Exercise-induced muscle damage (EIMD)

Exercise-induced muscle damage (EIMD) is disruption to muscle fibers and surrounding tissue after intense exercise, particularly unfamiliar work.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 54.},^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.} It ranges from a few disrupted molecules to large tears in the cell membrane, connective tissue and contractile machinery.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.} The damage triggers local inflammation and raises blood levels of proteins such as creatine kinase.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.}

Eccentric muscle actions cause far more damage than concentric or isometric work, and higher training volume causes more damage.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.} Symptoms include lost strength, stiffness, swelling and delayed onset muscle soreness (DOMS).^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.}

In practice

• Damage drops with repeated exposure, known as the repeated bout effect: a second bout of the same protocol cut swelling to a third of the first, and damage was practically inconsequential by the end of a 10-week study.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.}
• Introduce eccentric-heavy work and new exercises gradually, since conditioning the muscle reduces damage.^{3Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 47.}
• Israetel and colleagues advise aiming for some disruption: they consider training that causes no damage probably insufficient, but say maximal damage diverts resources from growth to recovery.^{4Israetel et al., Scientific Principles of Hypertrophy Training, p. 51.}

Does muscle damage cause growth?

Whether damage itself drives hypertrophy is unresolved: Schoenfeld concludes it may enhance adaptations, though excessive damage hurts them.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 65.} Israetel and colleagues say it’s unclear whether damage causes growth or simply comes from the same training.^{4Israetel et al., Scientific Principles of Hypertrophy Training, p. 51.} NSCA’s Essentials of Personal Training points to growth from concentric-only training and considers the volume of loading a likely stronger driver than eccentric damage.^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 91.}