Total hip arthroplasty relieves joint pain in patients with end stage osteoarthritis. However, postoperative muscle atrophy often results in suboptimal lower limb function. There is a need to improve functional recovery after total hip arthroplasty.To assess safety and efficacy of LY2495655, a humanized monoclonal antibody targeting myostatin, in patients undergoing elective total hip arthroplasty.Phase 2, randomized, parallel, double-blind, 12-week clinical trial with a 12-week follow-up period.Forty-two sites in 11 countries.Individuals (N=400) aged ≥50 years scheduled for elective total hip arthroplasty for osteoarthritis within 10 ± 6 days after randomization.Placebo or LY2495655 (35 mg, 105 mg, or 315 mg) subcutaneous injections at weeks 0 (randomization date), 4, 8, and 12 with follow up until week 24.Primary endpoint: probability that LY2495655 increases appendicular lean mass (operated limb excluded) by at least 2.5% more than placebo at week 12, using dual-energy x-ray absorptiometry. Exploratory endpoints: muscle strength, performance based and self-reported measures of physical function, and whole body composition over time.Participants: 59% women, aged 69 ± 8 years, BMI 29 ± 5 kg/m2. Groups were comparable at baseline. The primary objective was not reached as LY2495655 changes in lean mass did not meet the superiority threshold at week 12. However, LY2495655 105 and LY2495655 315 experienced progressive increases in appendicular lean mass that were statistically significant versus placebo at weeks 8 and 16. Whole body fat mass decreased in LY2495655 315 versus placebo at weeks 8 and 16. No meaningful differences were detected between groups in other exploratory endpoints. Injection site reactions occurred more often in LY2495655 patients than in placebo patients. No other safety signals were detected.Dose-dependent increases in appendicular lean body mass and decreases in fat mass were observed, although this study did not achieve the threshold of its primary objective.