Three methods of calculating force-time variables in the rebound jump squat.

The force-time qualities of the lower limb of athletes have been assessed using a variety of exercises and methodologies. The purpose of this study was to investigate the differences among 3 methods previously used to calculate various force-time measures during a rebound jump squat. Twenty-five professional rugby players performed 3 jump squats, each of which was analyzed using 3 different methods of calculation for a number of force-time variables. Method 1 analyzed the force-time curve from minimum force to maximum force; method 2 analyzed the concentric portion of the force-time curve only; and method 3 analyzed both the eccentric and concentric components of the force-time curve. Significant differences were found (p < 0.001) among all 3 methods of analysis (percent difference 1.1-364.3%) for all the force-time variables calculated. A number of variables had very high (r = 0.76-0.86) or practically perfect (r = 0.93-1.00) correlation coefficients among analysis methods, however, showing similar rank order of the population regardless of the analysis methods used. The results suggested that force-time variables that assess rate of force development relative to peak force produce significantly different values, but these values are highly correlated whether the concentric phase is included in the analysis or the eccentric and concentric phases are included in the analysis. When time-dependent variables are investigated, however, the starting point of calculation results in the measurement of functionally independent physical qualities.