This book discusses the architecture, functioning, and biomechanics of the human body, its bones, joints, muscles, tendons, and ligaments. The book explains energy extraction from food and drink, what efforts the body is capable of, and how our efforts depend on the coordination among the respiratory, circulatory, and metabolic systems. This text shows how the body monitors itself, how it reacts to work loads and the environment such as heat or cold, humidity and wind. The book also explains how to measure a person's ability to work at high efficiency: by observation of breathing rate, heart beat frequency, oxygen consumption, and by careful evaluation of subjective judgements.The text discusses, in practical terms, effects of environmental conditions and how shift work arrangements during day, evening, and night affect task performance.