Low-intensity resistance training (LIRT)

Low-intensity resistance training (LIRT) means lifting light loads. Here, intensity means the load: the percentage of 1RM lifted.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 101.} Schoenfeld classes light sets as 15 or more reps.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 101.} The textbooks use the phrase but don’t give it one fixed cutoff.

With light loads, effort matters most. Evidence suggests hard sets at loads as low as 30% of 1RM recruit most of a muscle’s motor units.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 138.} So the lighter the load, the more effort counts.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 138.}

A meta-analysis compared loads at or below 60% of 1RM with heavier loads, all taken to failure. Muscle growth was virtually the same.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 104.},^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 105.}

There seems to be a floor, though. In one study, 20% of 1RM grew less than half as much muscle as 40, 60 or 80%.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 105.} Schoenfeld says training below about 30% of 1RM may not be enough for full muscle growth.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.}

Light loads lean on metabolic stress, and they build local muscular endurance the most.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.} They may also favor growth of Type I fibers.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.} The trade-off is maximal strength. Lighter loads build the ability to repeat easier efforts, at the cost of top-end force.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.}

In practice

• ACSM puts muscular endurance training at or below 67% of 1RM. It calls that below the level needed for a strength stimulus.^{6Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 356.}
• For people new to lifting, lighter loads with more reps give form practice. At first, they also give enough stimulus for strength gains.^{7Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 357.}
• Take light sets close to failure. Stopping well short of it at loads under 60% of 1RM blunts muscle protein synthesis.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 104.}
• Schoenfeld advises some light-load work in a muscle-building program. It helps develop Type I fibers and lets muscles handle more reps at moderate loads.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.}
• Light loads are the base of blood flow restriction training. With a wrap, loads of 20 to 30% of 1RM can match heavier loads for muscle growth.^{8Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 124.}

Where sources differ

The sources frame the role of BFR differently. Helms says a wrap lets loads of 20 to 30% of 1RM match heavier loads for growth.^{8Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 124.} Schoenfeld cites studies where light sets to failure grew muscle about equally with or without BFR.^{9Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 103.} The lowest useful load also varies. Schoenfeld flags loads below about 30% as possibly too light.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.} Helms uses 20 to 30% with a wrap.^{8Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 124.}