The 100 rep belt squat set pushes both the aerobic and anaerobic energy-producing systems to their limits. Cells contain enough ATP to last for about two seconds, so stored ATP would be exhausted after the first rep. This is where the phosphagen system begins operation, as (adenosine triphosphate) ATP must be replenished. As ATP is broken down to supply energy, a phosphate group is split off to form ADP (adenosine diphosphate) and a free phosphate group. Creatine phosphate is able to donate its phosphate group to ADP to regenerate ATP. The second system to come into play is the lactic acid system, also known as the glycolytic pathway. Anaerobic glycolysis is the pathway used to convert carbs into ATP in the absence of oxygen. This is a rapid way to produce energy, but is not very efficient. The third system called into play is the aerobic energy-producing system. This is where oxygen is used for the burning of carbs and fat to produce energy. The first 20 reps should pass fairly easily because during that time all of the energy systems will be working at capacity to provide enough energy to support the activity. After that, however, a decrease in energy will become noticeable as the phosphagen and lactic acid systems begin to deplete. The aerobic system then becomes the primary energy source. This pathway is able to supply energy for long-term demands, even for hours. The rate of energy production by this pathway is limited by the vascular (which limits oxygen delivery) and mitochondria. At 50 reps, the anaerobic systems will be completely shut down and the aerobic system will be working extremely hard to keep up with energy demands. After 60 reps, all of the energy-producing systems will be depleted and the majority of work done from here on out will be by the spotters as they assist you in lifting the weight. One other energy-producing system that doesnít play much of a role in belt squats is the glucose-alanine cycle. During long-term aerobic activity, after glycogen stores and blood glucose are depleted, the body begins to break down muscle tissue into free amino acids, which are added to pyruvate to form alanine. Alanine is then transported to the liver where it is converted to glucose which is then transported back to the muscle to be used as energy. In addition the branched-chain amino acids leucine, isoleucine and valine can be used directly as fuel by the muscles.