 Fifteen patients with a unilateral TKA, operated between 1 to 2 weeks prior to the first investigation, will be included. The participants are investigated twice. During the first investigation, the absolute load kilograms corresponding to 10 Repetition Maximum RM a load that can be lifted exactly 10 times will be determined for unilateral operated leg knee extension. At the second investigation, at least 72 hours later, the patients will perform 1 set of 4 repetitions with a relative loading of 8, 14, and 20 RM each, in a randomized order, estimated from their 10 RM test at the first investigation. Time under tension contraction velocity and range of knee joint motion for each repetition will be controlled for. During the concentric phase of each repetition, the patients rate their knee joint pain verbally, using a numerical rating scale placed 1 meter in front of them. After a short break, the patients perform 1 set of knee extension with a relative loading of 10 RM until of contraction failure. As for the loading effect described above, they rate their knee joint pain during the concentric phase of each repetition. 