Lifting weights has a number of well-documented benefits, including increased strength and muscle size, greater muscular endurance, improved bone mass, and increased muscle mass. You can, however, have too much of a good thing. The National Association of Strength and Conditioning warns that lifting weights every day, or too many days in a row without a break, can cause a number of problems and may lead to a collection of symptoms commonly referred to as overtraining syndrome.
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Poor Training Progress
Weightlifting causes catabolism, or muscle breakdown. After your workout, your body goes through a process of repair and growth so that, when you repeat your workout a few days later, your muscles are bigger and stronger than before. This is called anabolism, and only occurs when you rest. If you exercise too frequently, your body will not have time to adapt to your workouts and, as a result, you progress may be very slow, or even nonexistent.
Your muscles are anchored onto your bones using tendons, non-elastic cords of connective tissue that transmit the forces of your muscles to your bones so your limbs will move. Tendons have a very poor blood supply, and are subsequently slower to heal and repair than muscles. Tendons are prone to becoming inflamed if exposed to too much, or too frequent, exercise. This inflammation is called tendinitis, which can be very painful, and if not allowed to heal, may require treatment such as complete rest, deep -- and painful -- massage, steroid injections or surgery.
A joint describes the meeting of two bones. The ends of the bones are protected with a tough, thin, slippery material called hyaline cartilage. Like tendons, hyaline cartilage has a very poor blood supply and, if damaged, will take a very long time to repair, if it heals at all. Overly frequent workouts using repetitive exercises will cause wear and tear to the hyaline cartilage and, once worn, may result in osteoarthritis and joint pain. Once a joint has become arthritic, it will only deteriorate further, becoming less functional and more painful as time passes -- and might ultimately require replacement through surgery.
Impaired Immune System
Exercise is a form of stress to which the body will attempt to adapt. Increases in strength, power, or muscle size are all positive adaptations to the stress of exercise. However, like mental or emotional stress, there is only so much exercise stress you body can cope with before it begins to break down. Working out every day will cause an accumulation of stresses to which the body will be unable to adapt. One of the symptoms of over-training/under-resting is immune-system impairment. As your body attempts to deal with the stress of exercise, its recovery resources will become limited and, as a result, you are likely to fall ill more often, and be ill for longer than normal. An impaired immune system is likely to leave you with feelings of fatigue and lethargy, weight and strength loss, and lack of enthusiasm for working out.