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Rate of force development: neuromuscular determinants - Prof. Duchateau

Invited Session "Rate of force development: methodological, physiological and practical issues" Rate of force development: neuromuscular determinants Duchateau, J. Universite Libre de Bruxelles The maximal rate of force development during a fast (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 factor. This is supported by a simulation study performed in our laboratory showing that the incidence of a few initial motor unit discharges at high frequencies (100Hz) plays a major role in the increase of the rate of force development. Initial conditions under which the ballistic contraction is performed influences both motor unit discharge and rate of torque development. For example, the performance of a fast isometric contraction is decreased when preceded by a sustained submaximal contraction but improved when a brief unintentional (silent period) or voluntary period of relaxation is inserted between the sustained and b.allistic phases of the contraction. Furthermore, chronic changes of the neuromuscular system, due to training with fast contractions and ageing, can also influence the capacity to reach high initial motor unit discharge rate and thereby the maximal rate of force development. The purpose of this keynote will be to review the neuromuscular determinants of the rate of force development and emphasize how the conditions from which fast contractions are initiated can influence motor unit activation and thereby the performance of such action. The potential mechanisms underlying chronic modulation of motor unit discharge during ballistic contraction will be also evoked.