Swimming Is a Safe Exercise for Patients with Total Hip or Knee Arthroplasty - In Vivo Measured Hip and Knee Joint Loads During Swimming

Background: Joint loading is essential for implant stability, joint mobility, and long-term joint function following total hip arthroplasty (THA) and total knee arthroplasty (TKA). Swimming, as an aquatic therapy, is commonly suggested as a post-surgical rehabilitation exercise for both THA and TKA patients. However, in vivo hip and knee joint loads during this activity remain unknown. Material and methods: Six THA and five TKA subjects with instrumented joint implants were included for the measurement of in vivo hip and knee joint loads. All loading parameters were measured during breaststroke swimming (BS) with both arms and legs actively involved at 0.5, 0.6 and 0.7 m/s and breaststroke kick (BK) and crawl kick (CK) with only legs included at 0.5 and 1.0 m/s in all individuals. The joint forces and mo-ments were expressed as percentage of body weight% (BW) and % BWm (body weight × meter), respectively. A dynamic time warping algorithm was used to mean “subject-specific” and “activity-specific” time-load patterns for each loading parameter. Results: Within the mean “activity-specific” time-load patterns, the absolute maxima of resultant contact force (FRes, hip and knee), torsional moment around the femoral shaft axis or tibial axis (MTors, hip and knee), bending moment at the middle of the femoral neck (MBend, hip), and torsional moment around the femoral neck axis (MTne, hip) were mainly identified in the catch or sweep phase of BS and BK and in the downbeat phase of CK. The absolute maxima of FRes lay in the range of 157-193% BW (hip) and 93-145% BW (knee) during BS. Higher swimming velocity resulted in greater FRes (hip and knee), MTors (hip and knee), MBend (hip) and MTne (hip) during BS. A significantly positive correlation between the maxima of FRes and the BS velocity in the hip (r = 0.541, P < .05) and the knee (r = 0.414, P < .001) was identified. A significantly higher FRes was observed in the CK than in the BK in both hip and knee joints. Conclusion: This study is the first comprehensive in vivo measurement of hip and knee joint loads during swimming. The results suggest swimming is a safe rehabilitation exercise due to low loading patterns observed in patients with THA and TKA. Swimming velocities and styles can affect joint loads. Therefore, recommendations on swimming should be determined individually.