In the second week, the riders added a third HIIT session and increased the length of some of their intervals to eight minutes. In the third week, they trained five times with a mix of four- and eight-minute jumps. Finally, on week four, they effectively halved the amount and intensity of their exercise to recover. The researchers repeated all of the tests every week.
Then they compared how people’s bodies had changed week by week.
The results were encouraging at first. By the end of the second week, the riders pedaled harder and appeared to be getting fitter, with better daily blood sugar control and more total mitochondria in their muscle cells. Each of these mitochondria were also now more efficient and producing more energy than when they started.
But by the third week something started to go wrong. The volunteers’ ability to generate electricity while cycling was flattened, and their subsequent muscle biopsies revealed sputtering mitochondria, each of which was producing only about 60 percent as much energy as the previous week. Drivers’ blood sugar control levels also slipped, with bobbing peaks and dips throughout the day.
After a week of riding at lower intensity, her mitochondria started popping up again and producing more energy, but still 25 percent less than the second week. Her blood sugar level also stabilized, but not to the same extent as before. However, the riders were able to pedal with the same – or even greater – force as in week two.
Overall, the month-long experiment suggests that “HIIT training shouldn’t be excessive if health improvement is desired,” says Mikael Flockhart, a PhD student at the Swedish School of Sports and Health Sciences who conducted the study with his advisor , Filip Larsen and others.
The study didn’t focus on athletic performance, but even for serious athletes, he says, stacking multiple high-intensity interval workouts weekly with little rest between them likely leads to a tipping point after which performance, as indicators of metabolic health, also begins to slide.
The researchers aren’t sure what changes in their volunteers’ bodies and muscles caused the negative results at week three. They tested several possible molecular causes, says Flockhart, but didn’t isolate an obvious, single instigator. He and his colleagues suggest that a cascade of biochemical changes in people’s muscles during the toughest week of exercise overwhelmed the mitochondria, and the weakened mitochondria contributed to disruptions in people’s blood sugar control.