The nuclei are pushed aside by long ribbonlike organelles, the myofibrils, which nearly fill the cytoplasm. Many oval nuclei can be seen just beneath the plasma membrane, which is called the sarcolemma in muscle cells. The fourth function of muscle, generation of body heat, is a by-product of muscle activity as ATP is used to power muscle contraction, nearly three-quarters of its energy escape as heat and this heat is vital in maintaining normal body temperature. As the skeletal muscles pull on bones to cause movements, they also stabilize the joints of the skeleton muscle tendons are extremely important in reinforcing and stabilizing joints that have poorly fitting articulating surfaces. We are rarely aware of the skeletal muscles that maintain body posture, yet they function almost continuously, making one tiny adjustment after another so that we can maintain an erect or seated posture despite the never-ending downward pull of gravity. Mobility of the body as a whole reflects the activity of the skeletal muscles, which are responsible for all locomotion they enable us to respond quickly to changes in the external environment. Producing movement is a common function of all muscle types, but skeletal muscle plays three other important roles in the body as well.
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