The Limiting Joint During a Failed Squat: A Biomechanics Case Series

This investigation examined the characteristics of a failed back squat. Subjects were instructed to perform 3 repetitions of a barbell squat with a 3 repetition maximum load while instrumented for biomechanical analyses and standing atop force platforms. Inverse dynamics calculations were used to determine the net joint moment (NJM) power, work, and energy of the hip, knee, and ankle. Five subjects failed to complete all 3 repetitions, allowing for comparisons between a successful and the failed repetition. Although the NJM power and work were lower at all 3 joints during the failed attempt, the only statistically significant differences were at the hip. These findings suggest that the energy generated by the hip joint NJM limited performance of the task. However, examination of the NJM energy generation over time on an individual basis uncovered some features that were masked by the aggregated group mean data. For some subjects, the knee NJM limited the movement. Additionally, negligible to modest compensations occurred between the hip and knee NJM: a decreased energy generated by one NJM was often accompanied by an increase in energy generated at the other. A limiting joint, or "weak link," may explain the failure to complete a lift. Interventions should address the limiting joint on an individual-specific basis and incorporate assistive exercises that target these deficiencies.