It’s often easy to overlook the effect that exercise has on the skeletal system because the bones and other associated skeletal organs are very much out of sight and out of mind. The skeletal system consists of the bones, ligaments that connect bones to other bones and cartilage that protects the bones from wear and tear. Exercise has a number of effects on the skeletal system both in the short and the long term.
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Increased Synovial Fluid Production
The bones and joints are avascular, that is, they have little or no blood supply. To keep joints healthy, stop cartilage from drying out and keep cartilage lubricated and nourished, the joints produce an oil-like substance called synovial fluid. According to "Sports Injuries: Their Prevention and Treatment, Third Edition" by Per Renstrom, synovial fluid is produced by the synovial membrane within the joints and is a short-term or acute response to exercise. This means that joints require regular exercise to stay lubricated, nourished and healthy.
Increased Joint Range of Movement
Exercise increases the production of synovial fluid, which keeps joints lubricated and makes them supple. Synovial fluid production increases the range of movement available at the joints in the short term. Often, after long periods of immobility, the joints “dry out,” stiffen up and lose some of their movement range. According to "Principles of Anatomy & Physiology, Ninth Edition" by Sandra R. Grabowski and Gerald J. Tortora, exercise increases the range of movement available at the joints as more lubricating synovial fluid is released into them. Mobility exercises such as arm circles and knee bends keep joints supple by ensuring a steady supply of synovial fluid.
Increased Bone Density
Weight-bearing exercise such as strength training and running put stress through your bones. In response to this stress, bodies produce cells called osteoblasts, which build new bone and make bones stronger and denser. Increased bone density can, say Grabowski and Tortora, prevent a condition called osteoporosis, which is the weakening of bone and an increased likelihood of suffering fractures. Osteoporosis is more common in older females but can affect either sex at any age.
The bones are held together with nonelastic avascular strap or cord-like structures called ligaments. Without ligaments, the joints would be very unstable and would probably bend the wrong way! When exposed to regular exercise, ligaments become stronger and more resistant to injury. Because ligaments have no or a very poor blood supply, any adaptations are very slow to develop.