High-intensity workouts such as P90X can burn more calories faster, but health and fitness experts say there are risks, especially for people who are very overweight or who are not used to that level of exercise. One P90X user was hospitalized with rhabdomyolysis, which happens when muscle tissue breaks down and releases toxins, and orthopedic surgeon Dr. Stephen Fealy said he sees at least one person each week whose high-intensity workout has led to injuries.

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