Effects of weighing phase duration on vertical force-time analyses and repeatability.

Force plate analyses of various activities sometimes require the average (WPav) and standard deviation (WPsd) of force across the Weighing Phase (i.e., quiet period) to calculate kinetic, temporal and kinematic metrics. Yet, the influence of weighing phase duration on these analyses has been scarcely investigated. This study investigated the effects of weighing phase duration on the agreement between vertical force-time variables and the repeatability of WPav and WPsd. Durations of 0.5, 1.0 and 1.5 s were compared to 2.0 s. Limits of agreement (LOA) for system weight, onset threshold, onset time, net impulse, take-off velocity and take-off displacement were calculated for 137 counter-movement, squat and single leg jumps. Repeatability coefficients for WPav and WPsd estimated the consistency between repeated trials. Shorter weighing phase durations produced small differences in WPav (LOA < ±0.25%), which accumulated during integration, affecting net impulse, take-off velocity (LOA ±2%) and take-off displacement (LOA ±23%). Differences were substantial using 5xWPsd as the onset threshold (LOA approximately ±25% to ±72%), consequently influencing onset time (LOA approximately ±6% to ±18%). WPav repeatability was high but the within-trial differences could augment with integration, requiring weighing phases longer than 2 s. WPsd had poor repeatability and its use requires further investigation.