Intra-rater reliability and agreement of muscle strength, power and functional performance measures in patients with hip osteoarthritis.

OBJECTIVE To investigate the reliability and agreement of measures of lower extremity muscle strength, power and functional performance in patients with hip osteoarthritis at different time intervals, and to compare these with the same measures in healthy peers. DESIGN Intra-rater test-retest separated by 1, 2, or 2.5 weeks in patients, and 1 week in healthy peers. SUBJECTS Patients with hip osteoarthritis (age range 61-83 years) with 1 (n = 37), 2 (n = 35), or 2.5 weeks (n = 15) between tests, and 35 healthy peers (age range 63-82 years). METHODS Maximal isometric hip and thigh strength, leg extensor power, and functional performance (8-foot Up & Go, stair climbing, chair stand and 6-min walk) were measured in patients, and quadriceps strength, leg extensor power and functional performance were measured in healthy peers. Systematic error, reliability and agreement were calculated. RESULTS Most hip strength measurements for the most symptomatic extremity, and nearly all strength measurements for the least symptomatic lower extremity, declined after 1 week (p < 0.05), but not after a 2.5-week interval. In healthy peers, quadriceps strength was unchanged. Regardless of the time interval, leg extensor power was unchanged, while functional performances improved at retest for all participants. CONCLUSION In patients with hip osteoarthritis leg extensor power is unaffected by the time interval between tests, in contrast to muscle strength and functional performance.