•Energy pathways used by your muscles are determined by the levels of activity that the muscles are engaged in. At rest, supplies of glucose are converted to glycogen and stored in the muscles. During moderate activity the need for ATP increases and the body turns the stored glycogen back to glucose which, along with oxygen, is then used in the process of cellular respiration to produce energy.
• During a high activity exercise such as running, the oxygen demands for the body increases. When a person has finished running, you can see them take many deep breaths. This happens in order to bring the oxygen levels in the body back to normal.
If the body's supply of oxygen is low, the cells perform anaerobic respiration which produces lactic acid which produces a limited amount of energy (ATP) and eventually the lactic acid will build up in the muscle causing soreness and fatigue. Anaerobic respiration is not as efficient as aerobic respiration is providing energy, however it is an excellent alternative way for the body to produce energy when oxygen is limited and the mitochondria cannnot keep up with the demands.