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The functions that different muscles perform place different demands on them. Some muscles, such as postural muscles, must sustain a load continuously over long periods of time. Muscles used in walking or running must vary their strength of contraction over a wide range of positions. In contrast, the jumping muscles of the frog must exert maximum force instantaneously. The muscles that control your fingers generally do not have to sustain long, strong contractions, but they must be able to undergo quick yet graded contractions for fine manual dexterity. What is “quick” for humans, however, doesn’t begin to compare with insect flight muscles that can contract as fast as 1,000 times per second. How are different muscles adapted for specific functions and demands?
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A twitch is a unit of contraction in one muscle fiber initiated by a single action potential.
Many twitches can be summed in response to many action potentials to produce a large and/or sustained force.
Slow and fast twitch muscle fiber types differ in the rate at which they develop and maintain tension.
Three systems supply ATP to muscles for contraction, and together they determine the work capacity and endurance of a muscle.