Knee extensor torque and BMI differently relate to sit-to-stand strategies in obesity

Background. Obesity alters whole body kinematics during activities of daily living such as sit-to-stand (STS), but the relative contributions of excess body mass and decreased relative strength are unknown. Methods: Three-dimensional motion analysis data was collected on 18 obese subjects performing sit-to-stand (chair height: 52 cm). Isometric knee extensor strength was measured at 90 knee flexion. Forward stepwise linear regression was used to determine the association between the independent variables BMI and the knee extensor torque with the dependent variables: foot position and trunk kinematics. Findings: BMI, but not knee extensor torque, was inversely related to shank angle and positively related to stance width. Relative knee extensor torque, but not BMI, was inversely associated with initial trunk angle, peak trunk flexion angle, and peak trunk extension velocity (r(2) = 0.470-0.495). BMI was positively associated with peak trunk flexion velocity, but no other parameters of trunk kinematics. In the final regression model, BMI was the primary predictor (r(2) = 0.423) and relative knee extensor strength served as a secondary predictor (r(2) = 0.118) of peak trunk flexion velocity. Interpretation: BMI and knee extensor strength differently contribute to sit-to-stand performance strategies in obese subjects. Muscle strength may be an important determinant of whole-body kinematics during activities of daily living such as STS.