Jump Phase Definitions

Understanding the phases of a jump is essential for interpreting force-time data correctly. The definitions below follow the consensus used in the peer-reviewed literature (Linthorne 2001, Meylan et al. 2017, Hahn et al. 2011).

Phase overview

Force
  │
3×BW│         ╭──────╮
    │        ╱  prop  ╲
2×BW│       ╱          ╲
    │      ╱            ╲            landing
  BW│─────╱──────────────╲──────────────╭────────╮
    │    │  unweight brake │  flight     │        │
  0 │────┴────────────────┴─────────────┴────────┴─── time

1. Quiet standing

The athlete stands still before the jump. Used to estimate bodyweight (BW). The app averages force during a stable 2-second window after the countdown ends.

2. Unweighting

Starts: Force drops below BW
Ends: Velocity reaches its maximum negative value (or force returns above BW)

The athlete initiates the countermovement by relaxing lower-limb extensors. Force falls below BW, meaning the athlete is accelerating downward. This is the eccentric initiation.

3. Braking

Starts: Velocity at its maximum negative value (peak downward velocity)
Ends: Velocity = 0 (lowest point of the crouch)

The athlete decelerates the downward motion. Force is above BW, generating a net upward impulse. This phase requires high eccentric strength — braking impulse is a key metric.

4. Propulsion

Starts: Velocity = 0 (bottom of crouch)
Ends: Force drops to 0 (takeoff)

The athlete drives upward. Force peaks well above BW during this phase. Propulsive impulse determines jump height.

5. Flight

Starts: Force = 0 (feet leave plate)
Ends: Force > 0 again (feet contact plate)

No ground contact. Flight time is used to calculate jump height via:

h = g × t² / 8

where g = 9.81 m/s² and t = flight time in seconds.

6. Landing

Starts: Force returns above 0
Ends: Force returns to BW (stable standing)

The landing spike is typically 3–5× BW. Peak landing force is not routinely reported but is relevant in fatigue and injury-risk contexts.

Phase detection in ZForce / Hub

The algorithm uses velocity (integrated from net force) to locate phase boundaries — not raw force thresholds. This makes it robust to individual differences in bodyweight and movement style.

References

• Linthorne, N.P. (2001). Analysis of standing vertical jumps using a force platform. American Journal of Physics, 69(11).
• Meylan, C. et al. (2017). The reliability of jump kinematics in sprint-trained and endurance-trained athletes. Journal of Strength and Conditioning Research.
• Hahn, D. et al. (2011). Segment-specific force-time characteristics of countermovement jumps.