lled space called the synovial cavity. The entire complex is acted upon by muscles and ligaments. The muscles are attached to bone by their tendons and serve as the prime movers of the joint. While muscles can promote joint stability, this function however is primarily performed by the ligaments. The ligaments both "restrict joint motion at each joint" and "coordinate relative motion between joints" (6:442).
Both muscle tendons and ligaments are made of connective tissue. The characteristics of different connective tissues can be quite variable (8:58). The tissue may differ with regard to its density or, additionally, the spatial arrangement of its collagen fibers. Collagen is a fibrous protein which generally occurs embedded within a protein polysaccharide matrix. Collagen fibers tend to be of very high tensile strength (8:58). In fact, the range of motion in many joints is determined by the tension of their connective tissues.
Synovial joints generally have different degrees of freedom of movement (6:442). Besides being limited by connective tissue, range of motion can also be prevented by the direct contact of either adjoining bones or soft tissues. In addition, joint movement may depend upon whether a motion is active or passive. Active motions are those performed entirely by the subject; with passive motion, there is typically an application of some external force. In most circumstances, range of motion is greater with passive motion. Only in, for example, elbow extension, where motion is limited by bone to bone apposition are passive and active ROMs equal (6:446). Where muscles appear to be important for ROM, research has shown that it is actually the muscles' connective tissue which limits motion myofibrillar elements have little capacity for resisting tensile fo
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