Interpreting a Countermovement Jump Force Trace: A Practical Guide

The countermovement jump (CMJ) is the workhorse test of force plate monitoring. It is quick, low-fatiguing, and one of the most sensitive practical markers of neuromuscular fatigue and readiness. But the number on the screen, jump height, is the least informative part of the test. The force trace underneath it tells you how the jump was produced, and that is where the coaching value lives.

This guide walks through the phases of a CMJ force trace and the metrics worth tracking, so the data drives decisions instead of just filling reports.

The phases of the trace

A standard CMJ force trace, force on the vertical axis and time on the horizontal, moves through six phases:

1. Weighing phase. The athlete stands still. Average the force here to establish bodyweight; every relative metric depends on this baseline being quiet and stable.
2. Eccentric unloading phase. Force drops below bodyweight as the athlete sinks into the squat. Depth and tempo here shape everything that follows.
3. Eccentric loading phase. Force rises sharply above bodyweight as the athlete decelerates the downward movement and reverses. Peak force often occurs here.
4. Concentric loading phase. Force stays elevated while the athlete drives up. The area under the curve during concentric loading (concentric impulse) largely determines takeoff velocity.
5. Flight phase. Force drops to zero. Flight time is the classic basis for estimating jump height.
6. Landing phase. Force spikes as the athlete absorbs the landing. Landing forces are worth watching for athletes returning from lower-limb injury.

The metrics that matter

You do not need all of them every week. Most programs get the most value from this set:

• Jump height. The headline outcome, usually estimated from takeoff velocity. Best interpreted as a change from the athlete's own rolling baseline rather than against a normative table.
• Modified reactive strength index (mRSI). Jump height divided by time to takeoff. It captures how quickly the athlete converts the countermovement into propulsion. Two athletes with identical jump heights can differ substantially in mRSI, and that difference points to different training emphases.
• Peak force (relative to bodyweight). How hard the athlete can push, independent of speed. Changes here reflect maximal strength qualities more than fatigue.
• Concentric and eccentric impulse. The area under the force curve in each phase. Eccentric impulse tends to drop earlier under fatigue, which is why it earns its place in monitoring batteries.
• Eccentric deceleration utilisation. The ratio of eccentric loading phase characteristics that reflects how efficiently the athlete reverses the movement. Clumsy reversals often show up here before jump height drops.
• Left-right asymmetry. On dual force plates, the split between limbs during propulsion. Under about 10 percent is common in healthy athletes; track the trend, not the single reading.

Reading the trace like a coach

Patterns in the trace's shape are as useful as the summary metrics:

• A slow, deep countermovement with a long time to takeoff suggests a strength-dominant profile. Expect a lower mRSI and target rate-of-force-development work.
• A shallow, fast countermovement with high mRSI but modest height suggests a reactive profile that may be under-powered. Add force-producing strength work.
• A damped eccentric loading phase and reduced eccentric impulse in a normally sharp athlete is a classic fatigue signature, often visible before jump height falls.
• A growing propulsion asymmetry that was not there in previous weeks deserves a conversation with the physio before the next heavy session.

Testing consistency beats testing volume

CMJ data is only as good as the protocol behind it. Three habits keep the numbers honest:

1. Keep the protocol identical. Same instruction (hands on hips or hands free, but pick one), same warm-up, same number of trials, same rest between trials.
2. Take a consistent number of trials and use a defined summary, best trial or average, and never switch between them mid-season.
3. Test at the same point in the weekly cycle. A Monday-morning CMJ after a rest day tells you something; one taken the day after a heavy gym block tells you something else.

From trace to decision

In practice, CMJ monitoring earns its keep through simple weekly logic:

• Baseline within normal range, asymmetry stable, proceed with the planned session.
• Jump height or eccentric impulse down beyond an athlete's usual variation, consider pulling intensity back or substituting a low-neural session.
• Persistent asymmetry or a trace that looks different from the athlete's normal shape, flag it to medical staff before loading further.

With native VALD ForceDecks integration, Prospect pulls every CMJ result, including the full force-trace metrics and asymmetry values, straight into each athlete's profile, alongside training load and wellness data. That means the Monday jump test is read in the context of the week's load, not as an isolated number on a tablet.