flow necessary to supply oxygen to muscles during exercise. Smooth muscle also supplies the visceral organs in the gastrointestinal tract. Skeletal muscle will generate heat because of the breakdown of ATP. Whenever ATP is broken down, heat is produced. This heat is easily noticed during exercise, particularly when there is sustained muscle contraction. When there is extreme cold, muscles will shiver due to random skeletal muscle contractions. This shivering expends ATP energy and produces heat necessary to keep the body warm when it is challenged by a cold environment.
TYPES OF MUSCLE FIBERS There are three different types of muscle fibers in the human body. There are slow oxidative fibers or SO fibers that contract relatively slowly, using the process of aerobic respiration in order to produce ATP. There are also fast oxidative fibers or FO fibers that contract quickly and use both aerobic and anaerobic (glycolytic) respiration. These tend fatigue more quickly than SO fibers. Finally, there are fast glycolytic fibers or FG fibers that have fast contractions but use anaerobic glycolysis to gain energy. These muscles fatigue the fastest. You should know that humans contain different proportions of these types of muscles. The speed of contraction will depend on how quickly the ATPase associated with the myosin molecule can cross-bridge and reload. Fast fibers will hydrolyze ATP approximately twice as quickly as slow fibers, resulting in a much quicker cycling process. If a fiber uses ATP made by aerobic pathways, it is considered oxidative. This will be a fiber that is more resistant to fatig0ue. Glycolytic fibers will use ATP made by glycolysis, which does not require oxygen, work more quickly, and fatigue the fastest. The oxidative fibers will contain more mitochondria than glycolytic fibers because these organelles are the source of ATP energy used by these cells. On the other hand, these tend to be small diameter muscle cells that do not produce a large amount of tension. There is intense blood supply to these cells and a greater amount of myoglobin, which houses the oxygen necessary for these cells. This is why these fibers are called “red muscle.”
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