Increased hip abduction in high body mass index subjects during sit-to-stand.

Obesity is associated with increased risk of total hip arthroplasty (THA) dislocation. Differences in kinematics and kinetics at the hip during activities of daily living such as sit-to-stand (STS) may contribute to this risk. Nine high body mass index (BMI) subjects (mean BMI 31.2) and ten normal BMI control subjects (mean BMI 22.1) were analyzed using force plates and an optoelectronic motion capture camera system during controlled STS movement. Flexion/extension, abduction/adduction, and internal/external rotation angles and moments at the hip and knee were calculated using a musculoskeletal model. No differences were found at the knee. Peak hip abduction angles were on average 50% greater in the high BMI group compared to the normal group (p=0.038). The hip was roughly 50% more abducted throughout the entire STS cycle in the high BMI group. Peak normalized hip abduction moments were approximately twice as large in the high BMI group (p=0.005). Further research is required to determine if this increase in abduction angle and moment observed during STS is a contributor to risk for complications following THA in obese subjects.