As with the bones, the shape of the joints reflects their function (and their function reflects their shape). Joints come in a spectrum of shapes, depending on the mobility or stability they require. For example, the hip joint is a ball and socket while the knee joint is a hinge. A ball and socket type hip joint confers the greatest mobility in all planes and is useful for activities such changing direction while walking and running (or reaching in various directions to grasp objects, as with the shoulder). A hinge type knee joint provides greater stability and is useful for propelling the body forward (or drawing an object towards the body, as with the elbow).
Other joints such as the intervertebral between the vertebrae allow for limited mobility between individual vertebrae but great stability to protect the spinal cord. Mobility of the spinal column comes from combining the limited movement of individual intervertebral joints as a whole.
ball and socket
knee f if r
hip joint capsule with synovium (posterior view)
Articular cartilage covers the joint surfaces, allowing smooth gliding of one bone over the other. In fact, articular cartilage is one of the smoothest surfaces known to man. Applying excessive pressure to this fragile cartilage can injure it, ultimately resulting in arthritis.
The meniscus deepens the articular surface and broadens the contact area of the joint. This aids to stabilize the joint and distributes the force of gravity and muscular contraction over a greater surface area. The meniscus is composed of fibro-cartilage, giving it a flexible rubbery consistency.
Articular Structure hip articular cartilage
The joint capsule is connective tissue sheathing that surrounds and seals synovialjoints. It is susceptible to stretch injury when executing extreme movements in yoga postures.
Synovial tissue lines the inside of the joint capsule. This tissue produces synovial fluid, a viscous lubricant for the joint surface that decreases friction during joint movement. Synovial fluid circulates throughout the joint, transporting nutrients to the articular cartilage and removing debris from the joint space. The various contortions resulting from yoga postures aid fLex and expand thejoint capsule, stimulating circulation of synovial fluid.
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