Mechanical tension

Mechanical tension is force acting on muscle tissue; technically, it’s force normalized to the area it acts over, measured in pascals.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 39.} Muscle cells detect that force through receptors that trigger molecular signals for growth.^{2Israetel et al., Scientific Principles of Hypertrophy Training, p. 41.} Tension alone has been shown to stimulate mTOR, a growth signal.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 39.}

Of the three proposed mechanisms of hypertrophy, tension, metabolic stress and muscle damage, tension is considered the most important.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 65.} Muscle sensors respond to both its magnitude and duration.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 65.} Without tension, muscle shrinks, as seen in astronauts and people confined to bed.^{2Israetel et al., Scientific Principles of Hypertrophy Training, p. 41.}

In practice

• Israetel and colleagues emphasize reasonably heavy loads: the larger, most growth-responsive motor units activate when force demands are high, so very light work produces little growth per rep.^{4Israetel et al., Scientific Principles of Hypertrophy Training, p. 42.}
• The heaviest loads create enough fatigue to reduce the volume a client can do.^{4Israetel et al., Scientific Principles of Hypertrophy Training, p. 42.}
• Avoid rest intervals so short that clients lose reps or load: this sacrifices progressively increasing tension and volume, the primary driver of growth.^{5Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 174.}
• A slow tempo can also reduce load and volume, as it did in one study discussed by Helms and colleagues.^{6Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 196.}