ABSTRACT
Skeletal muscles are composed of fibers that have the same basic structure as outlined in Topics I1 and I3. However, the jobs that different skeletal muscles in the body have to perform are very different. Take, for example, muscles that are involved in postural control during standing, as compared with muscles whose job is to move joints quickly against load, or to run fast for short periods of time. The postural control muscles must be able to hold joints in position for long periods of time, whereas the 'sprinting' muscles need not have this property. This difference in function is reflected in the composition of the muscles themselves.
