People come in all sizes: weak and strong, fit and untrained, female and male, young and old. Ergonomic knowledge of human characteristics stems from several disciplines, such as: anthropometry, work physiology, biomechanics, industrial psychology, and sociology. The human body must maintain an energy balance between external demands, caused by the work and the work environment, and the capacity of internal body functions to produce that energy. Thus, the pulmonary system (lungs), the circulatory system (heart and blood vessels), and the metabolic system (energy conversion) establish central limitations of a person's ability to perform strenuous work. Models have been developed to describe the central and local limitations. Simplified for convenience, they may be grouped as follows: physiological models, muscle strength models, biomechanical models, and psychophysical models. Different capabilities can limit a person's capability to perform strenuous material handling: local limitations are often in muscles and joints; and central limitations are often in circulation and metabolism.