Any movement that stresses your joints can result in popping and cracking sounds. Lifting weights often causes the shoulder joints to make these noises. There is no definitive answer as to why this happens, according to Johns Hopkins Medicine's Orthopaedic Surgery unit, though two theories have been proposed. The buildup and release of gases in the synovial fluid around your joints is suspected to cause popping. The slip of tendons across joints has also been suggested as the cause of cracking sounds. Joint-cracking without pain is not typically indicative of a medical problem.
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Synovial fluid -- the substance that lubricates your joints -- is composed of oxygen, carbon dioxide and nitrogen. These gases can bubble out of the synovial fluid when negative pressure is applied to a joint, such as when you lift weights. A pop or crack can result from the escape of these gases as demonstrated in X-rays, according to Michigan State University researcher Raymond Brodeur, writing in "Scientific American." Once the gases are reabsorbed into the synovial fluid, they can again be expelled, creating more cracking sounds.
The action of the joints used in lifting weights might contribute to the cracking sounds you hear. Tendons or ligaments might be slipping in and out of place s you move your shoulders, similar to the twang of a rubber band bowstring. Eroded bone and stressed supportive joint tissues can increase the likelihood of cracking sounds while moving your shoulders beyond their normal range of motion, such as over your head, while you raise heavy weights.
If you have osteoarthritis -- which often results in worn-down or roughened joint and cartilage surfaces -- you might experience more cracking while lifting weights than people without this underlying condition. If the cracking noises are accompanied by pain, a decrease of flexibility or weakness in the affected joint, you might be experiencing a joint problem. Consult your doctor if any of these symptoms occur.
Joint-cracking is usually harmless, and, contrary to popular belief, has never been proven to cause or exacerbate arthritis in that joint, although some research shows that it can lead to minor swelling in the hands and a weakening of your grip, according to the Library of Congress's scientific facts pages. If you don’t feel pain during the cracking sounds resulting from lifting weights, you probably have nothing to worry about.