High intensity interval training is all the exercise rave. It has legions of gung-ho devotees, especially those dedicated to being in top physical form. And now it appears that, if this activity is your thing, you better be.
Yet according to a study published in the Federation of American Societies for Experimental Biology Journal, scientists have discovered that the activity may do more harm than good, specifically for those who are not in prime condition.
“Our study raises questions about what the right dose and intensity of exercise for the average person really is,” says lead scientist Dr. Robert Boushel. “We need to be cautious about supporting sprint training in the general population.”
The literature suggests that this high-intensity activity — which involves repeated bouts of near maximal effort followed by short periods of recovery — increases mitochondria number and density, thus making the body's cells more efficient. This increases oxidative capacity — or the measure of how the body is able to use oxygen — which results in improved fitness. So, it’s curious as to why we would not recommend this type of exercise among those who seem to need it most.
Well, here's why: Researchers analyzed tissue samples from untrained subjects after a bout of upper- and lower-body high intensity interval training, also known as HIIT. Muscle biopsies were taken from the vastus lateralis (the largest muscle of the quadricep) and the triceps brachii. Interestingly, they found that mitochondria were “only firing at half-power post-training,” which means that after a bout of HIIT these non-athletes' capacity to consume oxygen was actually much less.
When mitochondria are working at a reduced capacity, high levels of free radicals (molecules with an unpaired electron) are known to circulate in the body. High levels of circulating free radicals “have been linked to a number of medical conditions, including cancer, premature aging and organ damage."
"If you're new to going to the gym, participating in high-intensity 'sprint' classes may increase your performance but might not be healthy for you," Dr. Boushel said. He recommends starting "slowly and gradually increase intensity over time." Seasoned athletes and those who are well trained, however, have built up the necessary "antioxidant enzymes in their bodies to protect against free radicals."
While HIIT has been shown to improve aerobic and anaerobic fitness, decrease blood pressure, improve insulin sensitivity and reduce body weight, while maintaining muscle mass, the increase in free radicals marks a dangerous side effect if one adopts this training too soon.
Again, untrained subjects showed less mitochondrial activity, and thus a weakened ability to fight off the free radicals that would not otherwise be present in trained individuals. And with the link between high levels of circulating free radicals and chronic disease clearly established, this is definitely something one should avoid.