###### Abstract  
Hip dysplasia patients after total hip replacement show worse functional performance compared to primary osteoarthritis patients, and unfortunately there is no research on muscle and joint loads that would help understand rehabilitation effects, motor dysfunctions and failure events. We tested the hypothesis that a higher functional improvement in hip dysplasia patients who received hip abductor strengthening after hip replacement, would result in different gait function and musculoskeletal loads during walking compared to patients who performed standard rehabilitation only. In vivo gait analysis and musculoskeletal modeling were used to analyze the differences in gait parameters and hip and muscle forces during walking between the two groups of patients. We found that, in a functional scenario of very mild abnormalities, the patients who performed muscle strengthening expressed a more physiological force pattern and a generally greater force in the operated limb, although statistically significant in limited portions of the gait cycle, and likely related to a higher gait speed. We conclude that in a low-demand task, the abductor strengthening program does not have a marked effect on hip loads, and further studies on hip dysplasia patients would help clarify the effect of muscle strengthening on loads.  
Keywords:total hip replacement; hip dysplasia; hip abductor muscles; musculoskeletal modeling; gait analysis; rehabilitation +
Footnote †: journal: Medical  
specific protocol for hip abductor strengthening with adequate presurgical planning would improve functional performance and patient satisfaction in DDH patients after THR.  
From a biomechanical perspective, gait analysis studies among THR patients demonstrated that DDH patients have worse functional performance and unsatisfactory clinical results compared to primary osteoarthritis patients [11; 12]. However, although a few studies analyzed the effect of hip abductor strengthening on joint loads in osteoarthritis patients after THR [13; 14], there is no literature regarding muscle and joint contact forces during movement in DDH patients after THR. Muscle and joint loads are related to the stress at the hip joint, with consequent failure events that may occur as a result of wear, edge loading, impingement and subluxation [15; 16].  
Quantifying muscle and joint loads after rehabilitation following THR of DDH patients would provide insights into the biomechanical effect of a program of exercises, identify potential compensation strategies, or help explain pain and residual motor dysfunctions. The state-of-the-art method to quantify musculoskeletal loads includes the use of multibody models of the musculoskeletal system and patient-specific gait analysis measurements, with consequent simulations of movement and prediction of the biomechanical variables [17; 18].  
Therefore, the aim of this study was to analyze the effect of a rehabilitation protocol for abductor muscle strengthening on gait function and musculoskeletal loads during walking in DDH patients after THR. We compared gait analysis measurements, joint contact forces and muscle forces of the hip between a group of patients who performed the specific rehabilitation protocol and another group of patients who performed standardization only, in the operated and contralateral limb. We tested the hypothesis that a higher improvement in clinical and functional outcome in patients after hip abductor strengthening, as shown recently [10], would result in differences in muscle and joint forces compared to patients who performed standard rehabilitation only.