Force Plates and Jump Testing
The vertical Jump Test is ubiquitous in the sports-performance world. Why? Simply put, the vertical jump is an easily measured movement that translates well. There are obviously other screening and testing procedures to consider, but it does not get much more foundational than the vertical jump test.The obvious question is what do we measure in a vertical jump? The answer depends on the equipment and technology that you are using to measure jumps. If your equipment is limited, then your measurements will be limited – this is a real-world compromise that cannot be overcome. This also means that so long as you know the limitations of your equipment, you should be on the right path.
There is no more accurate or objective way to measure vertical jumping than forceplates. With a force platform or set of force plates we can use relatively simple math to calculate a plethora of variables – with the highest degree of accuracy possible. For one, jump height is most accurate using force plates. Why? It’s all about the physics. Using Newtons laws, we can very accurately estimate how high an athlete’s center-of-mass will travel during a jump.
Why shouldn’t we just use vertical jump height?
Based on the impulse momentum relationship, vertical jump height is a direct outcome of the average net force applied during the concentric phase, and the time taken to apply it. As such, if we only consider vertical jump height, we may miss key information and, consequently, an opportunity to effect a positive change within our athlete’s program. For example, when using the vertical jump as a readiness-to-train measure, athletes may be producing less force due to the accumulation of fatigue but compensate by applying force over a longer period. In this instance, the athlete’s jump height would not change, and you would train as planned, but perhaps to the detriment of the athlete’s progress and safety. Alternatively, this overload may be planned, and this will allow you to determine if the desired effects of your programming are coming to fruition. It is important to note that timing compensations may not just be made within the concentric phase, and consideration must also be given to eccentric (or broken down further into unweighting and braking) compensations, which may be indicative of stretch shortening cycle fatigue. The same information could be used when profiling our athlete. If our athlete applies large amounts of force, over a longer period, they may be strong, but slow. If our athlete applies moderate amounts of force, over short periods of time, they may be weak, but fast. As such, our approach for training these two athletes may be different. However, it is very important to note that this information would be used in conjunction with the overall testing battery – jumping is simply one piece of the puzzle, and not all pegs are square.
A limitation of using just the impulse-momentum relationship; however, is that it does not consider any spatial compensations. For example, if we consider the work energy relationship, an athlete can not only compensate by applying the force over a longer period of time, but by applying the force over a greater range of motion. Although most athletes will self-optimize their countermovement jump depth, both simulations and experiments have demonstrated that by increasing range of motion, we provide more time to apply force, and as such jump higher. Therefore, with instant knowledge of jump height, average net force, concentric time, and displacement, we can not only choose when to terminate sets, but can determine why we are terminating the set. Just because jump height has not decreased, it does not mean that we are not seeing important compensations which may mean the set, session, etc should be terminated. Is the athlete too slow? Is the athlete using too great a range? Is the athlete simply not applying enough force? When this information is provided rep by rep, coaches can more specifically effect a change, and once educated, athletes can be motivated using these scores. Just because the athlete took too long, it doesn’t mean the set has to be over.
As forceplates become more affordable and easier to use in the performance setting, we will surely begin to see many additional developments with regard to what can and what should be measured in athletes.