Effects of fatigue on bilateral ground reaction force asymmetries during the squat exercise

Hodges, Stephanie, author
Reiser, Raoul Frederick, II, advisor
Puttlitz, Christian Matthew, committee member
Browning, Raymond, committee member
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Physical performance and risk for injury have been related to functional asymmetries of the lower extremity. The squat exercise is an important, functional movement serving as the major multi-joint, closed-chain lower extremity exercise in many strength and conditioning programs, as well as a large component in rehabilitation. Examinations of asymmetries during the squat exercise have revealed bilateral differences in the net joint torques, indicating unequal actions between the lower extremities. To date, no study has examined how fatigue affects these asymmetries. PURPOSE: The goal of this investigation was to examine functional bilateral asymmetries of the lower extremities during the squat exercise, exploring how they may change during the course of an individual set, as well as across multiple sets with fatigue. Based on a review of literature related to the effects of fatigue on functional asymmetries, it was hypothesized that asymmetries would increase with fatigue. METHODS: Seventeen recreationally trained men (n=9) and women (n=8) (mean ± SD age: 22.3 ± 2.5 yrs; height 170 ± 9.3 cm; mass 73.4 ± 13.8 kg) performed five sets of eight repetitions with 90% of their predetermined eight-repetition maximum (8RM 113 ± 35 % of their body mass). Anthropometric measurements and vertical ground reaction force (GRFv) data were collected. GRFv asymmetries were calculated by subtracting the % load on the right foot from that of the left foot. Initially highly symmetric subjects (± 1.7%) were removed from the analyses. The first two (R1&2) and last two (R7&8) repetitions of each set were examined. Eight subjects were reassessed for repeatability measures. RESULTS: R7&8 took longer to complete than R1&2 within a single set (p<0.001), however this finding was not observed across sets (p=0.867). The average asymmetry level across all sets for the absolute average was 4.3 ± 2.5% and 3.6 ± 2.3% for R1&2 and R7&8, respectively. There was no difference in whole group analysis (n=17) for any of the GRFv asymmetry analyses. Initially symmetric subjects were removed for the following analyses. For absolute peak instantaneous forces, the average asymmetry level across all sets was 3.8 ± 2.3% and 3.2 ± 2.3% for R1&2 and R7&8, respectively. Within a set, based on absolute asymmetry, levels of asymmetry dropped from beginning to end of a set (p=0.044). Exploring general shifts towards the left or right leg revealed a significant difference in peak instantaneous asymmetry with subjects becoming more symmetric with fatigue during the course of a single set (p=0.042). Of those who placed more weight on their right side at the beginning of a set, based on average asymmetry, a significant movement towards symmetry was seen for repetitions within a single set (p=0.036). Anthropometrics (segment lengths and circumferences) were generally not highly correlated to the asymmetries. Anthropometric measures were highly repeatable (Cronbach's α≥0.905, p≤0.003), as were time and asymmetry assessments of the squat GRFv asymmetries from visit to visit (Cronbach's α≥0.733, p≤0.056). CONCLUSION: These results suggest that functional asymmetries though low were present, and remained consistent, in this healthy population during a squat exercise. Contrary to the hypothesis, asymmetries did not increase with fatigue, however time to complete each repetition within a set increased with fatigue. Healthy subjects appear to load limbs similarly, subjecting them to a relatively similar training stimulus.
Department Head: Richard Gay Israel.
2010 Summer.
Includes bibliographical references (pages 55-60).
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