Isometric muscle action

An isometric muscle action, or isometric contraction, produces force without changing muscle length because the force equals the resistance.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 48.} The body or limb stays still.^{2Hoffman, NSCA’s Guide to Program Design, p. 24.}

Every dynamic lift includes isometric work: stabilizers hold posture, working muscles pause between lowering and lifting, and forearms grip the bar.^{3Hoffman, NSCA’s Guide to Program Design, p. 95.} In a back squat, the spinal erectors and abdominals hold the spine steady throughout the lift.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 21.} Some exercises, such as planks, are mainly isometric.^{3Hoffman, NSCA’s Guide to Program Design, p. 95.}

In practice

• Isometric strength gains occur mostly at the joint angle trained, with little carryover to other angles.^{5Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 129.},^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 90.}
• Zatsiorsky’s protocol uses maximal efforts of 5 to 6 seconds, 3 to 5 reps per position, and rest intervals of about 1 minute for small muscles or up to 3 minutes for large ones.^{7Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 130.} Gains peak within 6 to 8 weeks in trained athletes, so he recommends one or two mesocycles at most.^{7Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 130.}
• Functional isometrics involve pushing a bar into power rack pins for about 2 to 6 seconds and work best near a lift’s sticking region.^{3Hoffman, NSCA’s Guide to Program Design, p. 95.}
• Maximal isometric efforts with large muscles can raise blood pressure sharply; clients with heart disease, atherosclerosis or hypertension should avoid them.^{7Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 130.}