Lever systems

Lever systems use bones as levers, joints as axes and muscle contractions to apply force.^{1International Sports Sciences Association, Strength and Conditioning, p. 135.} A lever is a rigid structure fixed at a fulcrum, with effort from the active muscle (the agonist) and resistance from a load acting on it.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 61.} The arrangement of the fulcrum and those two forces determines its class:^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 61.}

• First class: the fulcrum is between the forces, as in the neck extensors holding up the head.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 61.}
• Second class: the resistance is between the fulcrum and effort, as in a calf raise.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 61.}
• Third class: the effort is between the fulcrum and resistance, as in a leg curl or biceps curl.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 61.},^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 70.}

Most joints act as third-class levers, with some first-class and relatively few second-class arrangements.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 62.}

In practice

The ratio of the two moment arms determines mechanical advantage; in a third-class lever, the resistance always has the advantage.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 62.} Most human muscles work at a mechanical advantage below 1.0.^{5Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 37.} In the NSCA’s triceps example, the resistance moment arm is eight times the muscle’s, so the muscle must produce eight times the load.^{5Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 37.} These high internal forces account for a large part of muscle and tendon injuries.^{5Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 37.}

Third-class levers trade force for speed over distance, helping baseball pitchers and volleyball spikers generate high hand velocity.^{6International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 36.},^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 70.} During movement, class can depend on an arbitrary choice of fulcrum, so the NSCA considers understanding mechanical advantage more useful than classifying every joint.^{5Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 37.}