Metabolic stress

Metabolic stress is the buildup of metabolites during exercise, particularly lactate, inorganic phosphate and hydrogen ions.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 47.} It peaks during efforts that rely heavily on anaerobic glycolysis, roughly those lasting 15 to 120 seconds, and causes fatigue within the muscle rather than the nervous system.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 47.} The burn and pump of a high-rep set reflect this buildup: sets of 15 or more reps last 45 seconds or longer and produce substantial metabolite accumulation, acidosis and a pump.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 101.}

Schoenfeld describes compelling evidence that metabolic stress promotes hypertrophy, while noting that an effect beyond mechanical tension hasn’t been established.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 65.} Proposed routes include greater fiber recruitment, myokine release, cell swelling and hormonal changes.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 65.}

In practice

Metabolic stress may matter mostly with light loads: some evidence shows no extra growth from it when training at around 80% of one-rep max.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 54.} In most training, tension and metabolite buildup rise together, making their contributions to growth hard to separate.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 54.}

Don’t shorten rest intervals so much that reps or load drop; that trades training volume, the primary driver of growth, for fatigue.^{5Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 174.} The post-workout hormone spikes once used to justify short rests aren’t what causes muscle growth.^{6Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 173.}