Activation is the amount of voluntary recruitment of a muscle during voluntary contraction. Full activation implies the recruitment of all muscle fibres at their tetanic frequency. In healthy subjects, and even in sports performances, full activation may be rarely achieved despite a subjectively maximal effort. Highly decreased activation has been observed in patients affected by various orthopaedic and neurological disorders. In these subjects, paresis may be caused or aggravated by primitive impairments of the central nervous system and/or, by stimuli arising from peripheral damaged tissues that inhibit the corticospinal or the intraspinal recruitment of motoneurones (arthrogenous muscle weakness). There are numerous investigations in the literature on activation measured during isometric contractions, while they are substantially missing as far as isokinetic concentric contractions are concerned. There are reasons to suppose that, contrary to what has been demonstrated for healthy subjects, in patients with various motor impairments the activation is diminished the more, the higher is the joint rotation speed. The present study aims to investigate the amount of activation of the quadriceps femoris during subjectively maximal isometric contractions at 40° knee flexion (0°=complete extension) and isokinetic concentric contractions at an angular velocity of 100°/s in patients with various orthopaedic and neurologic conditions. Activation will be measured on an isokinetic dynamometer, through the interpolated twitch technique. This consists of stimulating a representative sample of the muscle belly through an electric shock. If the shock does not generate an extra force during contraction, all muscle fibres belonging to the sample reached by the electric shock can be claimed to be recruited at their tetanic frequency. Otherwise, following the stimulus, a twitch can be observed revealing submaximal voluntary recruitment of the muscle.