2. Internal Tissue Loading

The musculoskeletal system is the load bearing structure within vertebrate animals. Boney structures resist gravitational forces and maintain the body's shape. As such, bones are the primary load bearing tissue within the body. Forces applied to the body, including gravity, attempt to compress or bend the bones. Ligaments hold together the bony structure by crossing articulations where bones inter-connect. Ligaments also act as a pulley system by guiding tendons around articulations. Tendons are the connective tissues that attach muscle to bone and therefore transmit muscle forces to the skeletal system to produce voluntary movements and exertions. A consequence of force exerted by the body, or acting against the body, is that adjacent tissues are subjected to mechanical loads. These include cartilage, disc, bursa, and nerve. A detailed examination of how each of the tissues is subjected to mechanical loading follows.

2.1. Bone

When an individual performs a movement or exertion, forces are generated within the body to initiate and control it. The bones must resist tensile, compressive, shear, and torsional forces, in addition to bending moments. Bone is an adaptable tissue that acts according to Wolfs Law, which states that bone material is added where there is increased stress and bone material is resorbed where stresses on the tissue are reduced.

Relatively little emphasis has been placed on the injuries created by the repetitive loading of bone during occupational activities. Although, recent studies have shown that stress fractures in the lower extremities are not uncommon in new military recruits (Linenger and Shwayhat, 1992; Anderson, 1990; Giladi et al, 1985, Jordaan and Schwellnus, 1994). This suggest that the bone remodeling associated with Wolf s Law is a slow process and that the vigorous training that occurs during the initial weeks of boot camp does not allow this adaptation to occur. Others have reported that osteoarthritis (OA) in the hip and knees is more prevalent in individuals employed in occupations that experience greater loading of the lower extremity (Kohatsu and Schurman, 1990; Lindberg and Axmacher, 1988; Lindberg and Montgomery, 1987; Vingard et al., 1991). Anderson and Felson (1988) found a relationship between the frequency of knee bending and OA. These same authors also report that knee strength demands were also predictive of knee OA in women aged 55 to 64 years. Taken together, these studies begin to demonstrate the link between workplace activities and changes in bone tissues.

2.2. Ligaments and Connective Tissues

By their nature, as the connective tissues linking bones within the skeletal system, ligaments are primarily exposed to tensile loads. A typical stress strain curve for ligamentous tissue reveals that the tissue initially offers little resistance to elongation as it is stretched, however, once the resistance to elongation begins to increase, it does so very rapidly. Thus, the ligaments, while loosely linking the skeletal system, begin to resist motion as a joint's full range of motion is approached. Adams et al. (1980),by severing ligaments in cadaveric lumbar motion



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