In the early phase after a total knee replacement (TKA), patients experience multi-level weakness in the operated leg, which is caused primarily by reduced central (CNS) activation failure of the muscles - especially the knee extensors. This considerable loss of muscle strength relates to reduced functional performance. The investigators recently reported that early-commenced physiotherapy, including progressive strength training performed in machines, seems feasible after TKA. The question is, if neuromuscular activity of the muscles in the operated leg, elicited during strength training in machines, can be reached during strength training in more simple forms? Many clinicians are faced with the problem of not having strength training equipment at their institution, or having to prescribe simple strength training exercises for home-based training.