The Effect of Thigh Muscle Forces on Knee Contact Force in Female Patients with Severe Knee Osteoarthritis

Thigh muscles greatly influence knee joint loading, and abnormal loading significantly contributes to the progression of knee osteoarthritis (KOA). Muscle weakness in KOA patients is common, but the specific contribution of each thigh muscle to joint loading is unclear. The gait data from 10 severe female KOA patients and 10 controls were collected, and the maximum isometric forces of the biceps femoris long head (BFL), semitendinosus (ST), rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) were calibrated via ultrasound. Four musculoskeletal (MSK) models were developed based on EMG-assisted optimization, static optimization, and ultrasound data. The ultrasound-calibrated EMG-assisted MSK model achieved higher accuracy (R² > 0.97, RMSE < 0.045 Nm/kg). Patients exhibited increased VL and VM forces (<i>p</i> < 0.004) and decreased RF force (<i>p</i> < 0.006), along with elevated medial and total joint contact forces (<i>p</i> < 0.001) and reduced lateral forces (<i>p</i> < 0.001) compared to controls. The affected side relied on VL and BFL the most (<i>p</i> < 0.042), while RF was key for the unaffected side (<i>p</i> < 0.003). Ultrasound calibration and EMG-assisted optimization significantly enhanced MSK model accuracy. Patients exerted greater quadriceps and hamstring forces bilaterally, shifting knee loading medially, and depended more on the lateral thigh muscles on the affected side. Hamstrings contributed more to joint contact forces, while quadriceps' contributions decreased.