Effects of foot placement on resultant joint moments in the lower extremity joints during the squat

The purpose of this study was to investigate the effects of foot placement on normalized resultant joint moments of the lower extremity joints in sagittal and frontal planes during the squat. A total of 42 skilled participants (21 male and 21 female) performed squat trials (five trials per condition) with a weighted barbell of 75% of 1 repetition maximum in six different conditions: 3 stance widths (narrow, medium, and wide) × 2 toe angles (toe forward and toe out). Normalized resultant joint moments of the lower extremities were extracted from each trial using three-dimensional motion analysis. A three-way (3 × 2 x 2) mixed designs MANOVA was conducted to compare the moments with stance width' (within), toe angle' (within), and `gender' (between) being factors. Ensemble-average normalized resultant joint moment patterns were also analyzed to identify the changes in the moment patterns. There was a significant interaction between stance width and toe angle in the normalized sagittal-plane resultant joint moments (p = .017). The toe out condition showed significantly larger hip extensor moment (HEX) and knee extensor moment (KEX) in the medium and wide stances and significantly larger ankle plantar-flexor moment (AP) in the narrow stance than the toe forward condition. Significant differences in HEX, KEX, and AP among the stance widths were observed during the toe out condition. Significant main effects of the factors (width: p < .001; angle: p < .001) were observed in the normalized frontal-plane resultant joint moments. In terms of stance width, the largest hip abductor moment (HAB), knee abductor moment (KAB), and ankle everter moment (AE) were produced in the narrow stance, followed by the medium stance, and the wide stance revealed the smallest values. In terms of toe angle, the toe out condition was characterized by significantly larger KAB and AE but smaller HAB than the toe forward condition. The maximum values of HEX, KEX, and AP were observed at the beginning of the upward phase. The maximum values of HAB and KAB occurred approximately in the middle of the downward and upward phases while the smallest HAB and KAB occurred approximately at the bottom of squat. The peak AE was observed around the bottom of squat. Based on the results of this study, an optimal foot placement during the squat would be toes out with wide stance condition.