Tissue Loads

When forces are applied to a material, like human musculoskeletal tissues, they create loads. Engineers use various names to describe how loads tend to change the shape of a material. These include the principal or axial loadings of compression, tension, and shear (Figure 4.1). Compression is when an external force tends to squeeze the molecules of a material together. Tension is when the load acts to stretch or pull apart the material. For example, the weight of a body tends to compress the foot against the ground in the stance phase of running, which is resisted by tensile loading of the plantar fascia and the longitudinal ligament in the foot. Shear is a right-angle loading acting in opposite directions. A trainer creates a shearing load across athletic tape with scissor blades or their fingers when they tear the tape. Note that loads are not vectors (individual forces) acting in one direction, but are illustrated by two arrows (Figure 4.1) to show that the load results from forces from both directions.

When many forces are acting on a body they can combine to create combined loads called torsion and bending (Figure 4.2). In bending one side of the material is loaded in compression while the other side experiences tensile loading. When a person is in single support in walking (essentially a one-legged chair), the femur experiences bending loading. The medial aspect of the femur is in compression while the lateral aspect is in tension.

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