KBox Eccentric Power Influence on Jump Height

Ricky Norton (Sports Performance Expert)

Tyler Standifird PhD (Assistant Professor in Biomechanics)


The purpose of this study was to understand the relationship between peak eccentric power and jump height in collegiate women’s volleyball players. Power is the rate at which work is done in the body, or the speed at which force can be developed. Force and the rate of the application of force can be a very important part of sports performance including jump height. For this study the idea was to compare the peak eccentric power produced by a group of athletes to their jump height. 


Fifteen collegiate level women’s volleyball players participated in the study. Each athlete performed a series of power output tests and also two jumping tasks. All of the tests were done on campus. The two jumping tasks included a standard counter movement jump (CMJ) and an approach jump (athlete takes as many steps as desired). Prior to jumping, subjects went through a general dynamic warm up and 10 air squat jumps. They each then performed a CMJ jump (first) followed by an Approach Jump (second) reaching for pins on the vertec. They were allowed to take as many attempts until they missed the lowest pin. Their total height on both jumps were then subtracted from their standing reach. Following both types of jumps, the athlete then performed 10 repetitions on the Eccentric KBox. The Average Eccentric Power of the 10 reps was then recorded. An analysis was completed on the data to compare the relationship between eccentric power and jump height.


A simple linear regression analysis was completed in order to understand the relationship between eccentric power and jump height. For both CMJ and Approach Jump, there appeared to be little relationship between KBox average peak power and jump height. The R^2 values for both of these jumps were low (0.16 and 0.17 respectively). Though this number does suggest that the KBox eccentric power did explain 16 and 17% of the variability in the jump performance. The data shows that an increase of around 40 watts of eccentric power would result in a one inch increase in jump height. 


The results of this case study suggest that peak eccentric power as measured by a KBox do not appear to be related to jump height in women’s collegiate volleyball players. The findings of this case study were contrary to what we had originally hypothesized. While power plays an important role in many sport specific activities, it may be that for jump height the speed of the force development may not be as important for performance. None of these athletes had ever used a KBox before and as a result there may be some learning aspect of the device. Athletes who are more comfortable with the apparatus might perform differently. These athletes did a set of ten squats and the average peak power of the 10 reps was taken, compared to the max rep for each type of vertical jump for height. Future studies may try and compare the one rep that showed the highest peak power on the KBox compared with maximal jump height or the maximum power produced in one repetition.  Also, eccentric power may be more closely related with how quickly an athlete can land and jump again, a skill necessary in volleyball and basketball, or the speed of changing directions in a cutting action as displayed in many sports. Future studies could look at how eccentric power is related to change of direction, in particular the speed of repeated jumping and landing tasks such as an RSI (Reactive Strength Index). This study was a good first step in starting to understand the relationship between eccentric power and jumping performance. As this understanding is further developed, trainers could know specifically how eccentric fly-wheel training on a KBox might influence performance and health. Future studies need to consider to explore this relationship.

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