Anatomic location places restrictions on muscle function by limiting the amount of shortening or determining the kinds of loads encountered. Skeletal muscle is generally attached to bone, and bones are attached to each other. Because of the way the muscles are attached and the skeleton is articulated, the bones and muscles together constitute a lever system. This arrangement influences the physiology of the muscles and the functioning of the body as a whole. In most cases, the system works at a mechanical disadvantage with respect to the force exerted. The shortening capability of skeletal muscle by itself is rather limited, and the
Antagonistic pairs and the lever system of skeletal muscle. Contraction of the biceps muscle lifts the lower arm (flexion) and elongates the triceps, while contraction of the triceps lowers the arm and hand (extension) and elongates the biceps. The bones of the lower arm are pivoted at the elbow joint (the fulcrum of the lever); the force of the biceps is applied through its tendon close to the fulcrum; the hand is 7 times as far away from the elbow joint. Thus, the hand will move 7 times as far (and fast) as the biceps shortens (lever ratio, 7:1), but the biceps will have to exert 7 times as much force as the hand is supporting.
skeletal lever system multiplies the distance over which an extremity can be moved (Fig. 9.13). However, this means the muscle must exert a much greater force than the actual weight of the load being lifted (the muscle force is increased by the same ratio that the length change at the end of the extremity is increased). In the case of the human forearm, the biceps brachii, when moving a force applied to the hand, must exert a force at its insertion on the radius that is approximately 7 times as great. However, the resulting movement of the hand is approximately 7 times as far and 7 times as rapid as the shortening of the muscle itself. Muscles may be subject to large forces and this can lead to muscle injury (see Clinical Focus Box 9.2).
Acting independently, a muscle can only shorten, and the force to relengthen it must be provided externally. These actions are achieved by the arrangement of muscles into antagonistic pairs of flexors and extensors. For example, the shortening of the biceps is countered by the action of the triceps,- the triceps, in turn, is relengthened by contraction of the biceps. In some cases, gravity provides the restoring force.
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