Work physiology is the science that studies how the human body responds to the physical stress of work or activity demands. These physiological responses are important in maintaining homeostasis in the body during work activities and reducing the adverse effects of physiological fatigue due to work. Homeostasis is defined as the maintenance of a constant or changing environment. In practical terms, it refers to the relatively constant internal environment of the human body during both stressed and relaxed conditions, due to many regulating anatomical and physiological systems. These organ systems and physiological responses regulate cellular metabolism, energy production, cellular waste product removal, voluntary muscle control, and the flow of blood and oxygen to working muscles. An understanding of the role of major organ systems in the human body during work activities and the relationships between work intensity and recovery intervals is essential to the science of ergonomics.
To accomplish work, the body requires energy, oxygen and nutrients. The human body consumes and uses carbohydrate, fat and protein nutrients to provide the required energy to maintain homeostasis both at rest and during work activity. During work, the primary nutrients utilized are fats and carbohydrates, with proteins contributing less than 5-15% of the total energy used. These nutrients, after having been converted to chemicals, enter the blood stream and circulate to the various internal organs and muscles. At the muscle sites, this chemical energy is converted into mechanical energy, or a muscle contraction, and heat. This process is known as metabolism.
Working muscle requires a constant supply of energy. The fundamental source of energy for these contractions is the high-energy Adenosine Triphosphate (ATP) molecule. The ATP molecule is the most important energy carrying molecule in the muscle cell. The ATP compound consists of three parts: adenosine molecule, a ribose molecule and three phosphate molecules linked together by chemical bonds. The bonds linking the phosphate molecules are high-energy bonds and when these bonds are broken, large amounts of energy are released. This energy is then used for muscle contractions. The energy can be liberated from the ATP molecule by a process known as phosphorylation. This metabolic process is shown below. Phosphorylation is the process in which the Adenosine Triphosphate molecule is broken down by the enzyme ATPase into Adenosine Diphosphate (ADP), a phosphate molecule (Pi) and energy.
The Adenosine Triphosphate needed for muscle work can be produced from either aerobic (with oxygen) metabolism or from anaerobic (without oxygen) metabolism. The aerobic metabolism of nutrients refers to the oxidation of glucose or glycogen molecules and fatty acids to form ATP, this process is called aerobic glycolysis. This metabolic pathway requires a continuous supply of blood in order to provide ongoing oxygen and nutrients.