This is a great video explaining how the body builds lactic acid. He also explains the importance of stretching and movement. When your muscles are being strained and stressed from poor posture, or over exertion, it cannot convert glucose into energy for the muscle. As the glucose sits there and waits to be converted, it turns into pyruvate, which if not converted quick enough will turn into lactic acid. Lactic acid is toxic metabolic waste that can decrease circulation, which causes pain. Frequent massage and bodywork can help decrease and eliminate lactic acid build up.