Firstly, the investigators aim to study the biochemical, structural and functional changes associated with acute muscle strain injury in the acute phase and during rehabilitation in vivo. Specific factors present after the injury and their effects on connective tissue cells will be examined in vitro.

Secondly, the investigators will investigate individuals with chronic pain, scar tissue and inferior function after a previous muscle strain injury and test a specific rehabilitation program to alleviate pain and disabilities associated with the preceding muscle injury. The investigators have the following hypotheses:

1. Humoral growth factors and pro-inflammatory macrophages released into the injured area post strain injury stimulate formation of connective tissue in vitro.
2. Early onset of rehabilitation will be superior to a delayed start of rehabilitation for the healing process, in regards to structural improvement of tissue and better clinical function.
3. Scar tissue following a muscle strain injury is characterized by disorganized structure of connective tissue.
4. Mechanical loading in the form of heavy eccentric muscle activity for 12 weeks will lead to a local re-modeling of scar tissue, and result in improved function and reduced symptoms in patients with chronic pain and impaired function after a previous muscle injury.

Inclusion Criteria:

• Acute muscle strain injury
• Chronic muscle strain injury
• Age above 18 years

Exclusion Criteria:

• Daily intake of non-steroidal anti-inflammatory drugs (NSAIDs) within three months prior to the injury
• Smoking
• Diabetes
• Connective tissue and/or rheumatic diseases
• Any observed organ dysfunctions