Figure 1.

Key times (KTa-d), phases (PhA-B), and performance (h) of a VSJ. KTa: initial position (the jumper stands still, heels on the ground, knee joint angle of ~90°, hands on hips); KTb: take-off (end of contact between the support and the jumper); KTc: peak of the flight phase (the mass center of the jumper, MC, reaches its maximal height of flight); KTd: landing (moment at which the feet touch again the ground); PhA: push-off phase (realized without initial countermovement, it starts at t0 when the segments of the jumper begin to rotate and ends at take-off); PhB: flight phase (it starts at take-off and ends at the instant of landing). The height of the jump (h) is given by the difference between the vertical coordinate of MC (♦) at take-off (z1) and at the peak of the flight phase (z2): h = z2 − z1. During the push-off the coordinate of MC evolve from z0 (initial position) to z1 (instant of take-off) and then to z2 (apex of the jump).