When I first embarked on my sports science journey as an undergrad back in 2007, jump mats were a regular part of athlete assessment. We used them to assess athletes’ lower limb ballistic and plyometric abilities, and repetition maximum tests for assessing strength. These tests had just a few setup requirements. We’d switch the jump mats on and they were good to go. They were not necessarily accurate, but we didn’t learn that until later. Then we’d set some range of motion and volume criteria for the repetition maximum test and, about 30-40 minutes later, we’d have a number for maximal strength.
We mainly used laboratory-grade force plates to answer specific research questions, while the jump mats were there to assess athletes’ physical qualities throughout their season.
Fifteen years later, force plates are no longer limited to universities or private specialist labs. Commercial force plate systems are now mostly robust, fully portable and include software that produces instant results – all at a reasonably affordable price within stretching distance of many pro teams’ limited sports tech budget.
Consequently, today’s sport and exercise medicine professionals are much more likely to use force plate systems regularly than they would have been as recently as three years ago.
This article will not address how force plate variables are calculated (which will probably be a relief to most). Nor will it review the published research on force plate use with athletes, largely because it’s difficult to collate much of the research due to inconsistent methods. I’ll assume that most people reading this article are practitioners who either already use a commercial force plate system with their athletes or are interested in acquiring one in the near future. I’ll attempt to summarise what my experience tells me many sports practitioners – including myself – wish they knew before embarking on their own force plate journeys.
It’s no secret that force plates are a little needier and more customisable than most equipment you’re likely to use to assess athletes. That’s probably led to many missed opportunities to progress applied force plate research, and may explain the apparent demand for more education on force plate use in sport . Like most things in sport, there a just a few key basic principles to abide by each time you conduct a force plate test, which can soon develop into an efficient setup routine.
Force plates demand you start on the right foot
Spoiler alert: there’s no getting around the fact that force plates need to be set up in a very precise way to be able to fully utilise their capabilities.
Designate your force plate gurus
In my experience, many practitioners don’t pay enough attention to the way they set up their force plates. I don’t think this is always intentional, and it probably reflects the old school ease some senior practitioners remember from their jump mats. I’ve met some fantastic coaches who probably shouldn’t go anywhere near the force plates because, well, they’re just not meticulous enough.
There is some merit in choosing the right people within the athlete performance team to collect the force plate data. Some of us are just a little more interested in them and like to nerd out by exploring how they work and what the data can tell us!
The best examples of using force plates in sport that I’ve witnessed have involved one or two members of the performance team taking ownership over the force plate testing. They either conduct all the testing themselves or oversee the training of key staff to ensure that it’s consistently done well. At the very least, they never compromise on the key points below.
Zero out the noise
The recommended sample frequency for most force plate tests is 1000 Hz, so the force plate measures force (or forces, if a multi-axis system) applied to it every 1 millisecond.
They also measure some amount of force even when nothing is on top of them. This is the signal noise, and it’s present in all force plate systems every time we test. We want to keep signal noise as low as possible to increase accuracy and sensitivity when athletes perform tasks on them.
Zeroing the force plate before each time an athlete steps onto it or it’s set up in a different location will help reduce signal noise, as will ensuring it’s always on a solid and flat surface during use. This will ensure we accurately hone in on the forces the athletes produce.
Tweet ThisPractitioners who adopt a team approach to force plate testing should create their own standard operating procedures for cueing and positioning to encourage consistency among key stakeholders
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Accurate body weight is an essential input
We also need an accurate measurement of the athlete’s weight each time we test them. Body weight feeds into the software calculations to take us from the forces that the plates directly measure to understanding the effect of those forces on the athlete’s motion (namely, their centre of mass velocity and position). Without a weight measurement, we can’t get much more information than what a simple jump mat can provide.
We weigh athletes by having them stand as still as possible for at least one second prior to performing the selected task. If they start somewhere other than on the force plate, like on a drop jump test, we average the force over the final second after they have landed the jump. Even though an athlete’s weight shouldn’t vary between tests performed within a given session, we still want to record the athlete’s weight during every trial they perform. We do this because the signal noise will vary slightly during each test, and most software can (and should) account for this in its analyses.
Interrogate your force plate system before trusting it
Commercial force plate providers might wince a bit at that title, but please just hear me out!
Practitioners have either scrambled a budget together to buy some force plates or they inherited some when they started their job. In either case, they should spend some time vetting the system before jumping into mass data collection with their athletes. No two force plate systems are identical, not even if they’re from the same manufacturer.
A spanking new set of force plates will have just undergone a rigorous manufacturing and calibration process, and are probably going to provide accurate force readings. Even so, still verify they’ve been validated against an industry gold standard. None of us expect our laptops or mobile phones to last or work optimally forever, so why should we expect force plate systems to? After all, we’re asking multiple athletes to push on them as hard and as fast as possible on a regular basis, sometimes after falling from a reasonable height (e.g., drop jumps) and sometimes for multiple repetitions in quick succession (e.g., multiple rebound jumps).
An eventual drift in accuracy is inevitable. But, like with most sports tech, I’ve never heard practitioners who regularly use force plates question it.
Every now and again, force plate users should incrementally place some calibrated weight plates (e.g., International Weightlifting Federation accredited bumper plates) on top of their force plates to check their accuracy. If there’s a discrepancy between the force plate readings and the weight plates (first multiply the mass of the bumper plates in kg by 9.812 to convert it to Newtons), contact the force plate provider and ask them to help with recalibrating them; or follow their user guides if they allow the end user to do it themselves. This is especially important if you are testing athletes across multiple systems, again, even if they’re from the same manufacturer. I’ve seen athletes do a countermovement jump test and rebound jump test just 15 minutes apart but on different sets of the same force plate system, and their body weight estimate from each test was different by 30 N (3.1 kg)! Such a weight discrepancy will vastly affect any derived kinematic and ratio-scaled force data.
Unfortunately, different force plate system brands can’t be used interchangeably without adding more measurement error to the mix. In addition to the reasons above, the software calculations will be different, not to mention the naming convention for the variables. As a result, it’s very difficult to compare athletes’ data if they’ve been tested with different systems.
Many sports clubs use different force plate systems to test their academy and senior athletes. If they intend to longitudinally track athletes’ force plate data as they progress through the ranks, the differing systems will introduce a discontinuity. Furthermore, some national-level sports have athletes who train in regional performance centres around the country, and the federation may wish to pool each region’s force plate testing data together to review the athletes’ performances. But to have a sufficient amount of confidence in the data, each region would need to use the same force plates in the same way, which seldom happens.
So, what if you want to switch force plate systems? Does it mean you’re stuck with whichever system you bought or inherited? Not necessarily. Establishing levels of agreement between your new and old force plate systems, or the different systems used within or between clubs or performance centres, at least provides some measure of confidence and aids data interpretation.
Don’t just jump! The importance of verbal cues
The final parts of the force plate data collection process are the practitioners giving the necessary verbal cues to their athletes and standardizing the athletes’ body positioning during the tests. Figure 1 summarizes the main verbal cues and body positions for the most common force plate tests.
Practitioners who adopt a team approach to force plate testing should create their own standard operating procedures for cueing and positioning to encourage consistency among key stakeholders. Many force-time variables are sensitive to even slight differences in body position, and verbal cues can significantly alter key metrics of interest. For example, a push vs. a pull command for an isometric mid-thigh pull test can change maximum force output by as much as 20% (based on my observations). Similarly, aiming for maximal jump height versus a short movement time will induce meaningful differences in the athletes’ jump strategy across a variety of jump tests.
Tweet ThisCues during testing matter. A push vs. a pull command for an isometric mid-thigh pull test can change maximum force output by as much as 20% (based on my observations)
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The immediate feedback athletes receive from the force plate software can lead to athletes learning how to modulate their test execution between successive trials to either beat their peers’ scores or their own previous result. While immediate test feedback for individuals or entire squads through “live” leader boards, for example, can be great for promoting athlete buy-in, self-competitiveness and squad camaraderie, it may lead to discrepancies between how practitioners want the athletes to perform the tests and what the athletes do to try to beat their mates, depending on the metrics that are displayed.
Force plate metric selection: Measuring and motivating what matters
Common force plate software produces a lot of variables – too many variables, in fact, which can make it difficult for practitioners who are new to force plate testing to decide which ones to monitor and report to their athletes. No one test or force plate variable can provide a holistic insight into athletes’ force producing capabilities. But any practitioner should understand the basics and try to keep the variable selection simple.
Despite the range of applications of force plate testing in sport (Figure 2), the variables that I report for each are pretty much the same. By better understanding the interaction of force and motion and the various ways in which we can describe the latter, one realises that there are a lot of duplicative variables in the output, or variables that illustrate the same pattern of change. This enables us to infer the pattern of changes in some variables based on how others changed. And so, thank goodness, there is no need to monitor 180 variables per test per athlete across the season.
Practitioners should connect how they cue the athlete to perform the force plate tests to the primary variable(s) that best support those cues. For example, if I cue a vertical jump test to focus on maximal height, then jump height will be the key outcome variable. But I will switch this to reactive strength index (for rebound jumps) or reactive strength index modified (if a countermovement jump) if I cue the athlete to minimise movement time whilst maximising jump height. Those variables best reflect the trade-off between jumping high while minimising time. We can’t expect the athletes to jump their highest if we constrain their movement time, as that limits their opportunity to generate a high vertical velocity at take-off, which dictates jump height.
Similarly, if we have a live leader board during the testing, we should display the variables that we are cueing for and measuring. That encourages athletes to try to beat their teammates or their own previous score by adopting a movement strategy that reinforces the verbal cues they are hearing.
For about five years my colleagues and I would report jump height as the sole outcome measure from countermovement jump testing with rugby league players, football players, netball players or whichever other athletes we regularly tested at the time. We never considered the large body mass variance across a rugby league squad that isn’t typical of non-collision sports. But, ironically, we’d always scale forces in tests such as the isometric mid-thigh pull or one rep max tests to player’s body mass because, otherwise, we’d bias the results towards heavier players.
Well, jump height is biased towards lighter players (see Figures 3 and 4). Yet, for some reason, we (and I imagine we weren’t the only ones) never did anything about it.
Consequently, we’d be ranking countermovement jump heights for rugby league players whose body masses typically ranged anywhere from about 80-120 kg. I bet you could guess that the 120 kg prop never came top of the jump height leader board! Clearly, it wasn’t sensible to compare just the jump height values across such a heterogeneous group. Despite being slow on the uptake, we later began to provide some alternatives.
Tweet ThisJump height is biased towards lighter players. Yet, for some reason, we (and I imagine we weren’t the only ones) never did anything about it.
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Initially, we reported what I refer to as propulsion net impulse, also known as concentric net impulse. Propulsion net impulse describes how hard and for how long the athlete pushed their body mass upwards before leaving the ground during a vertical jump. Propulsion net impulse may be divided by body mass to give take-off velocity which, as mentioned above, dictates jump height (see Figures 3 and 4). In other words, if an 80 kg and 120 kg athlete both apply the same propulsion net impulse, the 80 kg will attain a higher take-off velocity and so jump higher.
A collision sport athlete’s body mass can be an asset for certain aspects of their game. Their jump momentum, which is equal to the propulsion net impulse (see Figures 3 and 4), can help explain the interaction between the player’s speed and mass characteristics. That lets practitioners interpret the data based on each athlete’s different positional or match demands.
Jump momentum may also be a valuable metric for monitoring youth athletes over time. For example, if an athlete weighed 45 kg and jumped 35 cm, and is retested when they weigh 50 kg and again jumped 35 cm, they have shown a positive adaptation: they can jump the same height despite being heavier. If jump height alone was the only thing we looked at, then a coach might tell this athlete that they showed no improvement in “jump performance,” which would not be an accurate statement in the broader athletic context.
Variables that link cause and effect are known as strategy variables. Isaac Newton’s 335-year-old laws provide the foundation for commercial force plate providers’ software calculations. The end user just needs to identify a select number of variables that link strategy (mostly within different phases) and the outcome most pertinent to vertical jump tests.
Practitioners should stick to reporting maximum force output alone for multi- or single-joint isometric tests (whether unilateral or bilateral), at least until athletes have become well acquainted with the protocols. Then they can explore time-specific force and impulse values, or navigate the cumbersome world of rate of force development.
Figures 3 and 4 shows how jump strategy variables influence other jump strategy variables, such as those during the braking (rebound jumps) or countermovement phase(s), their effect on the propulsion phase and subsequent outcome variables.
Even if we use a dual force plate system for bilateral jumps, we only tend to explore independent left and right leg force contributions if something unexpected has happened, such as injury or a large drop in performance. There is little merit in assessing bilateral force asymmetries in uninjured healthy athletes from a performance standpoint. The bilateral force data will likely remain on whichever cloud system you are using, so it can always be looked at if and when you need it.
The decision process in Figures 3 and 4 applies to unliteral vertical jump tests, too, as I haven’t seen any benefit from reporting alternate variable choices.
Tweet This“There is little merit in assessing bilateral force asymmetries in uninjured healthy athletes from a performance standpoint”
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In all cases of rebound jump or drop landing tests, practitioners should report the athlete’s fall height either from the box (in the case of the drop jump) or previous jump(s). This is rarely done, but it will considerably vary both within and between athletes, and greatly influences the intensity of the ground contact phase of the rebound jumps. Some examples of how data from individual and combined force plate tests (such as when calculating countermovement jump peak force divided by isometric mid-thigh pull peak force to give dynamic strength index) may be visually reported, interpreted and fed back to athletes and relevant performance staff are in Figure 5.
Be meticulous, consistent and patient or risk chasing phantom data
The good thing about being an outsider who comes into a professional club is observing and giving feedback on things that those working at the club may not see or feel they can comment on. I get to come in as an external advisor and can objectively suggest subtle changes to improve force plate use among departments and practitioners… and then disappear! It’s impartial, non-accusatory and tends to foster a (hopefully) lasting cohesion between practitioners without treading on anybody’s toes.
The force plate industry is booming, and practitioners are becoming very creative with how they want to use them to support their decision making. While this is exciting for somebody like me, it’s important that we all remember to do the basics well (no matter how boring this can be), invest time into better understanding the data; and play the long game to build an athlete-centred force database founded upon sound biomechanical principles blended with the art of practical application.
