The effect of repetition on force platform metrics during the bilateral bodyweight squat

Abstract

INTRODUCTION: Functional Movement Assessments (FMAs) have been gaining popularity for screening athletes to determine weaknesses that could increase risk for injury. A common movement among many FMAs is the bilateral bodyweight (BWT) squat. Currently, FMAs typically require only 3-5 repetitions of the squat when unfatigued. However, it is known that fatigue affects performance and increases injury risk. It is possible that performing more repetitions might simulate the effects of fatigue, increasing the sensitivity of the FMA. PURPOSE: The goal of this investigation was to analyze the vertical ground reaction forces (GRFvs) and center of pressure (CoP) stability metrics during the down phase, up phase and whole movement across the course of a 20 repetition set of the BWT squat in a recreationally competitive group of young adult women. It was hypothesized that on average, due to changes in some individuals, there would be an increase in asymmetries and normalized values from the early repetition ranges to the later repetition ranges of the bilateral BWT squat in the down phase, up phase, and in the whole movement, that there would be evidence of fatigue in the later repetitions, and that the measures would be repeatable from day-to-day. METHODS: Fourteen recreationally active women (mean ± SD age: 20.5 ± 2.1 yrs; height 167.2 ± 7.3 cm; mass 66.9 ± 10.6 kg) with competitive sport backgrounds performed 20 bilateral BWT squats to a thighs parallel to the floor position while force platform data were recorded under each foot. Asymmetries and normalized metrics were compared across the early repetitions (2-7), middle repetitions (8-13), late repetitions (14-19), and entire set (2-19). Six subjects were reassessed for repeatability. RESULTS: Although not all measures analyzed produced group level changes from the early repetition ranges to the later repetition ranges, there was evidence of some people expressing different force platform metrics in the middle and later repetitions compared to the early repetitions in both GRFv (absolute average GRFv asymmetries during the up phase (p=0.008) and whole movement (p=0.025), relative minimum GRFv asymmetries during the down phase (p=0.018), normalized net minimum GRFv during the up phase (p = 0.005)) and CoP (relative AP sway asymmetry during the down phase (p=0.006) and whole movement (p=0.006), relative AP path length asymmetry during the down phase (p=0.002) and whole movement (p=0.002), normalized net AP sway (p=0.031)). High repeatability (Cronbach's alpha > 0.8) was found in the majority (107 out of 168) of metrics from day-to-day. CONCLUSION: Evidence of change, but limited evidence of fatigue, was found from the early repetition ranges to the later repetition ranges of the BWT squat. Furthermore, the high Cronbach's alpha values show that many of the metrics are repeatable from day to day. Differences also existed in response to repetitions between the up and down phases. Therefore, it is important to further investigate beyond the typical 3-5 repetitions as well as within each phase of the movement in order to determine if greater sensitivity for risk of injury while performing the BWT squat can be obtained.