Muscular endurance is generally defined as the capability of either one muscle or group of muscles to contract multiple times under sub-maximal loads by means of energy reproduction within the cell in the presence of oxygen. Training to increase muscular endurance is fairly simple in both theory and application, and is often employed by athletes of all kinds. It is important to note the difference between cardiac, respiratory, and muscular endurance because, while all three are intimately related, they are very different systems.
Muscles possess the ability to use a a few different substances as energy, including adenosine triphosphate (ATP), glucose, fat, protein, and lactic acid. When a muscle is first called into action and begins to contract, it quickly converts stored glycogen into glucose, which provides a quick, powerful burst of energy that powers the muscle for roughly 30 seconds. At this point, if the muscle is still required to contract, it uses aerobic glycolysis to turn glycogen into ATP, which will fuel the muscle for the remainder of the exercise period. It is important to note, however, that after all of the creatine phosphate and glycogen stores have been used, the muscle will start metabolizing fat, and eventually muscle fiber itself for energy. Only the cardiac muscle fibers of the heart can readily and efficiently use lactic acid as a form of energy.
Muscle endurance is beneficial on the most basic level because it allows skeletal muscle to contract for extended periods of time. Without muscular endurance, humans and animals would not be able to run or walk for more than about 30 seconds. The ability to quickly and efficiently create and use energy in individual cells is the only way that humans are able to survive, because even if we did not move for more than 30 seconds at a time, our heart and lungs have to continue beating and pumping 24 hours a day.
In order to increase muscular endurance, both the amount of ATP uptake and the rate of lactid acid removal must increase. In order to increase ATP uptake, one or both of the following must happen — either the muscle itself must grow bigger, or the mitochondria must supply more ATP to the smaller muscle fibers. In either case, the NAD+ carrier molecules must remove more lactic acid.
In order to increase the size of a muscle, a training regimen must be designed and implemented that forces the muscle to exert near maximal force for as many repetitions as possible to induce muscle hypertrophy, such as in weightlifting. In order to improve ATP synthesis, both an increase in mitochondria count and efficiency must take place. These may both be achieved by forcing the muscle to contract for ever-extending periods of time, and simultaneously at an ever-increasing intensity, such as during marathon running.
