High-intensity resistance training (HIRT)

High-intensity resistance training (HIRT) means lifting heavy loads. Here, intensity means the load: the percentage of 1RM on the bar.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 101.} The textbooks use the phrase but don’t give it one fixed cutoff.

Schoenfeld calls loads above 85% of 1RM high intensities of load.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 101.} These loads put high mechanical tension on the muscle.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 101.} A heavy set is short, under 15 seconds. So its energy comes mainly from the phosphagen system, and relatively few metabolites build up.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 101.}

Heavy, low-rep work builds the ability to lift heavier loads, through greater adaptations in the nervous system.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.} Some evidence suggests it grows Type II fibers more than light loads do.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.} For whole-muscle size, though, load matters less. In a meta-analysis, loads above and below 60% of 1RM, taken to failure, gave virtually the same growth.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 104.},^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 105.}

Heavy loads of about 80% of 1RM help build both muscle and bone.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 513.} As people age, they lose muscle and the ability to produce force. A program of high-intensity resistance training can greatly ease, or even reverse, these effects.^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 97.}

Heavy lifting has costs. Relatively few sets can cut muscle glycogen by 20 to 60%.^{7Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 72.} It also drives blood pressure up. Peaks of 320/250 mmHg have been reported in a leg press at 95% of 1RM.^{8Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 119.}

In practice

• To build strength, ACSM advises loads at or above 85% of 1RM.^{9Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 357.}
• Say the goal is building muscle to get as strong as possible. Schoenfeld then sees little reason to go below about 70% of 1RM. A deload may be the exception.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.}
• Heavy sets of 1 to 5 reps build strength. That strength then lets clients use heavier loads in sets of 6 to 12.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.}
• Clients new to lifting may start lighter. Lighter loads with more reps give practice with form and, at first, enough stimulus for strength gains.^{9Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 357.}

Where sources differ

Where “heavy” starts depends on the book. Schoenfeld calls loads above 85% of 1RM a high intensity of load.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 101.} ACSM says loads for building strength should be at least 85%.^{9Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 357.} The NSCA personal training text calls about 80% of 1RM relatively heavy.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 513.} For strength-focused muscle work, Schoenfeld treats about 70% as the practical floor.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 109.}