The countermovement jump

Unweighting, braking, propulsion, flight, landing — and why jump height is an impulse story. · 7 min read

Five phases

Quiet stand. Unweighting: COM accelerates down, GRF falls below body weight. Braking: GRF rises, eccentric knee/hip extensors absorb the dip. Propulsion: concentric extension, GRF stays high until take-off. Flight: GRF is zero; COM is a parabola. Landing: the reverse problem, with a second peak that often exceeds take-off.

Jump height from flight time is h = g t² / 8. It assumes equal take-off and landing postures. COM apex from kinematics does not. Report both when they disagree — the difference is a methods lesson.

CMJ versus squat jump

The squat jump removes the countermovement. Eccentric utilisation ratio (CMJ / SJ) is typically 1.05–1.15 in trained subjects. Below 1.0 usually means the athlete dipped on the ‘squat jump’ or cannot use the stretch-shortening cycle, not that they are ‘weak’.