Objective: To determine what combination of relative trunk and tibia inclination and knee flexion biases the hip and knee extensors during the squat. Design: Repeated measures. Setting: University research laboratory. Participants: 16 healthy individuals. Main Outcome Measures: The average hip/knee extensor moment ratio (HKR) was calculated during squat descent, and trunk-tibia inclination difference and knee flexion angle at peak knee flexion. A regression tree for repeated measures determined what combination of trunk-tibia inclination difference and knee flexion angle predicted HKR. Results: HKR was predicted by trunk-tibia inclination difference (primary predictor) and knee flexion (secondary predictor). Four squatting strategies emerged: knee extensor bias (average HKR = 0.3), neutral bias (average HKR = 0.9), hip extensor bias (average HKR = 1.5), and high hip extensor bias (average HKR = 2.2). When trunk inclination exceeded tibia inclination ( >= 2.2 degrees), hip-biased squatting was evident. Squat depth affected only hipbiased squatting: high hip extensor bias at moderate depths (<96 degrees knee flexion), and a less pronounced hip bias at greater depths ( >= 96 degrees knee flexion). Conclusions: Our results provide a framework to bias the squat exercise based on intent. To maximize or minimize hip relative to knee extensor demands, clinicians should be cognizant of the trunk inclination relative to tibia inclination and the influence of knee flexion.
