For calculation of sample size the average loss of strength from baseline (before surgery) to 48 hours after TKA-surgery being 0.8 Nm/kg body weight (SD=0.39, Holm et al. 2010), a risk of type I errors 5% and a risk of type II errors 20% (80% power) were used. The sample size is estimated to reveal a group difference in knee extension strength of minimum 40%.