I rest on regular sets not 5x5 and for a total of at least 2 min. to let the ATP back into the muscles.
BIOG 1105-1106 | Cornell Introductory Biology, Individualized Instruction
Muscle Contraction and ATP
Muscle Contraction Requires Large Amounts of ATP Energy
The energy for muscle contraction comes from ATP, which in turn comes from the metabolism of glucose and fatty acids. But so little ATP is actually stored in the muscles that just a few twitches could quickly exhaust the supply. How do muscles overcome this limitation? Although there is little ATP, there is another phosphate compound stored in the muscles, creatine phosphate, which is formed by linkage of a phosphate group to the substance creatine. Creatine phosphate cannot be used directly to power muscle contraction, but it can transfer its phosphate group to ADP to form ATP:
Creatine phosphate + ADP + H+ ---> Creatine + ATP
The newly formed ATP then acts as the direct energy source for contraction. The muscle stores enough creatine phosphate to enable it to contract strongly during the several seconds it takes before the machinery of glycolysis and cellular respiration can produce additional ATP.
If the demands on the muscles are not great, much of the energy used to replenish the supply of creatine phosphate and ATP may come from the complete oxidation of glucose and/or fatty acids to carbon dioxide and water, which requires oxygen. During the unavoidable delay before adjustments of the gas-exchange and circulatory systems increase the oxygen supply to the active muscles, some of the oxygen necessary for aerobic respiration in red muscles may come from oxygenated myoglobin. Myoglobin is a special oxygen-storage protein in muscle. Like hemoglobin, it forms a loose combination with oxygen while the oxygen supply is plentiful, and stores it until the demand for oxygen increases. Consequently, muscle has its own built-in oxygen supply.
But during rigorous muscular activity, such as strenuous exercise or the lifting of a very heavy object, the energy demands of the muscles (especially white muscles) are great (see Table below) and the oxygen from myoglobin is quickly used up. Because sufficient oxygen cannot be gotten to the tissues fast enough, the muscles obtain the extra energy they need from anaerobic processes. This is accomplished by producing lactic acid through fermentation, and incurring what physiologists call an oxygen debt. Some of the lactic acid accumulates in the muscles, but much of it diffuses into the muscle capillaries and is transported in the blood to the liver. When the rigorous activity is over, a period of hard breathing or panting helps supply the liver with the large quantities of oxygen it requires for aerobic respiration (see figure below), thereby paying back the oxygen debt. In the liver, the lactic acid is converted back into pyruvic acid, most of which is oxidized to carbon dioxide and water. The ATP energy thus obtained is used to replace the ATP and creatine phosphate stores, and to synthesize glucose and glycogen from the remaining lactic acid. Note that it is the liver cells, not the muscle cells, where lactic acid is reconverted into pyruvic acid.
Lactic acid, precisely because it is an acid, can damage the muscle fibers if it is not removed promptly. This is why a “cool down” period is so important after strenuous exercise. The continued circulation of blood through the muscle aids in lactic acid removal. “Sore” muscles are the result of damage to the muscle proteins due to lactic acid accumulation. Endurance-training programs for athletes are designed to increase the oxygen availability in muscles and thereby encourage aerobic metabolism. During such training, the number of mitochondria within the muscle fibers increases, the stores of myoglobin enlarge, and the growth of new blood capillaries within the muscle is stimulated, thereby increasing blood flow through the muscle. As a result, trained athletes are capable of carrying out more strenuous activity without greatly increasing their lactic acid production and accumulation.
Muscle spasms and cramps result from involuntary strong contractions of a muscle. The cramp or spasm is accompanied by sudden pain, which probably results from mechanically stimulating pain receptors within the muscle or from compressing the blood vessels and interfering with the delivery of oxygen to the fibers. Most cramps and spasms will clear up of their own accord within a few minutes. Muscle cramps are sometimes associated with a calcium deficiency, especially in pregnant women, but this does not appear to be true in all cases. The mechanism of cramping is still not understood.
