From this study:

Hip abduction strength more important to function than quadriceps after Total Knee Arthroplasty. (2011)

Measurements
Strength of quadriceps muscles and hip abductors was measured using an isokinetic dynamometer. Performance-based physical function was assessed with 4 measures: self-selected walking speed, the Figure-of-8 Walk Test, the Stair Ascend/Descend Test, and the 5-Chair Rise Test. Self-reported physical function was assessed with the Western Ontario and McMaster Universities Osteoarthritis Index Physical Function Subscale.
Results
In hierarchical regression models, after accounting for demographic and anthropometric factors, quadriceps muscle strength was associated with performance on the Stair Ascend/Descend Test. After accounting for demographic, anthropometric, and quadriceps strength, hip abductor strength was associated with performance on the Stair Ascend/Descend Test, the Figure-of-8 Walk Test, and the 5-Chair Rise Test.
Conclusions
After TKA, hip abductor strength influenced physical function in participants more than did demographic or anthropometric measures or quadriceps strength. Longitudinal studies with larger samples are warranted. If findings are replicated, they will justify targeting the hip abductors during rehabilitation after TKA.

My comments:

I always like to read a study where I learn something new. While I think restoration of total leg strength is important it does surprise me that hip abduction correlated more so with physical performance tests than did quadriceps. I would imagine if they looked at other muscles, hip extension, adduction and knee flexion would also have considerable correlation with physical performance tests post Total Knee Arthroplasty. This finding along with some additional research on greater trochanteric pain syndrome (formerly known as trochanteric bursitis) has made me more likely to have patients use both my hip abduction machine and perform hip abduction in side lying both in the clinic and in home exercise. One plus with the hip abduction machine is that it puts very minimal stress on the knee and is subsequently very well tolerated very early on post-op.