The Power of EPV: Part 1

About the Authors

Dr. Daniel Cohen is the co-founder of ForceDecks and a consultant and researcher with over 20 years of experience evaluating healthy and injured elite athletes using force platforms. He currently works with teams across the English Premier League, NBA, NFL and NHL. He is also an Adjunct Associate Professor in Human Performance and Innovation at the University of Limerick’s School of Education.

Dr. Morgan Williams is a Data Scientist at VALD and Adjunct Associate Professor at Griffith University’s School of Health Sciences and Social Work. As part of the VALD Data Science Team, Dr. Williams uncovers new insights into VALD data and identifies how they can be used to enhance practice.

Eccentric peak velocity (EPV) made it into Dr. Daniel Cohen’s top three favorite force plate metrics. It is a key metric for unlocking powerful insights into data quality, countermovement jump (CMJ) technique, strategy and lower limb function.

The CMJ and single leg jump (SLJ) tests can be used to assess an individual’s stretch-shortening cycle (SSC) function and eccentric qualities. Inspecting EPV across jump trials can be a way to check how the movement has been performed and whether the attempt is sufficient to qualify as a CMJ. An under-capacity EPV may result in a “quasi-squat jump” and under-represent an individual’s SSC function and eccentric capacities.

Not checking EPV to verify the CMJ is, therefore, analogous to ignoring a countermovement check in a squat jump assessment, where the purpose of that check is to ensure the test evaluates concentric-only, non-SSC function and does not include a countermovement. So why in the CMJ – which aims to assess SSC function and eccentric qualities – do we not apply a threshold or criteria for an acceptable jump?

We suggest that since an insufficient EPV creates a sub-optimal demand for eccentric work and minimizes the intensity of the SSC, EPV should be considered a candidate criterion metric.

…since an insufficient EPV creates a sub-optimal demand for eccentric work and minimizes the intensity of the SSC, EPV should be considered a candidate criterion metric.

Across this series of blogs, we highlight EPV’s value in revealing powerful insights from CMJ and SLJ assessments. Combining years of research and experience with VALD’s Data Lakehouse, we examine EPV’s value in data hygiene checks, and in assessing CMJ technique and lower limb function. We will also discuss potential context-specific “target” values and examine how different cues can influence EPV during testing.

In part 1, we provide some background to CMJ force plate assessments, clarify eccentric phase terminology, identify where within the countermovement EPV is located and highlight how it relates to other metrics in this phase.

The CMJ Test

The CMJ is the most tested protocol across VALD Health, Performance and Tactical environments. To date, millions of CMJ tests have been uploaded to the VALD Data Lakehouse. Some of the factors that explain the popularity of CMJ testing include the following:  

• There is a substantial body of evidence that supports CMJ’s use to profile and monitor load response in healthy and rehabilitating individuals. 
• The CMJ test is widely used because of the speed and ease with which it can be conducted. 
• The range of CMJ metrics provided across the eccentric, concentric and landing phases correlates to a myriad of performance qualities present across a range of sporting and daily activities. 
• The CMJ test provides the capacity to simultaneously obtain asymmetries and compensatory strategies.  

…millions of CMJ tests have been uploaded to the VALD Data Lakehouse.

As a fundamental movement skill, jumping is an essential part of daily activities. Therefore, it is familiar to many and well-practiced from an early age. Indeed, the recognizable coordination pattern of the countermovement is observed in children as young as three years, with that general pattern retained across the lifecycle (Clark et al., 1989). 

This familiarity with jumping explains the excellent reliability of jump height (the performance outcome), even in untrained or unfamiliarized individuals who tend to display poor reliability in a range of other CMJ metrics. 

Jump height can be measured using a range of tools, including jump mats, optical systems and tape measures. Force plates (capable of sampling at higher frequencies) provide a detailed force-time measurement, capturing not only jump height but also force application throughout the CMJ.

ForceDecks instantaneously generates a comprehensive range of metrics that not only provides a core metric in jump height but also describes how force is applied prior to take-off and on landing. The complete metric list with definitions can be found in the VALD technical glossary.

To best extract performance and injury insights from these metrics, it is useful to understand where the metrics are derived in the movement, how they are calculated and how they change with stimuli. This knowledge enables the alignment of metrics with specific questions regarding an individual’s status (profile) and their response to load. 

To best extract…insights from these metrics, it is useful to understand where [they] are derived in the movement, how they are calculated and how they change with stimuli.

When discussing eccentric metrics such as EPV, it is helpful to be aware of the terminology differences that exist in describing this phase and its sub-phases. This avoids confusion when reading and interpreting the varied sources of information available. 

Eccentric Terminology, Metric Definitions and Calculations

In the VALD technical glossary, EPV is described as the “maximum velocity during the eccentric phase.” Elsewhere, EPV has also been referred to by others as “peak negative velocity” or “peak braking velocity.” Just like concentric peak velocity, EPV is a single time point identified on the velocity-time curve and is the instantaneous maximum (negative). 

[In ForceDecks] EPV is described as the “maximum velocity during the eccentric phase.” 

The higher the negative value is for EPV, the faster the eccentric phase will be… Keep this in mind when displaying or finding “maximum” values in your data. 

There are some confusing eccentric phase terminology issues that exist that are important to be aware of. They include: 

“Eccentric” vs. “Downward” Phase

In ForceDecks’ software, the “eccentric” phase of the jump is defined as the period beginning at the start of descent (start of movement) until the instant of zero velocity, i.e., maximum countermovement depth (Gathercole et al., 2015). However, the entire phase is not exclusively an “eccentric” muscle action. Hence, this phase is also termed the “downward phase.”

“Downward” eccentric and “upward” concentric phase terminology simply describes the direction of movement of the center of mass (Figure 1a). 

Sub-Phases 

Inconsistencies also exist across other software and literature in eccentric sub-phase terminology. In ForceDecks, the eccentric phase has three sub-phases: 

• Unloading 
• Braking 
• Deceleration 

As shown in Figure 1b, ForceDecks’ deceleration phase (Jakobsen et al., 2012) begins at EPV and ends at zero velocity (the end of the eccentric phase). Elsewhere, this is termed the “braking phase.”

…ForceDecks’ deceleration phase begins at EPV and ends at zero velocity (the end of the eccentric phase). Elsewhere, this is termed the “braking phase.”

The ForceDecks’ “eccentric braking phase” starts at minimum eccentric force and ends at zero velocity. Therefore, it includes the deceleration sub-phase and the sub-phase preceding it – the “yielding” phase recently proposed by Harry et al. (2020) based on CMJ joint power analysis (Figure 1b).

[In ForceDecks,] the “eccentric braking phase” starts at minimum eccentric force and ends at zero velocity.

Where During the CMJ Is EPV Located?

EPV marks the beginning of the eccentric deceleration phase. The downward descent continues, but beyond the EPV time point, body mass (BM) begins to decelerate until zero velocity is briefly reached. The brief pause marks the point of maximum countermovement depth (CMD), which is immediately followed by a reverse in direction and initiation of the “upward” concentric phase.

EPV marks the beginning of the eccentric deceleration phase.

The location of EPV and other eccentric phase metrics and events are shown here in the ForceDecks “raw data” output (Figures 2a and 2b):

Now that the definitions and terminology have been clarified, let us move on to the importance of EPV as a marker and as a “gatekeeper” to the use of other eccentric metrics. 

Why Should EPV Be a “Go-To” CMJ Metric? 

Metric selection is a continuous hot topic. It is fueled by the wide range of CMJ metrics reported in literature and available in ForceDecks, combined with the practitioners’ desire for a shorter list of metrics in their dashboard. 

As shown in the table below, metric selection will fundamentally depend on the purpose of the assessment and the status of the individual – is it for “profiling” or “load-response monitoring (LRM)” and is the individual in the “healthy” or “rehab” category? 

A reference list of available metrics and their prioritization across use cases can be found on page 31 of the Practitioner’s Intermediate Guide to Force Plates.

Jump height is almost always a key metric, as it reflects the task outcome. However, some execution-related metrics are often overlooked. We suggest that EPV should also feature as a criterion metric since it functions as a “technique check” or task execution metric.

…EPV should also feature as a criterion metric since it functions as a “technique check” or task execution metric.

By using EPV, the practitioner can: 

Furthermore, EPV can be viewed as the “gatekeeper” to eccentric metric use. It serves as a quality control measure, as an adequate EPV is required to ensure that the outputs of various eccentric metrics accurately reflect an individual’s true capacity.

In other words, if an individual descends very slowly, they minimize the deceleration demands placed on the neuromuscular system during the “eccentric” phase and, in turn, the need to express their maximal capacity. This scenario is analogous to testing the braking performance of a car only at very low speeds.

Figure 3 illustrates the negative effect of a CMJ performed with an EPV below an individual’s true capacity on commonly reported eccentric metrics. In this example, metrics from an individual’s CMJ trial with the fastest EPV are compared to the trial with the slowest EPV. These were from the same assessment, under the same conditions and instructions.

On inspection, the notably slower EPV (-1.27m/s) results in sub-optimal outcomes across all the eccentric variables (including FT:CT) and a consequent underestimation of the individual’s ranking when profiling with these metrics. Concentric metrics were minimally affected.

It is important to understand that while the slower EPV meets a proposed target of -1.2m/s, key eccentric metrics are below capacity for that individual. Therefore, practitioners should recognize that merely meeting an external criterion does not necessarily reflect an individual’s full capacity.

While faster EPVs are associated with a greater CMD at a cohort or population level, the two are not always directly related across trials. As such, CMD should also be reviewed across trials as part of a “technique check.”

It is evident how EPV does not fit neatly into a standard schema used for most CMJ metrics (i.e., a continuum of poor to excellent values – whereby we always aim to drive values higher). However, this does not mean it is any less important.

On the contrary, in our opinion, it should be standard practice to obtain real-time between-trial feedback on this metric as well as mean and peak values. Without this information, you are “blind” as to how the jump was performed.

[EPV] should be standard practice to obtain real-time between-trial feedback on this metric as well as mean and peak values.

Our experience is that EPV is not most practitioners’ “go-to” metric and is often not even on their radar. This may partly be due to the lack of information specifically demonstrating how it can be used, despite it appearing in landmark literature over a decade ago (e.g., Cormie et al., 2010).

This lack of awareness and guidance available to practitioners provides the motivation for writing this series of blogs since we feel that by omitting EPV from CMJ metric selection, practitioners could:  

• Miss critical information about jump technique and their approach, intent and execution of cues or instructions. 
• Misclassify or misrepresent an individual’s “eccentric qualities.” 

“For healthy individuals, Cohen et al. (2020) recommend an EPV of at least -1.2m/s to ensure a valid jump. Slower eccentric speeds may not accurately reflect the individual’s deceleration strategy.” 

So, Should -1.2m/s Be Your EPV Target?

In part 2, we will explain why EPV should be a “go-to” metric regardless of the context and purpose of testing. We will start by looking at EPV differences between professional soccer players undergoing rehab after ACLR and healthy players that shaped this -1.2m/s EPV “target” (Cohen et al., 2020). We will then look beyond this dataset and show how assessing EPV serves multiple purposes, including data hygiene, technique or intent assessment and lower limb function – both in rehab and healthy individuals.

To achieve this, we will tap into the very large CMJ dataset in the VALD Data Lakehouse, combined with other data sources and years of experience. This combination will help us critically evaluate potential EPV targets and thresholds for various contexts and purposes.

If you would like to learn more about how EPV can be used to enhance CMJ assessments, improve data reliability and ensure accurate interpretation of eccentric phase metrics, please reach out here. 

References 

1. Clark, J. E., Phillips, S. J., & Petersen, R. (1989). Developmental stability in jumping. Developmental Psychology, 25(6), 929–935. https://psycnet.apa.org/doi/10.1037/0012-1649.25.6.929 
2. Cohen, D. D., Burton, A., Wells, C., Taberner, M., Alejandra Diaz, M., & Graham-Smith, P. (2020). Single vs double leg countermovement jump tests: Not half an apple! Aspetar Sports Medicine Journal, 9, 34–41. https://www.researchgate.net/publication/339831206_Single_vs_Double_Leg_Countermovement_Jump_Tests_Not_half an Apple 
3. Cormie, P., McGuigan, M. R., & Newton, R. U. (2010). Changes in the eccentric phase contribute to improved stretch-shorten cycle performance after training. Medicine & Science in Sports & Exercise, 42(9), 1731–1744. https://doi.org/10.1249/mss.0b013e3181d392e8 
4. Gathercole, R. J., Stellingwerff, T., & Sporer, B. C. (2015). Effect of acute fatigue and training adaptation on countermovement jump performance in elite snowboard cross athletes. Journal of Strength & Conditioning Research, 29(1), 37–46. https://doi.org/10.1519/jsc.0000000000000622 
5. Harry, J. R., Barker, L. A., & Paquette M. R. (2020). A joint power approach to define countermovement jump phases using force platforms. Medicine & Science in Sports & Exercise, 52(4), 993–1000. https://doi.org/10.1249/mss.0000000000002197 
6. Jakobsen, M. D., Sundstrup, E., Randers, M. B., Kjær, M., Andersen, L. L., Krustrup, P., & Aagaard, P. (2012). The effect of strength training, recreational soccer and running exercise on stretch–shortening cycle muscle performance during countermovement jumping. Human Movement Science, 31(4), 970–986. https://doi.org/10.1016/j.humov.2011.10.001