How a joint works

The ends of two bones meet to form a joint, which is enveloped by a fibrous capsule lined by a synoviai membrane (synovium). The synovium manufactures thick, mucus-like synovial fluid, which lubricates, nourishes, and cleanses the cartilage on the ends of the bones. Joints are moved by muscles whose tendons attach to the bones. Flexor muscles bend joints (as in making a fist or bending the elbow or knee). Extensor muscles straighten joints (as in opening the hand or lengthening the elbow or knee). These muscles and their tendons provide important support for the joints, as do ligaments, dense fibrous bands connecting bone to bone.

Maintenance of joint health

To stay healthy, joints must do what they are supposed to do: move and bear weight. (“Weight-bearing” means that there is mechanical stress during active movement.) Since cartilage has no blood supply, it must be bathed by normal synovial fluid in order to receive oxygen and nutrients and to discharge its waste products. The thick, viscous synovial fluid also acts as a lubricant for the gliding surfaces, just as grease acts to lubricate moving metal surfaces in an engine. Movement stimulates the synovial membrane to produce synovial fluid. Therefore, the old concept that an injured joint must be rested is now understood to be incorrect.

Movement is crucial to induce the manufacture of synovial fluid, to distribute it over the cartilage and to circulate it through the entire joint space. In this way, movement helps cartilage to heal. If the synovium is unhealthy (as in the case of rheumatoid arthritis and related conditions), abnormal synovial fluid is produced which is less efficient at nourishing: cleansing, and lubricating the joint cartilage. Movement is vital for increasing circulation of blood and lymph into and out of the joint structures and adjacent soft tissues. This increased circulation reduces swelling, removes waste products, and increases delivery of nutrients and oxygen to facilitate healing of joint tissues.

Cartilage and bone require the stimulus of weight-bearing for normal metabolism and repair. Cartilage depends on the squeezing-releasing action of weight-bearing to force fluids containing waste out of its matrix and allow fresh fluid to enter. The electrical currents created in bone by weight-bearing are vital for stimulating bone re-modelling and repair — an ongoing process in which old bone is removed and replaced. Further, this weight-bearing stimulus induces bone to “hold on” to its calcium molecules. Without weight-bearing, bone loses excess calcium and becomes fragile and prone to fracture and collapse — a condition called osteoporosis.

Yoga

Running/walking is a weight bearing activity however it pounds and compresses the joint rather then moving it gently. Yoga therefore is vitally important to a runner/walker’s joints to help them gently move the joint nourishing it and helping to clean and lubricate the joint. Yoga is also considered a weight bearing activity so can help aid in the loss of calcium.