The body has three ways of producing ATP: from creatine phosphate, anaerobic cellular respiration and aerobic cellular respiration. (Jenkins et all, 2007) At rest, the muscle fiber produce more ATP than is required by the body. This excess ATP is used to synthesize creatine phosphate – an energy rich molecule found only in the muscle fiber. Creatine is a small molecule that muscle cells assemble from fragments of amino acids. (Martini, 2008) The enzyme creatine kinase catalyzes muscles fibers to break down excess ATP and transfer a phosphate group to creatine, forming creatine phosphate and ADP. (Jenkins et all, 2007) During contraction, muscle fibers transfer the phosphate group from creatine phosphate to ADP, forming ATP (Jenkins et all, 2007, pg. 290). Together, the ATP levels and creatine phosphate levels are called the creatine phosphate system. The creatine phosphate system can supply the energy needs of working muscle at a high rate, but only for 8 to 10 seconds.
After the initial 8-10 seconds (Craig Freudenrich, Ph.D., How exercise works?), when the supply of creatine phosphate is depleted (Jenkins et all, 2007), the muscle depends on the complex carbohydrate reserves stores in the muscle tissue called glycogen. Glycogen is a chain of glucose molecules. A cell splits glycogen into glucose (Freudenrich, Ph.D., 2010). This glucose passes easily from the blood into contracting muscles fibers and a series of reactions known as glycolysis quickly breaks down each glucose molecule into two molecules of pyruvic acid (Jenkins et all, 2007) in the cytoplasm of a cell (Martini, 2008). This process of glycolysis uses two ATP but produces four ATP for a net gain of two (Jenkins et all, 2007). In the absence of oxygen, most the pyruvic acid is converted into lactic acid, which ideally in the presence of oxygen would have entered the mitochondria and facilitated in the production of a large amount of ATP. The presence of lactic acid in the muscles is what makes our muscles hurt, causes muscle fatigue and soreness. Hence it is essential to warm-up before starting an exercise regime. This process of producing ATP anaerobically – without oxygen- is called Anaerobic Cellular Respiration (Abrahams, 2007). The blood then removes this lactic acid from skeletal muscle and carries much of it to the liver for reconversion into glucose (Jenkins et all, 2007). This system produces just enough ATP anaerobically to last about 90 second and hence provides the heart and lungs some time to get their act together. It is also handy because the rapidly contracting muscle squeezes off its own blood vessels, depriving itself of oxygen-rich blood. (Freudenrich, Ph.D., 2010)
When oxygen is available after about two minutes of exercise, the pyruvic acid molecules from the glycolysis enter the mitochondria, where aerobic cellular respiration completely oxidized glucose for a total of 36 ATP molecules from one glucose molecule that has entered glycolysis. This process is called the aerobic cellular response. This process of aerobic cellular respiration provides enough ATP for prolonged activity as long as sufficient oxygen and nutrients are available. Therefore for any activity longer than 10 minutes, the aerobic cellular respiratory system provides most of the ATP and for longer endurance activities such as a marathon race; all of the ATP requirements are met by this system (Jenkins et all, 2007).
Aerobic respiration can also use fatty acids from fat reserves in muscle and the body to produce ATP. In extreme cases (like starvation), proteins can also be broken down into amino acids and used to make ATP. Aerobic respiration would use carbohydrates first, then fats and finally proteins, if necessary. Aerobic respiration takes even more chemical reactions to produce ATP than either of the above systems. Aerobic respiration produces ATP at the slowest rate of the three systems, but it can continue to supply ATP for several hours or longer, so long as the fuel supply lasts (Freudenrich, Ph.D., 2010).
In summary, the sliding filament action of the actin and myosin fibers fueled by ATP facilitates muscle contraction and expansion. The aerobic cellular respiration is the most effective way of producing ATP since it provides for 36 ATP molecules from one glucose molecule that had entered glycolysis. There is also no lactic acid build up in muscle allowing us to exercise without muscle soreness or cramps.