The joints of the human body, like those used in the construction of buildings, furniture, and machines, connect different segments together and often allow movement between those segments. The design of the joint reflects these demands. The dictum form follows function, coined by the American architect Louis Sullivan and promoted by the Bauhaus school of design of post–World War I Germany,1 suggests that the appearance (form) of an object or building is governed by its function. Although a wooden stool and a chair with a padded seat, arms, and back are both for sitting, the differences in form hint at the intended functions, respectively, of short-term versus longer-term support. The function of a step stool is to support weight so the elements are joined to serve stability and can be made of a sturdy material such as a heavy wood. However, if the step stool needs to be portable and collapsible while still stable when opened, the design and material composition will become much more complex. An aluminum stepladder is lighter and has a metal strut with a locking device as well as hinges at the intersection of the two sides of the ladder. The ladder can be folded when the brace is unlocked; when the brace is locked, the ladder is still stable (Fig. 2–1). If the ladder is opened but the brace is not locked, stability is compromised by excessive mobility. Human joints that must meet both stability and mobility demands are more complex than joints that favor stability. The more complex the demands on and design of a human joint, the more stresses the joint must withstand and the more at risk the joint is for failure.
Folding stepladder joint. A. The sides of the stepladder are free to move (mobile) when the brace is unlocked. B. The sides of the stepladder are prevented from moving when the brace is engaged, providing necessary stability.
Form includes an object’s structure as well as its composition, and a change in either structure or materials (or both) can influence function. The prosthetic foot shown in Figure 2–2 reflects the relationship between physical form (structure and composition) and function. The design of the curved foot allows the structure to bend as body weight is applied whereas the heel extension provides stable support. The bending of the foot can both absorb shock and provide an elastic recoil that contributes to the subsequent propulsion. Alterations in the material (usually a carbon fiber composite) affect the amount of elastic recoil. A person with a lower extremity amputation can order limbs made of different materials, depending on their functional demands. Springier materials that are harder to stretch and have more recoil are used for activities with large loads and high speeds (e.g., running, jumping), whereas materials with ...