Biological joints come in many types and forms. Some allow movement to different degrees, some do not. Synovial joints like the one shown in section alongside allow free movement. The ends of the articulating bones are covered with cartilage, a smooth resilient material which facilitates movement at the joint. The joint surfaces are lubricated by synovial fluid, shown in blue in this diagram. A capsule surrounds the joint and retains the synovial fluid.

The ligament shown in the diagram to the left is the ulnar collateral ligament on the inside (medial aspect) of the elbow joint. Although the joint between the humerus and ulna is a hinge joint allowing rotation around only one axis, the collagen bundles within the ligament have a complex three-dimensional arrangement. This is to ensure that the required degree of tension is achieved at each position of the joint during flexion and extension. A simplified arrangement has been used for the joints of the adaptable mast.