Key Parameters
Jump height
Section titled “Jump height”| Method | Formula | Notes |
|---|---|---|
| Impulse-momentum | h = (net impulse)² / (2 × mass² × g) | More accurate — uses the full force-time curve |
| Flight time | h = g × t² / 8 | Simpler — assumes symmetrical takeoff and landing posture |
The ZForce app reports both. Impulse-momentum is preferred for scientific use.
Peak force
Section titled “Peak force”Unit: N (Newtons) or N/kg (relative)
Definition: Maximum instantaneous vertical ground reaction force during the test.
Relevance: Reflects neuromuscular capacity. Higher peak force correlates with sprint and jump performance.
Typical values (CMJ, bilateral, trained athletes):
- Recreational: 1800–2400 N
- Trained team sport: 2400–3200 N
- Elite: 3000–4500 N
Impulse
Section titled “Impulse”Unit: N·s (Newton-seconds)
Definition: Area under the net force-time curve (force minus bodyweight).
| Impulse type | Phase | Meaning |
|---|---|---|
| Braking impulse | Braking | Eccentric loading capacity |
| Propulsive impulse | Propulsion | Drive to takeoff; directly determines jump height |
| Net impulse | Full jump | Total momentum change at takeoff |
Rate of Force Development (RFD)
Section titled “Rate of Force Development (RFD)”Unit: N/s
Definition: Rate of change of force from rest. Measured at discrete time windows:
| Window | Relevance |
|---|---|
| RFD 0–50 ms | Early neural drive (before EMG can act on muscle) |
| RFD 0–100 ms | Relevant to short ground contacts (sprinting, DJ) |
| RFD 0–200 ms | General strength expression |
Higher RFD → faster force production → better performance in ballistic tasks.
RSI — Reactive Strength Index
Section titled “RSI — Reactive Strength Index”Definition: Measures ability to rapidly change from eccentric to concentric contraction.
| Variant | Formula | Best for |
|---|---|---|
| RSI (Drop Jump) | flight time / contact time | True reactive strength |
| RSI-mod (CMJ) | jump height (m) / time to takeoff (s) | Proxy when no DJ available |
Typical RSI-mod values (CMJ):
- Recreational: 0.2–0.4
- Trained: 0.4–0.6
- Elite: 0.6–1.0+
Contact time
Section titled “Contact time”Unit: ms
Definition: Time from first ground contact to takeoff (Drop Jump). Shorter = more reactive.
Flight time
Section titled “Flight time”Unit: ms
Definition: Time airborne between takeoff and landing. Used to calculate jump height.
Asymmetry Index (AI)
Section titled “Asymmetry Index (AI)”Unit: %
Formula: |L − R| / max(L, R) × 100
Definition: Percentage difference between left and right for bilateral metrics (force, impulse, peak power).
Peak power
Section titled “Peak power”Unit: W (Watts) or W/kg
Definition: Maximum instantaneous power = force × velocity at the same moment.
Relevance: A better predictor of athletic performance than peak force alone in many contexts.
Eccentric / Concentric duration
Section titled “Eccentric / Concentric duration”Unit: ms
Definition: Duration of the braking (eccentric) and propulsive (concentric) phases respectively.
Relevance: Eccentric duration affects how well elastic energy is stored and reused.