To assess the extent of the level of muscle activation during maximal voluntary effort, a method of superimposed trains of electrical impulses delivered at 100 Hz was employed. During a maximal voluntary contraction (MVC) in isometric knee extension, a submaximal superimposed electrical stimulation (ES) of differing train durations was induced to the quadriceps muscle, when maximal voluntary torque was achieved. For all train durations the force increased during ES. During 100 ms and longer trains, the additional torque reached a plateau. The same principle of submaximal electrical stimulation superimposed over MVC was used in explosive isometric knee extensions, comparing the rate of force growth in trials with and without ES. In all subjects ES had an augmenting effect during the increase in force and up to the maximal force. It was concluded that the subjects were not able voluntarily to activate fully their quadriceps muscle. This was true both for maximal force and for the increase in force. It seems that the existence of an activation deficit is a fact, the question is how to quantify it in a reliable and valid way.