Calculating SJ Performance
In most circumstances, SJ performance is reported as either jump height (cm or inches), or relative peak power output (W·kg-1). Jump height is an estimate of the height change in the athlete’s centre of mass. Jump height is best measured using the impulse-momentum data from a force platform [15, 17].
Other test variables such as those listed below may also be measured, but this requires special equipment such as a force platform. Therefore, these are not often measured in most environments. Though measuring those additional variables (e.g. impulse) provides a better picture of the athlete’s physical profile.
- Peak force (N)
- Relative peak force (N·kg-1)
- Peak power (W)
- Peak velocity (M·s-1)
- Rate of force development (N·s-1)
- Impulse (N·s)
In terms of measuring vertical jump performances, flight time is considered to be the most valid and reliable method for calculating jump height [17, 18]. Flight time is simply the total duration the athlete spends in the air with no ground contact. Flight time does not start until the athlete loses contact with the floor, and ends the moment they reconnect with it.
The method for measuring jump height using various equipment is as follows:
Contact Mat – If a contact mat is being used, then fight time is typically the outcome measure. However, some contact mat systems may calculate jump height for you. If not, then the test administrator can calculate jump height from flight time data using either of the calculations below.
Jump Height = 9.81 * (flight time)2 / 8 [References: 16, 19]
Jump Height = time2 * 122625 [Reference: 17]
Force Platform – Those using a force platform are advised to calculate jump height using the following formula :
Jump Height = (initial velocity)² / (2 * acceleration due to gravity)
High-Speed Camera – When using a high-speed video camera and appropriate software, flight time is typically calculated via slow-motion analysis. From this, because the flight time has been obtained, jump height can then be calculated using the formulas above.
Accelerometer (linear position transducer) – Similarly with the contact mats, accelerometers typically self-calculate jump height, peak power and peak velocity; meaning no additional work is needed.
Infrared platform (e.g. OptoJump) – This system calculates jump height by measuring flight time and then performing the abovementioned jump height formulas . Therefore, the test administrator is not required to perform any calculations.
IMPORTANT: One thing to remember which is essential when testing and comparing past and future results, is that different testing methods (e.g. force platforms or linear position transducers) produce very different scores . This is partly because the force platform calculates power using a force-time method, whereas the linear position transducer calculates it using a barbell displacement-time method.