A comparison of methods for determining the rate of force development during isometric midthigh clean pulls

Twelve female division I collegiate volleyball players were recruited to examine the reliability of several methods for calculating the rate of force development (RFD) during the isometric midthigh clean pull. All subjects were familiarized with the isometric midthigh clean pull and participated in regular strength training. Two isometric midthigh clean pulls were performed with 2 minutes rest between each trail. All measures were performed in a custom isometric testing device that included a step-wise adjustable bar and a force plate for measuring ground reaction forces. The RFD during predetermined time zone bands (0-30, 0-50, 0-90, 0-100, 0-150, 0-200, and 0-250 milliseconds) was then calculated by dividing the force at the end of the band by the band's time interval. The peak RFD was then calculated with the use of 2, 5, 10, 20, 30, and 50 milliseconds sampling windows. The average RFD (avgRFD) was calculated by dividing the peak force (PF) by the time to achieve PF. All data were analyzed with the use of intraclass correlation alpha (ICCα) and the coefficient of variation (CV) and 90% confidence intervals. All predetermined RFD time bands were deemed reliable based on an ICCα >0.95 and a CV <4%. Conversely, the avgRFD failed to meet the reliability standards set for this study. Overall, the method used to assess the RFD during an isometric midthigh clean pull impacts the reliability of the measure and predetermined RFD time bands should be used to quantify the RFD.