Insanity! Burnout! Super revved-up workouts are all the rage at gyms and online. One big trend: high-intensity sprint training.

Long-favored by special forces elites and athletes like superstar wide-receiver Jerry Rice, it calls for a warmup, short bursts of high-intensity running for about 30 seconds, followed by a couple of minutes of downtime.

But not so fast! "Shredded" or "ripped" might make sense if you're preparing coleslaw, but unless you are a trained athlete, diving right into such over-the-top workouts can backfire.

A study of untrained men reveals that high-intensity workouts don't make weekend warriors healthier or stronger, or provide protection from heart disease and body-wide inflammation.

Instead, that kind of exercise increases levels of stress in muscles and lowers your ability to fight off damage from free radicals — molecules that can damage your DNA and are associated with increased risk of cancer, premature aging, and organ damage.

If you're an exercise newbie, sprint training makes your cells' powerhouse, the mitochondria, work at half capacity and have less ability to fight off damage from free radicals.

In contrast, over time, well-trained athletes build up antioxidant enzymes inside their cells to protect them from free radicals.

So if you're starting to build muscle, increase aerobic capacity, and lose weight, grab a buddy and a pedometer and starting a walking routine, heading for 10,000 steps a day.

Only go for high-intensity exercise after you've done muscle-building resistance training and increased your workout intensity gradually.