Sports Science Alert (SSA) #025
Vertical Jump Testing & Physics
The vertical jump is one of the most widely used fitness tests, and rightly so as it is an excellent test of an athlete's leg power. There are quite a few different devices for measuring vertical jump, from the basic technique of using a wall to jump up against to expensive force plates. I prefer the direct measurement of jump height minus standing reach height, but many people use a technique that requires a more complex calculation to determine jump height, usually based on hang time.
Basic physics is used to calculate jump height based on time in the air. Timing mats and force plates can be used to measure 'hang time', but you can also use video. My colleague at www.whatsmyvertical.com has created a nifty tool to assess the height a video-recorded jump.
It is possible to measure vertical jump height using video because jump height is a function of the time between take-off and landing (hang time). Using video of known frame rate, one can determine the hang time by subtracting the timestamp of the take-off from the timestamp of the landing. Playing around with basic physics equations you end up with this equation that can be used to calculate the jump height:
jump height = 4.9 * (1/2 hang time)^2
Each method of measuring vertical jump has its limitations, and it all comes down to what equipment you have available. Even the basic jumping against a wall method can be cheated by athletes who don't extend fully their standing reach height. With the methods using hang time, the scores can be manipulated by not landing back on flat feet or with the legs extended.
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