Kinematics Limit the Potential to Capitalize on Available Muscle Strength During Sit-to-Stand for Patients Before and After Total Knee Arthroplasty.

Sit-to-stand (STS) is a fundamental activity of daily living that is essential for independent living but remains difficult for many older adults, including those with knee osteoarthritis (KOA), even after a total knee arthroplasty (TKA). Investigating age- and disease-related changes in STS kinematics and muscle function is key for understanding whether there are kinematic changes that limit how patients' muscles are used to perform STS. This study compared STS kinematics, muscle function, and lower extremity muscle strength between individuals with KOA who received a TKA and similarly aged control individuals. Dynamic musculoskeletal simulations of STS were performed for seven age-matched healthy control adults (HC) and seven adults before and six months after TKA. Computed muscle control and induced potential analysis were used to determine individual muscle forces and potentials to contribute to center of mass (COM) accelerations, respectively. Individuals before and after TKA demonstrated deficits in lower extremity isometric muscle strength but generated larger iliacus and soleus muscle forces during STS, compared to HC. Moreover, despite the persistent strength deficits observed both before and after TKA, individuals' greater dorsiflexion and knee flexion, compared to HC, reduced the mechanical advantage of the gluteus maximus, vasti, and plantarflexors to accelerate the COM upward and/or forward during STS. This reduced mechanical advantage to accelerate the COM, when combined with a greater mechanical advantage from the quadriceps to horizontally decelerate the COM before and after TKA, may contribute to the persistent STS performance deficits often observed in individuals with KOA and after TKA.