This review will deal with two evolving concepts which describe and attempt to unify various observations concerning the behaviour of motor units that have been reported during the past decade. The two concepts are: The common drive which describes the behaviour of the firing rates of motor units, and appears to provide a simple schema for controlling motor units; and the firing rate/recruitment interaction which appears to enhance the smoothness of the force output of a muscle. The evolution of these concepts has been expedited by the development of recent techniques such as our decomposition technique which enables us accurately to decompose the myoelectric signal into the constituent motor unit action potential trains. For details refer to LeFever & De Luca (1982), Mambrito & De Luca (1983) and Mambrito & De Luca (1984).

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