Background:

In the early phase after a total knee arthroplasty (TKA), patients experience multi-level weakness in the operated leg, which is caused primarily by reduced central nervous system (CNS) activation failure of the muscles - especially the knee extensors. This considerable loss of muscle strength relates to reduced functional performance. Early-commenced, progressive strength training of the knee extensors of the operated leg therefore seems rational. However, the concern is that this type of early-commenced, intense physical rehabilitation exacerbates post-operative symptoms, such as knee joint pain.

The investigators have recently reported that early-commenced physical rehabilitation, including progressive strength training of the knee extensors of the operated leg seems feasible after TKA. The classic exercise-physiology literature emphasizes loading and repetitions performed to contraction failure - among others - as important variables for muscle hypertrophy and strength gains. It is currently not known how loading and repetitions performed to contraction failure during knee extensions with the operated leg, influences post-operative knee joint pain in patients with TKA.

Purpose and hypothesis:

The purpose of this study is to investigate how loading and repetitions to contraction failure influence knee joint pain during knee extensions with the operated leg early after TKA.

The hypothesis is that knee pain increases with increasing loading and fatigue.

Methods:

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.

Inclusion Criteria:

• Unilateral primary TKA
• Between the age of 18 to 80 years
• Understand and speak Danish
• Informed consent
• 1 to 2 weeks after TKA

Exclusion Criteria:

• Disease/Musculoskeletal disorder, which requires special rehabilitation modality
• Alcohol and drug abuse
• Lack of wish to participate or unwillingness to sign an informed consent