In order for muscle fibers to shorten when they contract, they must generate a force that is greater than the opposing forces that act to prevent movement of the muscle's insertion. When a
■ Figure 12.19 Force-velocity curve. This graph illustrates the inverse relationship between the force opposing muscle contraction (the load against which the muscle must work) and the velocity of muscle shortening. A force that is sufficiently great prevents muscle shortening, so that the contraction is isometric. If there is no force acting against the muscle contraction, the velocity of shortening is maximal (Vmax). Since this cannot be measured (because there will always be some load), the estimated position of the curve is shown with a dashed line.
weight is lifted by flexing the elbow joint, for example, the force produced by contraction of the biceps brachii muscle is greater than the force of gravity on the object being lifted. The tension produced by the contraction of each muscle fiber separately is insufficient to overcome the opposing force, but the combined contractions of numerous muscle fibers may be sufficient to overcome the opposing force and flex the forearm. In this case, the muscle and all of its fibers shorten in length.
This process can be seen by examining the force-velocity curve. This graph shows the inverse relationship between the force opposing muscle contraction (the load against which the muscle must work) and the velocity of muscle shortening (fig. 12.19). The tension produced by the shortening muscle is just greater than the force (load) at each value, causing the muscle to shorten. Since the contraction strength is constant at each load, a muscle contraction during shortening is called an isotonic contraction (iso = same; tonic = strength).
If the load is zero, a muscle contracts and shortens with its maximum velocity. As the load increases, the velocity of muscle shortening decreases. When the force opposing contraction (the load) becomes sufficiently great, the muscle is unable to shorten when it exerts a given tension. That is, its velocity of shortening is zero. At this point, where muscle tension does not cause muscle shortening, the contraction is called an isometric (literally, "same length") contraction.
Isometric contraction can be voluntarily produced, for example, by lifting a weight and maintaining the forearm in a partially flexed position. We can then increase the amount of muscle tension produced by recruiting more muscle fibers until the muscle begins to shorten; at this point, isometric contraction is converted to isotonic contraction.
When a force exerted on a muscle to stretch it is greater than the force of muscle contraction, the muscle will lengthen as it contracts. In other words, the muscle lengthens despite its contraction. This is known as an eccentric contraction. In this case, the muscle can absorb some of the mechanical energy of the external force, and thereby serve as a shock absorber.
For example, when you jump from a height and land in a flexed-leg position, the extensor muscles of your legs (the quadriceps femoris group) contract eccentrically to absorb some of the shock. In this case, most of the energy absorbed by the muscles is dissipated as heat. Less dramatically (and somewhat less painfully), these muscles also contract eccentrically when you jog downhill or hike down a steep mountain trail.
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