Muscle fiber types determine endurance and strength
Not all skeletal muscle fibers are alike, and a single muscle contains more than one type of fiber. The two major types of skeletal muscle fibers differ in the properties of their myosin molecules, and these myosin variants have different rates of ATPase activity. Those with high ATPase activity can recycle their actin-myosin cross-bridges rapidly and are therefore called fast-twitch fibers. Slow-twitch fibers have lower ATPase activity, so they can develop tension more slowly and spread maintain it over a longer period of time.
Slow-twitch fibers are also called red muscle because they contain lots of the oxygen-binding molecule myoglobin, they have many mitochondria, and they are well supplied with blood vessels. These characteristics increase their capacity for oxidative metabolism. The maximum tension a slow-twitch fiber produces is low and develops slowly, but it is highly resistant to fatigue. Because slow-twitch fibers have substantial reserves of fuel (glycogen and fat), their abundant mitochondria can maintain steady, prolonged production of ATP as long as oxygen is available. Muscles with high proportions of slow-twitch fibers are good for long-term aerobic work (that is, work that requires lots of oxygen). Champion long-distance runners, cross-country skiers, swimmers, bicyclists, and other athletes whose activities require endurance have leg and arm muscles consisting mostly of slow-twitch fibers (Figure 47.8).
Fast-twitch fibers are also called white muscle because, in comparison to slow-twitch fibers, they have fewer mito
Percent slow-twitch muscle in body ( i i )
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.