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Delayed Onset Muscle Soreness (DOMS) is a common side effect of exercise-induced muscle damage. Typically, DOMS manifests 24-48 hours post exercise and is limited to the affected muscle. While different theories exist, the most accepted thought is that the soreness is due to the irritation of mechanical nociceptors (pain receptors) due to the chemical changes associated with exercise induced inflammation. Type II muscle fibers (glycolytic) are most susceptible to DOMS due to the increased structural support of the Type I muscle fibers. Furthermore, Type I muscle fibers have more heat shock proteins and other mechanical proteins that help protect the muscle from mechanical stress. DOMS is also typically more severe in unconditioned subjects or when an athlete increases intensity or duration. It is also well accepted that the inflammatory response is directly related to exercise type, intensity and duration, and the mass of the muscles recruited. Diet macronutrient composition can also affect exercise induced muscle damage.
Once the inflammatory process has been initiated within the skeletal muscle, fluid, plasma proteins, and inflammatory cell populations infiltrate the affected tissues. This process normally begins as quickly as 45 minutes after exercise and peaks around 24 hours after exercise. It’s been observed 2-3 days post activity and may be present for up to 10 days after exercise. Neutrophils are the first to accumulate in the affected tissues and responsible for destroying damaged tissues through phagocytosis while working with the macrophages which are already located in the muscle. Additional macrophages as well as T-lymphocytes begin to accumulate and are responsible for removing cellular debris and muscle repair.
Endurance related trauma that leads to inflammation can lead to adaptation. Exercise induced muscle damage leading to the inflammatory process results in an increase in satellite cell behavior in skeletal muscles. Skeletal muscles are permanent cells which typically do not replicate. However, they are endowed with a stem cell called a satellite cell. These cells play an important role in muscle growth and repair. Satellite cells are attached to the outside of the muscle cell. When the muscle cell is damaged or has grown, the satellite cell divides and donates the additional nucleus to the muscle fiber. This enhances the muscle’s ability to synthesize new proteins thus supporting muscle growth and repair. In short, muscle damage leads to inflammation which increases satellite cell behavior leading to performance related adaptation. Exercise induced muscle damage has been shown to alter gene expression resulting in strengthening of the muscle tissues and prevention of further damage. This is known as the SAID principle (specific adaptations to implied demands).