Cellular Mechanisms Involved in Muscle Pathology

Inclusion Criteria: * 18 years and older, male and female, with an acute muscle strain injury (\<14 days post injury) in the calf or hamstring muscles (group 1). Pathological changes visible on an ultrasound scan as hypoechoic areas. * 18 years and older, male and female, with a chronic muscle strain injury (\> 6 months prior to inclusion) in the calf or hamstring muscles (group 2), sequela including recurrent strain injuries, pain and/ or decrease in muscle function. Pathological changes visible on an ultrasound scan as hypo-/ hyperechoic areas. * 18 years and older, male and female, with a full Achilles tendon rupture (\> 1 year prior to inclusion) with pathological changes: Fatty degeneration visible on an ultrasound scan (group 3). Exclusion Criteria: * Type I and II Diabetes, Connective tissue and/or rheumatic diseases, or any observed organ dysfunctions * Daily smoking * Persons with daily intake of non-steroidal anti-inflammatory drugs (NSAIDs) within three months prior to time of contact * Allergic reactions to local anesthesia * Use of anticoagulant treatment * Needle phobia * Any drug or alcohol abuse now or in the past * The absence of any pathological changes visible on an ultrasound scan as hypo-/ hyperechoic areas (group 1 and 2) * The absence of any pathological changes (fatty degeneration) in either the gastrocnemius or the soleus muscle visible on an ultrasound scan (Subject group 3)