Selecting Force Plate Metrics for Countermovement Jump Testing: A Comprehensive Guide
- Jo Clubb
- Jul 2
- 10 min read
This post break downs the complexities of force plate jump testing, guiding practitioners through selecting meaningful metrics from the countermovement jump for physical profiling, fatigue monitoring, and rehabilitation.
Force plate testing has become a valuable tool for assessing various physical attributes, monitoring neuromuscular function, and supporting rehabilitation. However, the abundance of metrics produced by force plate software can be overwhelming for practitioners, especially those new to the field.
Drawing upon scientific literature and practical insights, this article aims to demystify the process of selecting force plate metrics for jump testing, offering practical insights and scientific foundations to guide practitioners in selecting force plate jump metrics.
The Basics of Force Plate Technology
Force plates, equipped with sensors such as piezoelectric or strain gauge, measure Ground Reaction Forces (GRF) exerted by individuals during activities such as standing, walking, jumping, or running.
These sensors capture data in accordance with Newton's Laws of Motion, providing insights into the interaction between force and motion. For a deeper understanding of force plate technology, read my Force Plate 101 article for VALD or watch the video below.
VALD categorises their ForceDecks tests into Jumps, Functional, Balance, and Isometric. In jump testing, force plates play a crucial role in assessing an individual's ability to generate vertical force and power. Common jump tests include the Countermovement Jump (CMJ), Squat Jump (SJ), Drop Jump (DJ), and various single-leg jump variations. Each test offers unique insights into different aspects of neuromuscular function and performance.
Selecting Metrics that Matter
A force plate collects an abundance of metrics. These include measures of force, acceleration, velocity, and power, as well as asymmetry variables when dual force platforms are used. Many of these are calculated at different timepoints (e.g., at zero velocity, take-off, landing) during the exercise, or summated over different timeframes (e.g., concentric, eccentric, landing phases). Different measures are also calculated based on the test utilised.
It is easy to be distracted by exciting new and trendy metrics, but caution should be exercised as these can exhibit wide variation and less evidence supporting their use. Instead, build a foundation of evidence-based metrics, using exploratory metrics as the “cherries on top”. Apply the Pareto Principle by spending 80% of your time on foundational metrics and reserving 20% to explore new approaches and metrics.
For the rest of this article, I'll discuss a process for selecting pertinent metrics for the CMJ, although this framework could also be applied to other jump types. The CMJ is the most common jump test assessed in sport settings, owing to its familiar movement and ease of data collection. For more information about the CMJ, see this comprehensive guide.
Which metrics you focus on depends on the type of test conducted, the assessment objective, and the individual being tested. When applying CMJ testing, there are three primary assessment objectives: physical performance profiling, neuromuscular fatigue monitoring, and rehabilitation and return-to-play.
The table below, developed by Ryan McLaughlin of VALD, displays these purposes and the corresponding questions they seek to answer.
Let’s explore each of these areas in turn and discuss the key metrics to use as a starting point in each. As with all force plate data, there are many different roads to Rome! Here, I have selected a handful of primary metrics for each area based on both scientific and anecdotal experiences, but I am open to debating the rationale behind these choices.
Physical Profiling: What is the Outcome?
One of the primary objectives of force plate testing is physical profiling, which aims to assess an individual’s capacities and identify areas for improvement.
Practitioners must consider the specific demands of the individual, such as their sport, playing position, and competition schedule. In clinical settings this relates to the individual’s lifestyle and their activity objectives.
When profiling physical capacities, the focus is on an individual’s jump outcome, which gives insight into their physical capacities. While there are numerous potential metrics, the following are most commonly related to physical outputs:
Jump Height: Estimated using methods like Flight Time (FT), Impulse-Momentum (IMP-MOM), and Impulse-Displacement (IMP-DIS). The IMP-MOM method is recommended for its reliability. For more on this, read this open-access systematic review in Sports Medicine by Xu and colleagues (2023) or watch my video on calculating jump height.
Takeoff Peak Force: Represents the maximum force exerted during the jump in Newtons (N).
Peak Power: Calculated as force multiplied by velocity, and measured as both relative (W/kg) and absolute (W) values.
Impulse: Calculated as force multiplied by time. This can be represented as the area under the force-time curve. May be assessed over a specified time interval or the entire movement.
These metrics are often highly correlated. This research indicates that power only accounts for about 50-60% of jump height performance (Kirby et al., 2011). Understanding these relationships highlights which measures may be distinct enough to warrant tracking both for a more comprehensive assessment of an individual’s CMJ performance.
According to Kirby and colleagues (2011):
“The primary finding of this investigation is that relative net vertical impulse determines jump height performance... Therefore, increasing and monitoring relative net vertical impulse may be used to assess and improve vertical jump performance."
In line with this research, I recently saw ForceDecks creator Dr Daniel Cohen present and advocate for the metric Net Concentric Impulse 100, which he refers to as Con100. This measures the impulse generated in the first 100 milliseconds of the concentric phase. By fixing the time window, Con100 isolates explosive force production.
At present, Con100 is my 'exploratory metric' of most interest for physical profiling (as outlined in the framework figure at the end of this article). While I’ve associated it primarily with profiling in this framework, Dr Cohen has also highlighted its potential for detecting fatigue-related changes in movement strategy - reflected in changes in the area under the curve - as well as asymmetries and possible injury risk. It's certainly a metric worth exploring further.
Such outcome metrics are fundamental for assessing and tracking an athlete's force and power generating capacities. If using these metrics to make between-athlete comparisons, it is important to consider factors like body mass, age and maturation status, and sex. By repeating the CMJ test at appropriate intervals, practitioners can collect objective data on how an individual’s physical capacities are responding to a training programme.
Neuromuscular Fatigue Monitoring: What is the Strategy?
In addition to physical profiling, force plate testing also serves as a valuable tool for monitoring neuromuscular fatigue and readiness. This is particularly of interest in professional and high-performance sports settings where regular monitoring can support day-to-day training decisions. By analysing jump strategy and force-time characteristics, practitioners can potentially identify signs of fatigue-induced alterations in movement patterns.
While monitoring the output metrics described earlier remains useful, it's important to recognise that individuals can often maintain jump outcomes when fatigued by adjusting their movement patterns (Cormack et al., 2008). Therefore, monitoring metrics that represent jump strategy is essential.
These are predominantly time-based variables. Put simply, an individual may achieve the same jump height and force output, but take a longer (or shorter) time to achieve it.
Consider the following metrics to monitor neuromuscular fatigue:
Contraction Time: Time from the start of movement until take-off.
CMJ Depth: Depth of squat during the take-off phase.
Flight Time to Contraction Time Ratio (FT:CT): Calculated as Flight Time / Contraction Time.
Reactive Strength Index (RSImod): Calculated as Jump Height / Contraction Time.
It is worth noting that FT:CT and RSImod share a similar calculation and exhibit an almost perfect relationship, reducing the necessity to monitor both (McMahon et al., 2018). While RSImod may appear more popular currently, FT:CT actually came first.
Additionally, whenever analysing ratio metrics like FT:CT and RSImod, it is crucial to also consider its underlying component parts. Changes in contraction time, or jump height, can explain the change in FT:CT or RSImod, providing context for altered ratio metrics. Another reason for emphasizing the analysis of both components is the situation where both metrics change at a similar magnitude, but the ratio output stays the same.
My current exploratory metric in this bucket actually came to my attention since I've been working on this framework: Eccentric Peak Velocity (EPV). This represents the maximum velocity in the eccentric phase. Dr Daniel Cohen and Dr Morgan Williams, Data Scientist at VALD, have recently published an article discussing the value of this measure for VALD Health. In brief, they explain it should be used as a criterion metric since it functions as a way to check how the movement has been performed and whether the attempt is sufficient to qualify as a CMJ.
Changes in any of the strategy metrics discussed in this section may indicate fatigue-induced modifications in neuromuscular function, signalling the need for adjustments in training volume or intensity, and potentially a heightened focus on recovery.
However, the above is not a universal consequence of changes in jump strategy. Some individuals may demonstrate reduced CMJ height due to muscle soreness that prevents them from squatting to their usual depth. Conversely, others may increase CMJ depth to leverage a greater range of motion for vertical propulsion.
The key lies in individualised analysis and tracking of each athlete, employing appropriate calculations of change (e.g., z-score, standard deviations, effect size, percent change) to promptly identify significant shifts in these metrics.
Rehabilitation and Return-to-Play: What is the Progress?
In rehabilitation, force plate testing evaluates functional recovery post-injury and supports return-to-play decisions. The CMJ is frequently employed to assess lower-body capacity during rehabilitation, provided the injury allows. In this context, practitioners can apply the methodologies discussed earlier to assess both jump outcome and strategy, as well as add a specific focus on asymmetries.
Tracking the jump outcome, such as jump height, peak force, and power, allows practitioners to gauge how far an individual is from their baseline physical capacities, and track this progress throughout rehabilitation. It is important to note that progress may not necessarily follow a linear trajectory. As rehabilitation intensifies with increased load and physical demands, an individual's capacities may temporarily decrease.
While quantifying neuromuscular fatigue is less of a focus during rehab, metrics related to jump strategy remain valuable. The CMJ depth can represent the individual’s (in)ability to move through their pre-injury range of motion to the same depth, and when paired with contraction time, practitioners can evaluate an individual’s movement speed relative to their baseline.
A notable focus for force plate analysis during rehabilitation is on asymmetry metrics. These can pose a challenge to practitioners as there exists a ‘Calculation Conundrum’ with asymmetries measures. Understanding these calculations and selecting the most appropriate for specific use cases is essential. For more, watch the video below:
Beyond the Calculation Conundrum, there is a wide array of asymmetry metrics to consider. A straightforward starting point is mean force asymmetry, calculated for both the eccentric and concentric phases. I personally advocate for this metric over peak force asymmetry, as it provides a broader perspective, encompassing the force application over a whole phase rather than capturing a single millisecond in time, as the peak metric does.
Additionally, evaluating inter-limb asymmetry during landing can reveal whether an individual is offloading force asymmetrically or absorbing it bilaterally. Integrating video analysis of jumping and landing mechanics further enhances insights into movement compensations and injury-related deficits.
It is crucial to recognise that perfect symmetry is not the goal for most individuals, especially athletes who naturally exhibit asymmetries due to the demands of their sport. Ideally, baseline data from physical profiling tests should be available for comparison to pre-injury status; however, if unavailable, aiming for absolute symmetry (i.e. zero) is likely not necessary in athletic populations.
My current exploratory metric in the rehabilitation setting is the Force at Zero Velocity Asymmetry. The transition from the eccentric to concentric phase, when the velocity is theoretically zero, can highlight differences in force application between the limbs.
Also of note are VALD's P1 and P2 Concentric Impulse measures. Ryan McLaughlin breaks these down in the post shown right. The research mentioned includes this study led by Roula Kotsifaki at Aspetar that found significant differences in symmetry of concentric impulse in ACL reconstruction athletes compared to healthy controls.
Integrating force plate testing with clinical assessments provides a comprehensive understanding of how an injury has affected an individual's physical capacities. However, return-to-play decisions should not be made solely based on force plate data, but rather be part of a holistic approach to gathering objective and subjective information during rehabilitation.
Final Thoughts
Practitioners should adopt a systematic approach to metric selection, tailored to the objectives of each testing session and the status of the individuals being assessed (particularly in rehabilitation). The framework presented in this article, summarised in the figure below (6), serves as a foundational guide for choosing metrics across three key areas: physical profiling, neuromuscular fatigue monitoring, and rehabilitation.
While a case could of course by made for numerous other metrics, the goal is to start with well-established, reliable measures that effectively convey the CMJ test. Over time, practitioners will naturally develop their own philosophies and preferences in force plate analysis. Nonetheless, beginning with a solid base of evidence-based metrics is crucial. This approach then allows for the exploration of new and innovative data analysis methods through a scientific and critical lens.
Finally, while force plate data provides critical insights, it should be integrated with other clinical assessments and subjective observations to form a holistic view of an individual’s condition. This multifaceted approach ensures that all aspects of an individual's health, performance, and recovery are considered, leading to more informed and effective decision-making.
Frequently Asked Questions (FAQs)
What are force plates, and how do they work?
Force plates are devices equipped with sensors that measure ground reaction forces during movement. They capture data based on Newton’s Laws of Motion, providing insights into force production, movement efficiency, and neuromuscular function.
Why is the Countermovement Jump (CMJ) commonly used in force plate testing?
The CMJ is widely used because it is simple to perform, requires minimal technical instruction, and provides reliable insights into an individual’s lower-body power, neuromuscular fatigue, and movement strategy.
What are the most important force plate metrics for assessing jump performance?
Key metrics include jump height, peak force, peak power, and impulse. These variables help quantify an athlete's ability to generate force and power, which are critical for performance profiling and training adaptations.
How can force plate data help monitor neuromuscular fatigue?
Changes in jump strategy metrics such as contraction time, CMJ depth, and the Flight Time to Contraction Time ratio (FT:CT) can indicate neuromuscular fatigue. These insights help practitioners adjust training loads to optimise recovery and performance.
What role do force plates play in rehabilitation and return-to-play decisions?
Force plates help track functional recovery by assessing jump performance, movement strategies, and asymmetries. Monitoring these metrics over time ensures objective, data-driven decisions about an athlete’s readiness to return to sport.
How do I decide which force plate metrics to focus on?
Your choice of metrics should align with your assessment goal—whether profiling physical performance, monitoring fatigue, or guiding rehabilitation. Start with well-supported, foundational metrics and use exploratory metrics to refine insights.
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