Repeated bout effect

The repeated bout effect is the reduction in muscle damage that occurs when someone performs the same exercise program consistently.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.} Exercise-induced damage brings reduced force output, stiffness, swelling and delayed-onset muscle soreness (DOMS), with eccentric actions causing by far the greatest damage.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.}

Proposed explanations include stronger connective tissue, more efficient and synchronized motor unit recruitment, a more even workload across fibers, and greater help from synergist muscles.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.}

One extra bout of the same protocol reduced swelling to about a third of the first bout’s response.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.} In a 10-week lifting study with sets taken to failure, damage was substantial after the first session but markedly lower by the fifth.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.} Protection can last several months.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.} Zatsiorsky and colleagues describe the same pattern with downhill running: it readily causes soreness in unprepared clients, while conditioning the muscle reduces injury.^{2Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 47.}

In practice

• Expect the most soreness and damage when a client first tries an unfamiliar exercise, especially one with a large eccentric component.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.},^{2Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 47.}
• The effect reduces damage without preventing it: trained lifters still show some damage after unfamiliar exercise, and unfamiliar changes to training variables can bring damage back.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 56.}
• Upper-extremity muscles are more prone to damage than leg muscles, suggesting that frequent use in daily life offers some protection.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 55.}