The present experiments were carried out to assess the relationship between the mechanical behaviour of the leg extensor muscles in 12 male athletes during activities of a ballistic type and torque production, during: (1) torque production during isokinetic contraction: (2) ballistic type activities (vertical jumps from statis position = SJ, with counter-movement equal CMJ and after a fall from different heights equal DJ) and (3) a new anaerobic test (Bosco 1983a). This consisted of performing continuous jump work for a certain period of time (e.g., 60 s). Both ballistic activities and anaerobic power tests demonstrated the highest relationship with peak torques when this was developed respectively at pi and 4.2 rad x s-1. Peak torque was related to SJ (r = 0.71, p less than 0.01); to CMJ (r = 0.74, p less than 0.005); to best DJ (r = 0.60, p less than 0.05), to 15 s anaerobic power (r = 0.70, p less than 0.01) and 60 s anaerobic power (r = 0.68, p less than 0.01). The power output during ballistic activities was much higher than the power measured during zero acceleration performances. The difference noted was explained in the light of the fact that, when jumping, three different extensor muscle groups in each leg contract with re-use during the concentric phase of elastic energy stored during the eccentric phase. The relationship between isokinetic contraction and ballistic motion is discussed in terms of fiber type recruitment and their velocity dependency.