There's no problem with high-intensity training, per se. But you have to prepare for it. And many of those participating in these programs are extremely fit men and women. However, if you don't move around all that much, suddenly jumping, lifting, and squatting in quick rotation could really hurt. Before starting a high-octane workout, Zimmerman recommends a 90-day period to establish a "baseline" of strength and mobility. Yoga, in particular, is a good starting point, he says, for "getting your body acclimated to moving in ways that you're not familiar with, but in a slow, controlled manner."
"In general, people who slowly increase an exercise program don't run into major injury issues—those who jump right in and do too much have trouble," says Amy Powell, team physician at the University of Utah, where she teaches in the departments of medicine and orthopaedics. She notes that CrossFit actually uses a "ramp-up" orientation program to prepare enthusiasts for class, and the exercises can always be modified."
You may also consider meeting with a personal trainer to determine your fitness level and potential deficiencies before embarking on a high-intensity workout program, Bryant says, and advises paying attention to any signs of physical distress. Calling the "no pain, no gain" mentality "the biggest myth going," Bryant says that "pain is the body's definite, clear way of telling you to back off." Other signs of overdoing it are increased rates of sickness and fatigue. And if you're still feeling exhausted more than two hours after your workout, "you're probably training beyond your physiological capabilities." Also, stay attuned to your resting heart rate when you wake up, he says. A 20 to 25 percent increase indicates overtraining.
[See Getting Paid to Stay Fit]