Motor unit

A motor unit is a single motor neuron and all the muscle fibers it controls.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 21.} When the neuron fires, all of those fibers contract together.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 21.} It’s all or nothing: a stronger signal can’t make one unit contract harder.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 24.} An eye muscle may have as few as one fiber per neuron, while the quadriceps may have several hundred.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 24.}

Muscles control force in two ways: recruiting more units and firing them faster, which is called rate coding.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 86.} Recruitment follows the size principle: small, low-threshold units come in first, and larger, high-threshold units join as force demand rises.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 107.} Small hand muscles have recruited all their units by about 50% of maximum force and rely on firing rate after that, while the quadriceps keep recruiting up to 90% or more.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 86.}

In practice

• Within a set, more units are recruited and firing rates climb to make up for tired units.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 86.}
• A unit that isn’t recruited isn’t trained.^{5Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 90.} If a submaximal set stops well short of failure, high-threshold units may never be recruited.^{5Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 90.}
• Ballistic movements such as Olympic lifts and plyometrics have been shown to recruit fast-twitch units preferentially, departing from the usual size principle.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 107.}