Statistically significant differences were found in the hip abductor and adductor muscle lengths and moment arms between the legs in unilateral THA patients during gait. Significantly shorter abductor moment arms, less elongated adductor muscle, and longer adductor muscle moment arms during the stance phase of the gait cycle were observed in the implanted side than the contralateral non-implanted side (P < 0.05, Tables 2&3, Figures. 2&3). Among all the hip abductor and adductor muscles, the GMDP and PT moment arms were affected the most with the highest difference during most of the gait cycle (Figures. 2&3). The GMDP, AMP, and AL moment arms significantly decreased during the load-bearing response period (Figure. 3). Our findings indicated that THA would change the biomechanical parameters of the hip abductor and adductor muscles, which could result in muscle weakness and limit the effectiveness of regular rehabilitation.
The muscle lengths and moment arms are essential biomechanical parameters for muscle function evaluation. Dostal et al.  quantified hip muscle lengths and moment arms in a male cadaver by dissecting the proximal and distal muscle attachments. They reported that the GMD, GMI, and AM anterior bundles provided a relatively larger moment arm in each muscle. A clinical commentary study reviewed the hip muscle function based on a hypothetical model and reported that GMD has the largest abduction moment arm in the abductor muscles . Our results are in line with the abovementioned findings. Further, Bjørdal et al.  reported a larger abductor moment arm in the implanted side (65.4 ± 5.9 mm) than in the non-implanted side (58.0 ± 6.6 mm) on radiographs of 148 THA patients. In contrast, we found shorter abductor moment arms after THA. This difference may attribute to different measurement methods, surgical approaches, and implant systems used between studies.
Abductor deficiency has been reported after THA [2, 20]. Previous studies demonstrated that when the hip abductor is weak, the pelvis drops to the sound side, leading to gait instability [3, 7]. In this study, we found that the abductor muscle lengths of the THA side were longer than the contralateral non-implanted side during the support phase (Figure. 2), implying less abductor muscle contraction in the THA side. Furthermore, the moment arms of these abductor muscles were smaller than that of the contralateral sound side (Figure. 3), resulting in reduced abductor moment and muscle efficiency. On the other hand, previous studies reported that many THA patients experienced persistent adductor muscle contracture , which collaborates with the significant adductor muscle shortening than that of the non-implanted side in our study (Figure. 2). The abductor muscle weakness and contracture observed in THA patients may relate to the adverse biomechanical effects of THA.
The adductor and abductor muscles also assist hip flexion, extension, and rotation. The AM began to pull at the end of the loading response period. The AM and GMDP help the gluteus maximus muscle in hip extension, according to the previous studies . In addition to the AM and GMDP, AL can also act as a hip flexor and extensor . In this study, we found that AMP, GMDP, and AL of the THA side were shortened during the loading response period, implying concentrically contraction. Additionally, their moment arms are significantly smaller than the non-operated side (Figures. 2&3). The GMIP was considered as a secondary external rotator of the hip . In this study, we observed that the GMIP of the THA side had a significantly shorter moment arm, which might associate with the excessive internal rotation reported in THA patients . The adverse biomechanical effects may associate with the hip extensor weakness, lower hip range of motion, and abnormal hip internal rotation during gait (Figures. 2&3). It is essential to develop personalized treatment plans considering the effects of THA on each muscle to better restore their hip function.
The Sherrington’s reciprocal inhibition principle states that tensioned or shortened antagonist muscles may reflexively inhibit active muscles . The hip adductor muscle is the antagonist of the hip abductor muscle. If the hip adductor muscle becomes tight and shortened, it may, in turn, be a cause of hip abductor muscle weakness. Our study found that in THA patients, the hip adductors of the operated side were shortened compared with the non-operated side during gait, which may reflexively inhibit the hip abductor muscle. The current rehabilitation training for patients after THA focuses more on strengthening the hip abductor muscles, which may be limited. The previous studies reported asymmetric muscle activities even 2 years after THA [23, 24]. Therefore, the training of antagonist muscles should be enhanced together. We suggest that THA patients should first start to recover the hip adductor muscle tension and length, especially the PT, the most affected adductor, through muscle energy technique  and specific fascial release technique. Subsequently, strengthen the weak or inhibited abductor hip muscles, especially the GMD, the most affected abductor.
Janda  reported that tensed or shortened antagonist muscles often become active in unrelated movements, further worsening dynamic posture control. This compensation will naturally decrease hip extension and increase hip internal rotation and adduction . In this study, the decreased GMDP, AMP, and AL moment arms after THA may decrease hip extension, and GMIP may increase hip internal rotation during the gait cycle. This compensatory model will lead to the biomechanical changes of the entire lower limb, which in turn will lead to incorrect movements and aggravates injuries [27, 28]. It is suggested that postural control, combined with individualized muscle training, should be implemented in patients after THA to maintain the body’s overall balance by strengthening the coordination and control ability between the muscle groups.
Biomechanical changes in the musculoskeletal system after THA are closely related to surgical techniques and prosthesis design . Accurate surgical techniques as well as optimal prosthesis design and positioning, can reduce the impact of joint replacement on peripheral muscle function. A previous study suggested that accurate intraoperative femoral offset reconstruction is essential for maintaining the abductor muscle moment arm . The displacement of the centre of the femoral head may occur due to surgeon preferences (e.g., excessive correction of native femoral anteversion), inaccurate preoperative templating, or limited selection of available implants. Currently, the hip prosthesis design features a smaller femoral head, higher rotation centre, wider neck, and lower range of motion, which negatively affects the biomechanical parameters and peripheral muscle performance . We recommend that the next generation hip prosthesis geometry should be optimized for better functional recovery and clinical outcomes.
The present study should be interpreted in light of potential limitations. Only 10 patients who received THA for a year participated. However, using the muscle lengths difference between the implanted and non-implanted sides, the statistical verification power of the data is 97% through G-Power calculation. Future studies should recruit more patients who underwent THA with longer follow-up to compare the effects of component positioning, follow-up times, and rehabilitation program on muscle function recovery.