Mobility was measured in 79 people 6 months after knee arthroplasty using timed tests of stair climbing and walking speed. Recovery rates were calculated in comparison with a preoperative assessment. Changes in leg extensor power, knee flexion range, and pain were tracked simultaneously. Stair climbing speed improved on average by 53% during the first 6 months, and walking speed by 45%. In multivariate models, 2 preoperative factors emerged as being influential on recovery of mobility: leg extensor power and body mass index. A marked loss of flexion in the postoperative period was not a significant determinant of mobility speed. At 6 months, body mass index was the most significant determinant of stair speed, and leg extensor power was the most significant determinant of walking speed.