A comparison of methods for analyzing drop jump performance

PURPOSE: Drop jumping is a popular form of plyometric training. Different techniques are applied to determine parameter values quantifying drop jumps, such as the jump height or the durations of the phases of downward and upward movements of the center of mass (CM) during foot contact with the ground after dropping. The flight-time method estimates the jump height from the time between the instant of leaving the ground and the instant of landing. In video-based methods, markers are placed on the skin of the subject to define the positions of the body segments. The time-dependent positions of the CM and parameter values are then calculated utilizing models of the human body. If the vertical velocity of the CM can be estimated at one instant, the parameter values can be calculated from the vertical ground reaction forces. METHODS: The purpose of this study was to find out which technique yields the lowest errors compared with the results obtained by the double force plate technique. In this investigation, two force plates were used, one located under the drop platform. Twenty-five drop jumps were analyzed with eight different methods. There were large differences between the reference method and other methods. Using the height of the drop platform (0.39 m) to estimate the velocity at the end of the free fall, in conjunction with data from one force plate, resulted in a mean difference of 4.2% (SD: 9.6%) in the calculated jump height. Using video information to estimate the time that the velocity of the CM fell to zero after the drop phase, in conjunction with data from one force plate, resulted in differences in the jump height of up to 17%. RESULTS: Differences between the reference method and video based methods were comparatively small (mean value of differences in jump height: -0.007 m, SD: 0.013 m for the best of these methods) but not negligible. CONCLUSIONS: Nevertheless, video based methods turned out to be the most promising alternative to the reference method to determine accurate variables concerning drop jump performance.