Work-related musculoskeletal disorders arise from a complex interaction of events that may accumulate over time. In contrast to the acute trauma model, the cumulative trauma model assumes injury may result from the accumulated effect of transient external loads that may, in isolation, be insufficient to exceed internal tolerances of tissues. It is when this loading accumulates by repeated exposures, or exposures of sufficiently long duration, that the internal tolerances of tissues are eventually exceeded. The cumulative trauma model therefore explains why many musculoskeletal disorders are associated with work, because individuals often repeat actions (often many thousands of times) throughout the workday, or spend long periods of time (as much as eight hours or more daily) performing work activities in many occupations. Internal mechanical tolerance represents the ability of a structure to withstand loading. It is clearly multidimensional and is not considered a threshold but rather the capacity of tissues to prolong mechanical strain or fatigue. Internal tissue tolerances may themselves become lowered through repetitive or sustained loading.

A schematic diagram useful for elaborating the factors that can cause pain, discomfort, impairment, and disability is illustrated in Figure 1.2. External loads are produced in the physical work environment. These loads are transmitted through the biomechanics of the limbs and body to create internal loads on tissues and anatomical structures. Biomechanical factors include body position, exertions, forces, and motions. External loading also includes environmental factors whereby thermal or vibrational energy is transmitted to the body. Biomechanical loading is further affected by individual factors, such as anthropometry, strength, agility, dexterity, and other factors mediating the transmission of external loads to internal loads on anatomical structures of the body.

External loads are physical quantities that can be directly measured using various methodologies. External kinetic measurements, for example, include physical properties of the exertions (forces actually applied or created) that individuals make. These measurements have the