Vertical jump testing has become a staple of assess athletes. Decades ago the standard coach had to rely on the jump and reach test in order to testing jumping ability. Thanks to new technology and research, today coaches can now better analyze sport-specific jumping performance and more easily measure variables other than simple jump height. Jump height remains the most popular measure, but that is slowly changing as more technology enters the weight room. The advantages and disadvantages of different methods of vertical jump testing will be discussed below, with particular reference to the procedures required to obtain valid results. This is important because if the method you use is not valid or measuring accurately what it is intended to, your assessment will be of little value.
Methods of vertical jump testing
|Vertec||Contact mat||Force plate||Video recording|
|Variety of uses||★★||★||★★★||★|
Description: The Vertec device measures jump height in inches, while the Yardstick is the metric equivalent. Jump height is recorded as the difference between the standing reach (with the preferred arm outstretched) to the highest point reached during a jump.
Description: A contact mat essentially functions as a timer, and should not be considered as a substitute for a force platform. When a load is applied to the mat, a switch closes, and when the load is removed, the switch then opens. During a vertical jump, the timing starts at the instant the athlete leaves and timing stops at the instant one or both feet hits the mat on landing when the switch closes. Therefore the mat is only able to measure flight time and ground contact time.
Description: A force platform is capable of accurately measuring force applied to the ground over time. The most sophisticated force plate systems can display the force applied in the vertical, forward-backward, and side-to-side directions, and are usually confined to the laboratory. However, if you are only interested in just vertical jumping, some cheaper and more portable force platforms are available that only measure vertical force.
Description: A vertical jump can be recorded with the video function of a smart phone or tablet. The high speed video that is greater than 100 frames per second (fps) means that you can advance the video 1 frame at a time to easily identify the instant of take-off and landing. The difference between these instants in time represents the flight time of the jump. This is easily done with apps such as Hudl Technique or My Jump. The My Jump app then automatically calculates jump height from the flight time.
Using the flight time method to calculate jump height
Both the contact mat and video methods rely on flight time to calculate jump height, according to the following equation:
Height (cm) = (time of flight)2 x 122.625
For example, if the flight time=0.70 second, then the jump height would be calculated at 60 centimeters.
The jump height refers to the height of rise of the center of mass (CM). This equation is derived from projectile motion, assumes that the flight time while the CM is rising is equal to the flight time for the body to fall. For this assumption to be true, the body position at the instant of take-off and landing must be identical, as in this figure:
Therefore, to ensure a valid test, the athlete should be instructed to land with the body fully extended, before the impact is attenuated by dropping into a squat position. The arms should be kept in the same position throughout the jump, either with the hands on the hips, or with a light stick held on the shoulders. An arm swing should not be used, because it is impossible to ensure a consistent body position at take-off and landing.
It can be challenging for some athletes to land in a fully extended position, but this should be practiced and then monitored during testing. If the tester is not confident visualizing the instant of landing in real time, the jump can be video recorded and displayed one frame at a time for certainty. If the My Jump app is used, video recording is required to capture the instant of landing anyway.
Common faults in measuring flight time
The most common fault from my experience, is landing with the ankles in a dorsi-flexed position (heels near the floor), rather than with the toes pointed, like at take-off. Other faults that should be eliminated to ensure a valid jump are: landing with the knees bent (flexed), or leaning over at the waist (hip flexion). All these violations of the correct landing position result in the flight time during the fall being increased, and therefore an increased total flight time will overestimate the jump height.
You can easily demonstrate this yourself by landing in a tucked position, which can add several cm to the calculated jump height. To illustrate this issue, I asked a colleague to perform two CMJ attempts shown below. The first attempt was with strict instructions regarding the correct landing position, and I captured the instants of take-off (left) and landing (right) from a video recording at 120 fps. I then measured the flight time with Kinovea software, and calculated the jump height from the equation mentioned above, producing a result of 26.8 cm. The second attempt was with the instruction to land in a slightly tucked position, as shown below (right). The resulting flight time was again calculated, and the jump height produced was 35.9 cm, which was 9.1 cm higher (34%)!
This difference was mainly due to the prolonged downward flight time. Due to this error, I have heard some people instruct the athlete to remain fully extended during the entire flight, and then disregard a jump if the athlete bends the knees at all. This approach is actually incorrect and unnecessary, because the movements of the body in the air have no influence on the rise and fall of the CM, which behaves like any projectile. It is only necessary to be in a fully extended position at the instant of landing.
We also did a third attempt. As mentioned earlier, an arm swing should not be allowed when using the flight time method to calculate jump height, because it is impossible to ensure the arms are in the same position at take-off and landing. The pictures above indicate this, and the lower position of the arms at landing will cause an overestimation of the true jump height.
It has been claimed that the My Jump app produces valid measurements of jump height and force-velocity characteristics because it produces very similar results when compared to a gold standard force platform system. But when examining the validation studies, I noticed that the force platform procedure measured jump height with the flight time method. Therefore, these studies are essentially comparing the ability of the video recording with the My Jump app to calculate flight time, and to match the flight time from a force platform output. The problem with this is that the force platform measure of flight time is just as prone to error from an incorrect landing as the video recording. Two of the validation studies2 3 controlled the landing position of the jumps appropriately, while the other4, stated that the ankles should be dorsi-flexed!
A major take-home message is that the selected methods for assessing vertical jumping performance all have their advantages and disadvantages, so you should consider these for your needs. Secondly, if you are using any equipment that determines vertical jump height from flight time, you should carefully control the landing position to generate valid results. This is important whether you are assessing a single countermovement jump, a squat jump (starting from a static squat position), a drop jump, or multiple jumps.
- Young, W., Cormack, S., Crichton, M. Which jump variables should be used to assess explosive leg muscle function? International Journal of Sports Physiology and Performance, 6, 51-57: 2011.
- Jiménez-Reyes, P., Samozino, P., Pareja-Blanco, F., Conceição, F., Cuadrado-Peñafiel, V., José González-Badillo, J., Morin, JB. Validity of a simple method for measuring force-velocity-power profile in countermovement jump International Journal of Sports Physiology and Performance, 2016. DOI: 10.1123/ijspp.2015-0484
- Samozino, P., Morin, JB., Hintzy, F., Belli, A. A simple method for measuring force, velocity and power output during squat jump Journal of Biomechanics, 41, 2940–2945: 2008.
- Balsalobre-Fernandez, C., Glaister, M., Lockey, RA. The validity and reliability of an iPhone app for measuring vertical jump performance. Journal of Sports Sciences, 33(15), 1574–1579: 2015.