Lever Action of Muscles and Bones

A contracting muscle exerts a force on bones through its connecting tendons. When the force is great enough, the bone moves as the muscle shortens. A contracting muscle exerts only a pulling force, so that as the muscle shortens, the bones to which it is attached are pulled toward each other. Flexion refers to the bending of a limb at a joint, whereas extension is the straightening of a limb (Figure 11-31). These opposing motions require at least two muscles, one to cause flexion and the other extension. Groups of muscles that produce oppositely directed movements at a joint are known as antagonists. For example, from Figure 11-31 it can be seen that contraction of the biceps causes flexion of the arm at the elbow, whereas contraction of the antagonistic muscle, the triceps, causes the arm to extend. Both muscles exert only a pulling force upon the forearm when they contract.

Sets of antagonistic muscles are required not only for flexion-extension, but also for side-to-side movements or rotation of a limb. The contraction of some muscles leads to two types of limb movement, depending on the contractile state of other muscles

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems





Tendon Biceps Tendon

Quadriceps -femoris

- Gastrocnemius

Triceps contracts


Biceps contracts

Biceps contracts


- Quadriceps Quadriceps -femoris femoris relaxed contracts

- Gastrocnemius -contracts

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  • Bilba
    When a bicep contracts it exerts a force causing?
    8 years ago

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