Analysing the Countermovement Jump (CMJ) Force-Time Curve: Part 1

Force plates give us a huge range of metrics, but the visual inspection of the force-time curve from a countermovement jump (CMJ) is just as valuable—especially for identifying asymmetries, jump strategy, and data quality issues. In this post, we’ll explore how to read the force-time curve in detail, highlight what the different phases tell us, and walk through real examples of traces.

This post is part of our Athlete Testing Series, produced in collaboration with VALD, to share discussions on interpreting data from commonly used athlete assessments.

Understanding the Countermovement Jump

The Countermovement Jump (CMJ) is a foundational test utilised to assess an athlete's explosive power. It involves a dynamic movement starting with a downward phase (countermovement) before a rapid upward acceleration, driving the body off the ground.

It is most commonly assessed with hands on hips to avoid complication from upper-body coordination. An arms-free CMJ is generally referred to (and logged in VALD) as an Abalakov jump.

The CMJ is widely implemented, owing to its natural movement pattern for

athletes and the lower technical demands compared to other jump tests.

What Is a Force-Time Curve?

The force-time curve plots vertical ground reaction force over time during a movement like a CMJ. Using dual force plates, like those from VALD's ForceDecks, you can also separate left and right limb contributions.

The CMJ assesses the slow stretch-shortening cycle (SSC). The movement includes:

• Eccentric phase: The downward movement as the athlete squats (muscle lengthening)

• Amortisation phase: A brief transition between eccentric and concentric

• Concentric phase: The upward movement as the athlete jumps (muscle shortening)

This differs from:

• A squat jump, which omits the eccentric phase

• A drop or rebound jump, which tests the fast SSC with <250ms contact time

Key Phases on the Force-Time Curve

With reference to VALD's visual breakdown, we can divide the CMJ force-time curve into several key phases:

1. Quiet Standing

• The athlete stands still on the plate, producing a flat line representing body weight in Newtons.

• This is a crucial data quality check—movement here compromises the accuracy of body mass and start-time calculations.

2. Eccentric Phase

• Begins with the unloading phase, where force drops below body weight as the athlete initiates the squat.

• Transitions into the braking phase, where force increases again as the athlete resists gravity and decelerates.

• The point where force rises above body weight marks the start of deceleration.

3. Concentric Phase

• Starts at the lowest point of the squat and is typically split into:

  • P1: Early concentric, from bottom of squat to 50% of push-off time

  • P2: Late concentric, covering the second 50% of push-off

• Research shows asymmetry in P2 may distinguish between post-ACL athletes and healthy controls.

4. Flight and Landing Phases

• Flight is where no force is applied to the plate.

• Landing should show symmetrical, sharp impact peaks on both limbs.

What to Look for in Real Examples

As I discuss in the video above, when visually assessing the raw time curve I look for the following:

• Quiet standing phase: A stable baseline for body weight and a clear trial start point

• Symmetry: Of course, I look at the data outputs for asymmetry at different timepoints and across different phases, but assessing it visually can also provide insight into their (off)loading strategy from a continuous perspective across the jump)

• Curve shape: Is there a smooth, steep slope for take-off indicating an explosive, coordinated jump, or is it a low and slow curve, or perhaps an uncoordinated example with left and right limbs applying force at different times.

Each of these has implications for how you interpret jump height, power output, and asymmetry scores. Simply relying on numbers without inspecting the shape can mask important details.

For real-life examples, watch the video below 👇

Coming Soon: Unimodal vs Bimodal Curves

This is part one of a two-part series. In the next post, I'll dive deeper into the force-time curve and further unpack unimodal vs bimodal shapes.

FAQs about the Countermovement Jump (CMJ) Force Time Curve

What is a Countermovement Jump (CMJ)?

A CMJ is a vertical jump exercise used to evaluate an athlete's explosive capabilities, specifically the slow stretch-shortening cycle, through a dynamic downward movement (the countermovement) before an upward motion.

Why is the CMJ force-time curve important?

The curve offers detailed insights into an athlete’s ability to generate force, which is crucial for improving performance in explosive sports. While the cumulative measures and metrics are important, learning to visually assess the raw force-time curve can provide complementary insight into how your athlete executed the jump.

How can I tell if a CMJ trial is valid?

Look for a flat quiet standing phase, clear start of movement, and a clean takeoff and the force registering zero during the flight phase. Using reliable equipment (such as VALD's ForceDecks), a standardised protocol, and consistent execution is keep to collecting valid and reliable data.

What are the main phases of the CMJ force-time curve?

The main phases include the eccentric, concentric, flight, and landing phases, each offering unique insights into an athlete's performance. VALD break down the eccentric phase further into unloading, braking, and deceleration, as well as splitting the concentric phase into P1 and P2.

What’s the difference between a unimodal and bimodal force-time curve?

A unimodal curve has a single peak during takeoff, while a bimodal curve has two distinct force peaks—often reflecting inefficient or uncoordinated strategies. Look out for more to come on this topic soon!

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This article is support by VALD Performance. For more information, about their technology, visit their website.