The extent to which skaters can twist themselves into a pretzel is determined in part by the arrangement of joints that connect the bones of the skeleton.
Types of Joints
Joints are classified in terms of their structure as fibrous, cartilaginous, or synovial, and in terms of their operation as immovable or movable. Fibrous and cartilaginous joints can be either immovable or movable, while all synovial joints are movable. Some immovable joints are totally rigid (fused together) while others have a small amount of give. In the latter case, think of the bones as held together by very stiff rubber bands. Normally the joint does not move, but under large forces the rubber bands can stretch a little.
Movable joints vary considerably in the range of motion they allow. Some are only slightly movable, others permit a wide range of motion. The range of motion of the movable joints determines the positions the human body can assume, and plays an important role in athletic activity.
The movable fibrous and cartilaginous joints come into play in skating in several ways. The movable fibrous joints of the arm are involved in pronation and suppination of the hand (palm facing forward or palm facing back) and are thus involved in arm positions. Similarly in the legs, movable fibrous joints are involved in positioning the foot and turning out the toe. Perhaps most importantly to skating, however, are the movable cartilaginous joints of the spine.
The spine is made up of 26 bones. The first 24 are individual vertebrae, while the last two consist of several (5, and 4) small vertebrae fused together. The bones of the spine are separated by pads of cartilage called discs. The discs act as cushions that prevent the bones of the spine from grinding together, and also as small shock absorbers. The bones of the spine are joined together by a number of ligaments. The position of the spine is stabilized (held in place) by the muscles of the back. Running through a small channel toward the rear of the vertebrae is the spinal cord.
The normal erect position of the spine has two main curves giving it a somewhat S shape; and two lesser curves at each end, one where the spine connects to the skull, the other where it connects to the pelvis. The motion of the spine comes into play in skating in several ways, most notably in the position of the back in spins, pair and dance lifts, death spirals, and some connecting moves. In stressful skating elements such as jumps and lifts, the back is held more or less in its erect position and is subject to considerable forces. Because the spine relies on the muscles of the back to hold it in position, conditioning of these muscles should be included in a well balanced weight training program for skaters.
When the back is over-stressed a number of injuries can result. These can be simple muscle or ligament strains, or more severe injuries to the disc. Discs can be pushed out of place, and if they press on nearby nerves can cause considerable pain. In extreme cases the discs can rupture. Many back injuries respond well to therapy alone, but in the worst cases discs may have to be removed, and the adjacent vertebrae fused together. Because cartilage does not have a direct blood supply, disc injuries are very slow in healing. Back injuries are more common among male singles skaters due to the pounding their backs take in landing triple and quad jumps, and among male pair skaters due to the added strain in lifts and other moves where the man supports the lady.
The most freely moving joints are the synovial joints. These are the joints that most people first think of when you talk about joints. Where bones come together in synovial joints, the end of each bone is covered with a layer of cartilage. Separating the cartilage covered bones is a fluid filled sack called the synovial capsule which is filled with synovial fluid. During movement, synovial fluid is squeezed out of the cartilage, and then reabsorbed when pressure is released - like water in and out of a sponge. this process served to lubricate the cartilage, and flush out debris. Cartilage is nourished by absorbing nutrients from the synovial fluid. The bones in synovial joints are held together by ligaments, and are stabilized by the muscle groups ending at the joints. Also located in the synovial joints are small pads of fat called bursae.
Synovial joints permit a wide range of motion, some more than others. The wider the range of motion in a synovial joint the more the joint relies on attached muscles for stability, and the more the joint is susceptible to dislocation. The most widely movable joints in the body are the shoulders, with the hips a not too distant second. Some gymnasts have a range of motion in their hips nearly as great as in their shoulders. Next in importance come the synovial joints of the elbows, knees, wrists, ankles, and neck. These are just the main ones, there are several others.
The stability of a joint is determined by three factors: the shape of the bones where they come together, the ligaments which join the bones together, and muscle tone. In some joints the shapes of the bones are well matched resulting in a very stable joint (e.g., the hips), in others the opposite is true. The more ligaments a joint has, the stronger it is, but joints that rely on ligaments for bracing are not very stable. Ligaments can stretch about 6% before they snap. When stretched, they tend to stay stretched, like taffy - one reason why stretching exercises lead to a sustained increased range of motion in a joint. For most joints, however, muscle tone is the main stabilizing factor - the shoulder and knee joints, for example, are primarily stabilized this way. Muscle tone keeps the tendons that attach the muscles to the bones taut, reinforcing the related joints; thus, conditioning of the muscles and susceptibility of some joints to injury are related.
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