Selecting the right metrics for vertical jump testing

If you have ever run a jump test using a force plate, you will have noticed dozens of outcome measures that you can choose from to describe the jumping athlete. Distilling the 50+ jump metrics into a dashboard is a daunting task for a coach or your performance team.

This article will help you distill a kinetic analysis of the jumping athlete to the most important metrics, providing an overview of how to use a force plate to assess vertical jump strategy and performance in the context of monitoring neuromuscular readiness, strength testing, and monitoring athletes after injury to guide the return to sport (RTS) and return to performance (RTP) transitions. I’ll also share my experience developing the jump metric table below. Hopefully, we’ll bridge the gap between being a force plate novice and implementing vertical jump monitoring with confidence.

The jump metric table rates the reliability of the metric via three use cases, with better reliability shown in dark blue and those with lower reliability being light blue. In practice, you can think of these as “Use it” or “Think before you use it,” but clearly you should think before you use anything.

Matching metrics to goals for vertical jump tests

The performance goal of the vertical jump is straightforward: to jump as high as possible. Depending on the sport’s skill and movement patterns, practitioners may also need to assess the speed of the jump.

The forces we impart on the ground when we jump, that is, the external vertical ground reaction force (vGRF) determines our jump height through Newton’s laws of motion. We calculate the jump height via the net impulse, which is the area under the force-time curve, and the impulse-momentum relationship.

In addition to the jump height, coaches might be interested in quantifying how the athlete executed the vertical jump. This is the athlete’s jump strategy, and may change depending on training, fatigue and injury.

The three primary vertical jump tests for athlete monitoring are the countermovement jump (CMJ), drop jump (DJ) and squat jump (SJ).

The repeated short contact cyclic hop, a fast stretch-shortening cycle (SSC) movement, is also worth considering. Cyclic hopping has been studied in the context of human energetics, but it has received less attention after injury. Dan Pfaff’s Rudiment protocol is one of the best known examples of multi-directional cyclic hopping, and he uses it as a core qualitative assessment with his athletes.

Each of these jump tests has a single leg variation. Single leg jumping moves the athlete higher on the force-velocity relationship, requiring more strength and movement control.

Another variation for the CMJ and SJ adds an external load, via a trap bar, for example, to create a velocity-load profile. One challenge with a kinetic analysis of the jumping athlete is accurately determining the system mass and the onset of the jump. It is much easier to determine the onset of a vertical jump and accurately measure the system mass using the CMJ compared to the SJ, especially in loaded jumping.

In our 2022 article in the Strength & Conditioning Journal ^[1], we describe how practitioners can develop their protocols starting from a biological basis for the test or metric of interest. Two additional considerations for selecting a vertical jump test metric are whether the metric has a mechanical basis, and exercising caution around ratios.

To the first point, the net impulse in the eccentric deceleration phase is the impulse needed to bring our BCM velocity to zero prior to the ascent of the jump. The net concentric impulse determines jump height. On the other hand, discrete force-time measures don’t determine movement in the same way.

Regarding ratios and indices, simply put, we are notorious for our ratios and indices: reactive strength index (RSI), modified reactive strength index (RSImod), dynamic strength index (DSI), asymmetry index (AI), limb symmetry index (LSI), hamstring quadriceps strength ratio (H:Q) … the list goes on.

RSI and the asymmetry index are the commonly used ratios for jump monitoring. The key is that if a training intervention or injury has a causal connection with the components of the index or ratio, then practitioners can proceed – with caution! – to interpret the ratio metric.

For example, training might cause an increase in jump height and a decrease in the ground contact time. Both change together with the intervention. If you were tracking RSI over the course of training, you would have to track the component parts – jump height and ground contact time – to understand the change in the metric and, most importantly, the changes in the athlete.

Athlete monitoring with vertical jump tests

Vertical jump’s utility in the context of athlete monitoring is to determine an athlete’s reaction to training and to track performance fatigue.

Most of the time, the physiological response to an external load or training stimulus and the ensuing performance reaction are difficult to describe. Vertical jump is a standardized and repeatable performance test that can serve as a proxy for the physiological response to training, track the performance reaction and quantify fatigue. In tandem with questionnaire data to record the athlete’s perceived exertion and tracking the accumulation of the weekly training load, vertical jump testing completes a snapshot of an athlete’s response to training.

Granted, performance fatigue is just one component of interest in terms of quantifying an athlete’s reaction to training. But routine jump testing can help a practitioner determine the time course of performance fatigue over a period of training, and whether that aligns with expectations. It’s hypothesis testing for a coach’s training plan (assuming – hopefully – that we build our training plans on top of some sort of hypothesis).

I typically anchor a jump test to the day after a rest day. Occasionally I will also include a jump test before and after a workout of interest; or at the end of a microcycle prior to the rest day to assess the acute fatigue response.

Below is an example of tracking neuromuscular readiness and the reaction to training for individual athletes on a professional sports team. Velocity index is the ratio of the vertical takeoff velocity (the determinant of jump height) to the jump contraction time, using the takeoff velocity over jump height due to its better reliability.

It’s helpful to choose a few performance based jump metrics combined with jump strategy measures for athlete monitoring, then track these metrics alongside other anchors like RPE or training load.

Jump strategy

Jump strategy variables are important as an athlete may adapt their jump strategy when fatigued in order to maintain jump performance. Sometimes the change in jump strategy is visible to the naked eye. We can see an athlete who is jumping slowly or changing their countermovement depth. There may also be considerable variation in the force-time curves of the jumping athlete.

We can calculate various measures of jump strategy, but we need to do this over several movement cycles, especially for measures with higher variation. Ideally, we’ll track them over time to characterize the jump strategy and then detect the emergence of a new strategy. You can’t determine a jump strategy from a single jump test.

It’s also important to remember that, while we typically look at the intra-subject variation between two tests to establish a meaningful change for a given metric, there may be important changes that occur in jump strategy or jump performance that are “inside the noise” and become apparent with repeated testing. This is an advantage of vertical jump monitoring: it’s possible to get lots of serial measurements from a sport environment. Professional sports teams often accumulate thousands of measurements within a few years of routine monitoring.

The charts below show four jump metrics: takeoff velocity (CV = 1.9%), the external peak mechanical power (CV = 2.1%), the velocity index (CV = 10%)^[3] and the jump contraction time (CV = 10%). Since the velocity index is a ratio, the graph shows the component parts so we can track how the ratio changes across the training cycle.

Depending on the context there may be other useful measures for athlete monitoring. A few variables to consider include the countermovement depth, the net eccentric deceleration impulse, and lower limb stiffness; as well as measures that reflect more of a force vs. velocity dominant jump strategy. Examples of those could be the force and velocity at peak power, the force at maximum velocity, and the force at 0 velocity at the very bottom of the CMJ.

We may also want to aggregate vertical jump testing for a team to describe the neuromuscular readiness and performance fatigue at the group level. The figure below shows a professional team’s trend through the early season, midseason and into the playoffs. It’s interesting to note that the rise in external mechanical power towards the end of the season corresponded with relatively strong competitive performance for the team at large.

An important caveat is that vertical jump capacity is not important for all sports. However, in the context of the example below, external mechanical power in the vertical jump was highly associated with aspects of sport performance. Nevertheless, vertical jump monitoring is still helpful for identifying neuromuscular readiness and an athlete’s overall reaction to training over the course of a mesocycle or a season.

Vertical jumps for strength testing

The vertical jump and its variations are excellent for testing muscle strength capacity over time. Fast jumping movements like the cyclic hop and DJ can be used to assess fast SSC capacity. The CMJ can test slow SSC capacity. Each test has a bilateral and unilateral variation.

Since these tests are pretty standard, let’s focus on two additional performance tests, the loaded CMJ and the repeated SJ.

Loaded CMJ testing assesses high force capacity and loaded eccentric deceleration capacity, and creates takeoff velocity-load profiles. The usual protocol is three maximal CMJs with no external load, three maximal CMJs with an additional 30% of body mass and three more adding 60-80% of body mass. Familiarization is important, and incorporating variations of the trap bar clean pull and trap bar jump squat into regular training can ensure the athlete is ready for the test. There are many different methods to create loaded vertical jump profiles. One way, as in the graph below, plots the takeoff velocity on the y-axis and the external load on the x-axis. This provides relatively good reliability of takeoff velocity, and it sets the load being manipulated as the variable.

Repeat jump tests are valuable for examining power endurance capacity in sports or settings that require repeat, fast muscle actions. For example, I devised an 80 second repeated SJ test to assess power endurance, performance fatigue and asymmetries with elite alpine ski racers^[5].

The test can quantify the total mechanical power generated over the course of the test; fatigue-inducing factors; and the maximal mechanical power. The figure below shows how the SJ force-time curve might change with fatigue over the course of the test. Notice how the rate of force development and impulse decreases, while the jump contraction time increases with fatigue. You may also notice that the peak vGRF remains unchanged: again, discrete time point measures like peak vGRF do not determine jump performance.

A few additional considerations for incorporating the vertical jump for strength testing:

• Because time constraints are probably less of an issue compared to routine athlete monitoring, consider using several different jump tests to assess the envelope of function. Options include the bilateral and unilateral CMJ, a loaded CMJ test, repeated jump testing and the DJ.
• In addition to bilateral testing, unilateral variations of the vertical jump can assess lower limb strength capacity.
• As with routine athlete monitoring, opt for variables with better reliability. Bear in mind that detecting a change in jump strategy may also be of interest over the course of training.
• Consider using the jump tests in training so athletes become highly familiar with test execution, allowing us to improve test reliability.

Remember that fatigue can compete with neuromuscular adaptations during testing sessions. Coaches might miss the occurrence of a positive neuromuscular adaptation as fatigue is subsiding after a heavy training period. To make the most sense of strength testing, examine neuromuscular adaptations with entry/exit testing in conjunction with routine jump monitoring.

Monitoring return to sport and return to performance with vertical jumps

“The curious coach pays attention.” – Dan Pfaff

RTS and RTP monitoring represent an envelope of function, a non-linear process characterized by progress towards or regression away from the goal.

There’s a difference between the movement asymmetries that we measure in tests, like a bilateral vertical jump, and an asymmetry in an athlete’s strength capacity. Movement asymmetries are inherent in most human movements, and they tend to be variable. A strength capacity asymmetry could certainly exacerbate a movement asymmetry, and we often see this after injury when movement asymmetries in jumping can exceed 30% between the left and right.

Single leg strength and jump capacity asymmetries may also exceed 30% after injury. Both tend to normalize with time, but given the risk for an injury on one limb to impair the strength of the contralateral limb, it is crucial to be explicit about the goal of two relatively strong limbs, not low asymmetry per se. In fact, we can achieve better symmetry by having the non-injured limb detrain. I think we would agree that’s not the goal.

Restoring symmetrical strength and jump capacity are not goals in themselves. Instead, the goal is to retrain an athlete to a sufficient level of muscular strength and power so that they can withstand the rigours of their sport, while having an appropriate buffer (strength reserve) to meet the new demands of unforeseen events^[6].

When incorporating the vertical jump for RTS and RTP testing, practitioners should use expansive neuromuscular testing and a battery of tests to define rehabilitation status ^[7]. Unilateral and bilateral variations of the CMJ, SJ, DJ and the loaded CMJ test are top options. An important avenue of interest is the utility of the cyclic hop test to define recovery after traumatic knee injuries and ankle injuries.

Practitioners should establish a roadmap of functional criteria that define progress over time. This needs to happen before injuries do. It’s helpful to communicate these criteria to the athletes, the performance team and the coaches before an injury occurs.

Examine between-limb differences in the vGRF over the entire waveform and avoid using only discrete time point analysis. Variables like the left vs. right phase-specific impulse and unilateral jump performance measures that quantify the lower limb strength capacity after injury go a long way to maintaining the appropriate focus.

Finally, in all contexts but specifically in RTS and RTP, use the tests and metrics from Figure 1 as conversation starters towards designing more effective programs. They are not go / no-go criteria. Return to sport readiness after injury is far more complex than distilling a decision to a single jump test or jump measures.