Muscle strain injuries are a particularly frequent type of sports injury in soccer, athletics, badminton/ tennis and cross-fit fitness, thereby affecting a broad range of popular leisure time activities. Depending on severity, sports-active individuals may experience long-term functional impairment and pain. Additionally, individuals having sustained one strain injury have a substantially increased risk of injuring the same muscle again. Strain injuries lead to long-term, potentially permanent, loss of muscle mass, thereby weakening the muscle. Muscle atrophy is likely a major factor in the high re-injury risk. Further, strain injuries are associated with a long-term inflammatory response.

In the current study, the investigators seek to study interventions to prevent the loss of muscle mass and elaborate on strategies to address the prolonged inflammation observed at the site of the injured muscle.

The primary aim of this study is to investigate the effect of protein supplementation on the reduction of muscle atrophy following a severe muscle strain injury in comparison to a carbohydrate supplement. As a second purpose, this study aims to elaborate on findings of prolonged inflammation intra-/ intermuscular by large-scale protein analysis and the characterization of cells active at the site of injury.

The study includes the following hypotheses:

1. Protein supplementation administered in combination with a gradually increasing loading regime (rehabilitation with weekly progression in load/ intensity) will be effective in reducing the injury-related loss of muscle mass.
2. The environment at the site of injury is not only pro-inflammatory, but contains proteins associated with proteolysis.
3. Cells belonging to the group of fibro-adipogenic progenitors will be accumulating intra- and inter-muscularly.

Inclusion criteria:

• Sports-active men and women
• Acute muscle strain injury in either the calf muscles or the hamstring muscles
• Visible tear at the muscle-connective tissue interface visible on an US scan as a hypo-/ hyperechoic area

Exclusion criteria:

• Unwillingness to return to sports
• Claustrophobia
• Daily intake of non-steroidal anti-inflammatory drugs (NSAIDs) within 3 months prior to the strain injury
• Smoking
• Diagnosed or suspected type I or type II diabetes
• Diagnosed or suspected connective tissue and/or rheumatic diseases
• Any observed organ dysfunctions

Individual de-identified participant data or other documents such as study protocol will not be shared with any third party. A potential change in the data sharing plan after registration will be acknowledged as a statement in a submitted/ published manuscript. A change in the data sharing statement will furthermore be updated in the registry record.