segments, showed the supraspinous/interspinous ligaments segments are the first ligamentous tissues to become stressed with forward bending of the lumbar spine. Stability and movement of the spine or any other articulation within the low tensile region of the ligamentous stress-strain curve must be accomplished using muscular contraction. This is not to say that ligaments don't contribute to joint loading. Several authors have showed that with extreme flexion (forward bending) of the torso there is an electrical silence in the spinal musculature (Floyd and Silver, 1955; Golding, 1952; Kippers and Parker, 1984, Toussaint et al, 1995). This finding suggests that at times ligaments are used to resist the bending moments acting on the spine. The degree of ligamentous contribution to the forces placed on the inter-vertebral disc during manual material handling tasks has been debated in the scientific literature (Cholewicki and McGill, 1992; Dolan et al., 1994; Potvin et al., 1994). Nevertheless, there is consensus that ligaments are subjected to tensile stress with extreme movements, and hence, can contribute to the mechanical loads placed on the body's articulations including the inter-vertebral disc.
When ligaments act as a turning point for tendons (pulleys), they are exposed to shear forces and contact stresses. For example the transverse carpal ligament, in bridging the carpal bones in the wrist forms a pulley by which the path of the finger flexor tendons is altered when the wrist is flexed. Similarly, the palmer ligaments maintain the path of the tendons from the finger flexor muscles to the distal phalanges. Goldstein et al. (1987) showed that the tendon strain on the proximal side of the transverse carpal ligament was greater than the strain on the distal side of the ligament. This finding indicates that the friction between the tendon and the ligament results in the ligament being exposed to shear loads in addition to normal loads. Even though ligaments act as pulleys, the ligaments themselves are rarely the tissues damaged in work related musculoskeletal injuries. Instead, it is the tendons that experience the morphological changes which result in symptoms and injuries.
Tendons are a collagenous tissue that forms the link between muscle and bone. The orientation of the collagen fibers in tendons is in the form of parallel bundles (Chaffin and Andersson, 1991). This arrangement of fibers minimizes the stretch or creep in these tissues when subjected to tensile loading. With repeated loading tendons can become inflamed, particularly where the tendons wrap around bony or ligamentous structures. In more severe cases the collagen fibers can become separated and eventually pulverized wherein debris containing calcium salts creates further swelling and pain (Chaffin and Andersson, 1991).
Mechanical relationships between external forces, postures and internal tendon loading were demonstrated by Armstrong and Chaffin (1979) for the carpal tunnel of the wrist using the analogy of a pulley and a belt. A tendon sliding over a curved articular surface may be considered analogous to a belt wrapped around a pulley. That model reveals that the force per arc length F1, exerted on the trochlea is a function of the tendon tension Ft, the radius of curvature r, the coefficient of friction between the trochlea and the tendon m, and the included angle of pulley-belt contact q such that: