One of the hottest workouts is causing a lot of injuries. It’s high intensity interval training, HIITs, where you alternate between vigorous exercise and brief rest periods. The trend has exploded over the last few years. 

The P90X and Insanity workouts have sold over six million DVDs combined. And Cross Fit has more than 5,000 gyms across North America. People are hooked because high-intensity exercise blasts fat fast. In fact, a study found that people in a 20-minute HIITs program lost six times more body fat than those who followed a 40-minute moderate-intensity cardio program. And in an Auburn University study, a HIITs-style workout burned twice the calories of the typical cardio program. 
The only problem? HIITs exercises were created for super fitness fanatics; it’s like the graduate school program of workouts. And major problems can pop up when couch potatoes or extremely overweight people try it because it’s too much too soon. 
Dr. Stephen Fealy is an orthopedic surgeon with the Hospital for Special Surgery in New York. He says he’s seeing at least one patient a week who comes in with injuries from high-intensity interval training. The most common injuries are severe muscle and tendon strains. 
But symptoms can be even more serious. There are internet forums full of people reporting extreme injuries, like one moderately fit man in his 20s who did one session of P90X and suddenly couldn’t lift his arms and his urine was the color of cola. He was rushed to the ER and diagnosed with rhabdomyolysis. That’s when muscle tissue breaks down and releases toxins into the blood stream that damage your kidneys. 
So before you try any high intensity workout, talk with your doctor first to see if you’re a good candidate. And if you’re not, start slowly with a beginner level program.