addition to the increase of the force produced by each motor unit after training, recent evidences indicate that the maximal discharge rate attained by motor unit is enhanced without any change in synchronization. This neural adaptation occurs mainly at the beginning of a training program.

In many sports and activities of daily life, the ability to rapidly develop force is equally important, or even more important, than the maximal force itself. Movements that are performed with maximal velocity are usually defined as ballistic actions. The maximal rate of force development during a ballistic contraction is not only limited by muscle speed-related characteristics, but also by neural factors. Among them, the capacity of the nervous system to recruit a great percentage of the motor unit pool at the onset of muscle activation and to drive these units at a high discharge rate is an important aspect. In that context, it has been demonstrated that dynamic training consisting of rapid contractions against a small load (30-40% of maximum) augments both the rate of force development and the instantaneous discharge rate of motor units. In addition to the latter adaptation, the incidence of doublet discharges with an interval of less than 5 ms is increased. These neural changes, that contribute to enhance the maximal rate of force development, appear to adapt specifically to the training conditions. Together, these findings show that, in addition to intrinsic adaptations of the muscular system, the rate of force development during a ballistic contraction is modulated by chronic changes in motor units discharge pattern."

Implication. Actions that require effort (strength or more correctly "power") adapt specifically to the conditions and activities of training. The further removed from competition-specific actions are the well-intentioned exercises of strength and conditioning programs, the less valuable they will be for improving competition performances.

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