What is the fine-grained topic of the following text: BACKGROUND: Differences in the performance of gait and gait-related activities of daily living are known to persist after total hip arthroplasty compared to healthy controls, but the specific underlying deficits (spatiotemporal, kinematics and kinetics) are not completely understood. Only return the topic.
Underlying deficits in gait performance after total hip arthroplasty.