Background
I led the authorship of an article published in the National Strength and Conditioning Association’s Strength and Conditioning Journal in early 2018, titled “Understanding the Key Phases of the Countermovement Jump Force-Time Curve”. At that time, many researchers were using bespoke software for analyzing CMJ force-time curves, leading to inconsistent methodologies that complicated comparisons between studies. With force plates becoming more accessible, practitioners also struggled to apply research findings to their training programs due to the varied analysis approaches.
We aimed to explain the CMJ force-time curve phases clearly and suggest a standardized naming convention (or at least accurate descriptions) for use in any software analyzing the data. We didn’t anticipate that Hawkin Dynamics would adopt our phase names soon after publication, nor that I would become their Director of Research and Education six years later! I’m aware that although force plates are very familiar to me and more so now than ever to many practitioners globally, there are still many practitioners who are only just commencing their force plate journeys, and so I wanted to briefly summarize the six key phases of the CMJ force-time curve and simplify the science behind this powerful assessment option.
Why Test the Countermovement Jump?
The CMJ is a fundamental exercise that is commonly used to evaluate an athlete's neuromuscular function. It's a ballistic movement (some might call it explosive) that can tell us a lot more than just how high someone can jump. In fact, by focusing on jump height alone, we would omit most of the useful information that can be generated from testing a CMJ on a force plate. By analyzing the force-time curve generated during the jump, we can gain deeper insights into an athlete’s capabilities when they are fresh and injury free, when they are fatigued, and when they are injured, and use the generated information to specifically target potential areas for improvement. We can then regularly monitor an athlete’s CMJ ability to track progress or regressions in performance and adjust training programs accordingly. For more information on why one might test the CMJ, please check out the CMJ playbook here. For now, let’s get into brief descriptions of the six key phases of the CMJ.
1. Weighing Phase
The CMJ starts with what is known as the weighing phase. During this phase (shown below), the athlete stands still on the force plates to establish their baseline system weight (which is equal to body weight when doing a CMJ without an external load). While it might seem like a trivial step, accurately determining system weight is crucial. This value sets the stage for all subsequent calculations, including identifying when movement begins and calculating the main phases and kinematics (velocity, displacement etc.) of the jump. The athlete should remain as still as possible to ensure that the system weight measurement is accurate, as even minor fluctuations can affect the analysis. Luckily, Hawkin Dynamics enforces an accurate weighing phase of 1 second before allowing a CMJ test to run in our software. This is a non-negotiable step to generate accurate results. System weight is an important contextual metric that should always be considered when interpreting an athlete’s CMJ metrics and how these change over time.
2. Unweighting Phase
Next comes the unweighting phase, where the athlete begins to drop into a squat by flexing their hips, knees, and ankles. This phase starts as soon as the force recorded by the force plates drops below the baseline body weight plus the force threshold used to detect movement onset (which is equal to five standard deviations of the force registered during the weighing phase, for those who are interested!). The unweighting phase is all about preparing for the explosive jump—by lowering their center of mass, effectively free-falling under gravity, the athlete builds potential energy that is then converted into kinetic energy during the next phase.
3. Braking Phase
The braking phase is where the athlete decelerates their downward movement, effectively stopping their descent before reversing direction. The athlete must apply significant force over a short time to match the downward momentum generated during the unweighting phase. The peak braking velocity is an important contextual metric that represents the velocity at the beginning of the braking phase and, therefore, the velocity at the end of the unweighting phase. The ability to quickly and efficiently decelerate, or "brake," is a key indicator of an athlete's neuromuscular capabilities. The braking phase is critical because it sets the stage for the following phase (explained next). The average relative braking force indicates the athlete’s deceleration ability during the braking phase.
4. Propulsive Phase
Following the braking phase is the propulsive phase, where the athlete pushes upwards from their lowest position. The countermovement depth represents the lowest position of the center of mass attained at the transition point between the braking and propulsive phase and (shown below), thus, is a useful strategy metric. The propulsive phase begins when the athlete's center of mass starts moving upwards. The force applied during this phase and time over which it is applied propels the athlete off the ground and directly influences the jump height. A well-executed propulsive phase results from the athlete effectively utilizing the stored energy from the braking phase to achieve maximum upward velocity. The average relative propulsive force indicates the athlete’s acceleration ability during the propulsive phase.
5. Flight Phase
The flight phase is the simplest to understand—it's the moment when the athlete is airborne and begins at the instant of takeoff (end of propulsive phase). However, there's still a lot going on during this phase. The height reached during the flight phase depends on the effectiveness of the previous phases. The peak of the jump occurs when the upward velocity slows to zero before gravity pulls the athlete back down (shown below). Jump height is the most common metric reported for the CMJ and while it occurs during the flight phase, it is calculated from the takeoff velocity in our system rather than flight time. While airborne, the athlete’s body position can change and therefore influence how and when they land, which can result in errors when using the flight phase time to estimate jump height. You can read this blog for information about how and why we calculate jump height from takeoff velocity.
6. Landing Phase
The landing phase begins when the athlete contacts the ground again (which we call the instant of touchdown). A controlled, safe landing requires the athlete to flex their hips, knees, and ankles, which helps dissipate the forces (lower the peak force by extending the time over which force is applied) and reduces the risk of injury. The landing phase usually involves the largest peak forces and is the phase in which percentage force asymmetries are often of interest, such as the asymmetry between the left and right leg’s average landing force (called L/R average landing force in the Hawkin Dynamics system). The landing phase ends when the athlete’s center of mass velocity returns to zero, indicating that they have come to a complete stop and, therefore, finished the CMJ.
Practical Applications
Understanding these six phases of the CMJ can significantly enhance an athlete’s training and performance. By analyzing the force-time curve, coaches, and practitioners can identify strengths and weaknesses in an athlete's jump technique. For example, an athlete with a strong propulsive phase but a weak braking phase might focus on exercises that improve deceleration during what some people may refer to as “eccentric” focussed exercises. Moreover, consistent terminology and phase identification across testing sessions help to ensure that the data is collected in a reliable and meaningful way. This allows practitioners to track their athlete’s progress and adjust their training needs confidently.
The few CMJ metrics suggested in this post (in bold) make for a great starting point for those beginning their force plate journey and, indeed, for more experienced force plate users who are looking to focus on just a small number of CMJ metrics that provide context (like system weight), link phases (like peak braking velocity and countermovement depth), describe accelerative capabilities (like average relative forces), explain the jump outcome (like jump height), and highlight “force acceptance” symmetry between legs (like L/R average landing force).
Conclusion
The CMJ is more than just a measure of how high an athlete can leap. By breaking down the jump into its constituent phases and analyzing the force-time curve, as is standard practice in Hawkin Dynamics software, we can gain a comprehensive understanding of an athlete's neuromuscular capabilities. This detailed analysis allows for targeted training interventions, ultimately leading to improved performance and reduced injury risk. Whether you're a coach, an athlete, a clinician, or a sports scientist, understanding these phases is key to making the most of the CMJ as a performance assessment tool.