From 400-meter track repeats to CrossFit-style weight circuits, high-intensity interval training has blown up at gyms across America — and for good reason. There’s now ample proof that interspersing bouts of all-out effort with active recovery can help you get fit fast. But according to new research, there’s a caveat: If you’re not already fit, it may do your body more harm than good.
Twelve healthy, moderately active but untrained men knocked out a series of sprint interval training, or SIT, which uses explosive 30- to 40-second bursts to boost athletic performance. They put in just a few minutes a day with sessions on regular bikes and arm bikes over a two-week period. Afterward, the researchers analyzed the men’s muscle tissue to see what had happened at the cellular level. It didn’t look good. Their mitochondria, the muscles’ energy sparkplugs, were not firing properly, limiting their ability to scavenge the cell-damaging free radicals churned up by intense exercise.
“Although SIT is very effective at improving performance in trained athletes, our study shows it also produces a high concentration of free radicals,” says Robert Boushel, senior study author and director of the University of British Columbia’s School of Kinesiology. “If you’re untrained, this causes oxidative stress and suppresses muscle mitochondria.”
Additionally, in well-trained athletes, SIT increases antioxidant enzymes in the muscle that also blast free radicals. But the guys in this study had not built up enough of these enzymes to counteract the oxidative damage triggered by high-intensity sprint training. Whether these negative effects will have long-term health ramifications is unknown. However, unchecked free radicals can potentially cause you to age faster and open the door to disease.
It’s better to be on the safe side. “If you are a beginner at exercise training,” says Boushel, “we recommend you build up training intensity gradually over several weeks before starting SIT.” In other words, make sure you can knock out a full intense cardio session before you try condensing 30 minutes of exercise into five.