This document defines key terms related to energy, work, and power in exercise physiology. It identifies potential, chemical, and kinetic energy and explains that energy can exist in three forms: potential, chemical, and kinetic. Chemical energy stored in the body includes ATP, carbohydrates, and fats. Only a small portion of the total energy from food is converted to useful work, which can be measured using direct calorimetry to measure heat production. Chemical reactions can be endothermic, requiring energy input, or exothermic, releasing energy like the breakdown of ATP. ATP is the immediate form of stored energy in the body but the total amount is only enough to last 2-3 seconds of exercise before it must be