Biomechanics: Muscle contractions

A contraction is defined simply as the generation of tension within a muscle fiber. Muscle fibers generate tension through actin and myosin cross-bridge cycling. Under tension, a muscle belly can either lengthen, shorten, or remain the same length. The names of contractions are based upon how the muscle belly length changes during this tension.

Isokinetic contractions are those in which there is a consistent rate of speed.

Isotonic contractions are those in which there is consistent tension as the muscle length changes. These can be either concentric (muscle shortening) or eccentric (muscle elongation).

Isometric contractions are those in which the length of the muscle does not change.

TYPES OF MUSCLE CONTRACTIONS
TYPE OF CONTRACTION	DEFINITION / CHARACTERISTICS	EXAMPLE
Isokinetic	When the velocity of the muscle contraction remains constant while muscle length changes	Riding a stationary bike
Concentric (isotonic)	When a muscle is activated and required to lift a load which is less than the maximum tension it can generate, resulting in muscle shortening	Raising a dumbbell with a bicep curl
Eccentric (isotonic)	When the external force on a muscle is greater than the maximal force it can generate, resulting in muscle lengthening	Walking down stairs
Isometric	When a muscle is activated at a constant length	Holding a shopping bag

Figure 1. Riding a stationary bike, raising a dumbbell with a bicep curl, and holding a weight at a constant height are examples of isokinetic, isotonic, and isometric contractions, respectively.

Author: Arman Israelyan, OMS-III

Editor: Matt Lipinski, DO

Illustrator: Jillian Dunbar

Editor: Robyn Hughes, MScBMC

Biomechanics: Muscle contractions

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