Warning, these workouts can kill! Acute exertional rhabdomyolysis is a serious clinical illness caused by an injury to skeletal muscle that results in the release of myoglobin and other cellular contents, including creatine kinase (CK-MM fraction) and aldolase, into the circulatory system. It can kill. Several cases of Rhabdo have been associated with CrossFit workouts. Only newcomers have been affected and poor fitness has NOT been a factor. We cannot emphasize enough the importance and safety of starting slowly: http://www.crossfit.com/cf-info/start-how.html.