Squat jump movement onset thresholds influence on kinetics and kinematics

Background of Study: Differing movement onset thresholds have been used when analyzing the squat jump movement from force-time data obtained from a force platform. This makes comparisons difficult between investigations as this will impact the amount of the force-time curve that is analyzed. Objective: Thus, study examined the effect onset threshold had on kinetic and kinematic variables used in the assessment of the squat jump. Methods: Using a within-subject study design, fifteen recreational trained males performed three trials of squat jumps on a force platform. Each trial was analyzed using one of five different onset thresholds (2.5% SW, 5% SW, 10% SW, 20N, 5SD). Force, velocity, and power, as well as time to peak force, velocity, power and jump height were calculated using the vertical force data obtain from the force plate. Reliability was assessed using intraclass correlation coefficients and coefficients of variation. A one-way ANOVA was used to examine the impact of onset thresholds on all variables of interest. Results: The use of 10% SW and 5SD met minimum reliability criteria for all variables. Temporal related variables were impact to the greatest extent by differing thresholds with large (d > 1.20) significant differences. 10% SW showed the highest mean values of force, velocity, and power. Conclusions: The use of 5SD of the weighting phase is recommended as this showed high level of both absolute and relative reliability in addition to preserving a large portion of the force – time curve to be used in the analysis.