OBJECTIVE: To examine the safety and efficacy of a high-intensity (HI) progressive rehabilitation protocol beginning 4 days after total knee arthroplasty (TKA) compared to a low-intensity (LI) rehabilitation protocol. METHODS: A total of 162 participants (mean ± SD ages 63 ± 7 years; 89 women) were randomized to either the HI group or LI group after TKA. Key components of the HI intervention were the use of progressive resistance exercises and a rapid progression to weight-bearing exercises and activities. Both groups were treated in an outpatient setting 2 to 3 times per week for 11 weeks (26 total sessions). Outcomes included the stair climbing test (SCT; primary outcome), timed-up-and-go (TUG) test, 6-minute walk (6MW) test, the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), 12-item Short Form health survey (SF-12), knee range of motion (ROM), quadriceps and hamstring strength, and quadriceps activation. Outcomes were assessed preoperatively and at 1, 2, 3 (primary end point), 6, and 12 months postoperatively. RESULTS: There were no significant differences between groups at 3 or 12 months in SCT, TUG, 6MW, WOMAC scores, knee ROM, quadriceps and hamstrings strength, quadriceps activation, or adverse event rates. By 12 months, outcomes on the 6MW, TUG, WOMAC, SF-12, quadriceps and hamstring strength, and quadriceps activation had improved beyond baseline performance in both groups. CONCLUSION: Both the HI and LI interventions were effective in improving strength and function after TKA. HI progressive rehabilitation is safe for individuals after TKA. However, its effectiveness may be limited by arthrogenic muscular inhibition in the early postoperative period.