Validity and Reliability of Two Methods of Calculating Leg Power : Force plate only vs combined force plate and motion analysis

Abstract: Aim The aim of this study was to compare the reliability and describe the concurrent validity of two methods of measuring leg power; the use of a force plate only and the combination of force plate and motion analysis data. Methods Eight female second division volleyball players (Mean±SD: Age: 26.5±3.93 years, height: 1.70±0.053m, weight: 65.9±11.1kg) participated in two testing sessions, separated by one to six weeks. In each testing session participants performed three squat jumps (SJ) and three countermovement jumps (CMJ), with leg power for all jumps calculated both from force plate data only, and using force plate data combined with displacement data from motion analysis. Statistics calculated were t-test, intra-class correlation (ICC), and Bland-Atman plots. Results While the ICC of the calculated peak power using the two methods was moderate to high (ranging 0.715-0.847) for all jumps, the peak power was always significantly lower using the combined method indicating poor concurrent validity. Leg power decreased from test to retest in both CMJ and SJ, using both methods of calculation, although this decrease was only significant in the SJ using the combined method. The force plate only method showed no significant difference between test and retest, with a small bias (indicating a small systematic error) and a strong test retest correlation in both SJ and CMJ. However, both of these jumps also had quite a large range in the limits of agreements in the Bland-Altman plots. Peak power calculated using the combined method of force plate and motion analysis data showed a greater difference between test and retest values, significant in SJ and tending towards significance in CMJ. This indicates a greater systematic error than in the force plate only method. However, it still had a strong correlation and the range of the limits of agreement was slightly smaller than it was when using the force plate only, indicating a smaller random error. Conclusions The systematic differences between test and retest for the two tests indicates that the true peak power achieved by the subjects of the subjects in this study was lower in the retest. The ICC values still indicate results from this study suggest that reliability is acceptable for both methods. Limits of agreement and CI of the mean differences are presented to guide interpretation of individual and group data respectively. Either method could be used to test leg power, but the results from different methods should not be used interchangeably.