PURPOSE: Muscle weakness and difficulty descending stairs are common after unilateral total knee arthroplasty (TKA), but the relationship between each is unclear. The purpose of this study was to compare lower extremity muscle strength, lower extremity support moments during step descent, and assess relationships between each.

METHODS: The study included 40 subjects (20 post-TKA, 20 control). Knee extensor, hip abductor, and hip external rotator strength were measured and biomechanical analyses of step descent performed. Patients with TKA were assessed 3 and 6 months post-surgery.

RESULTS: At 3 and 6 months post-TKA, operated limb hip external rotator and knee extensor strength were impaired compared to the non-operated limb (p < 0.01); however, no between-limb differences were observed during step descent. Compared to the control group, hip external rotator and knee extensor strength, total lower extremity support moment, and knee support moment during step descent were impaired post-TKA (p < 0.05). At 6 months post-TKA, knee extensor and hip external rotator strength correlated with total support moment during step descent (r = 0.40, 0.41, p < 0.02). Hip abductor and external rotator strength negatively correlated with knee support moment during step descent (r = - 0.35, - 0.39, p < 0.03).

CONCLUSIONS: Persistent operative limb knee extensor and hip external rotator muscle weakness are noted following unilateral TKA. Despite unilateral weakness, bilateral alterations in step descent strategy occur following TKA. Patients with TKA utilize hip musculature to reduce knee muscle demand during step descent, possibly contributing to limitations in long-term step descent performance.