In two weeks, the riders extended the third session of HIIT, extending some of the time to eight minutes of drainage. Over the course of three weeks, they trained five times, pedaling for four minutes and eight minutes. Finally, during the fourth week of recovery, they effectively halved the amount and intensity of their exercise. Each week, the researchers repeated all the experiments.
Then they compared how people’s bodies changed from week to week.
At first, the findings were encouraging. Over the last two weeks, cyclists have been gaining strength և, seemingly getting better, with better control of their blood sugar in their muscle cells և more common mitochondria. Each of these mitochondria was now also more efficient, producing more energy than in the beginning.
But something went wrong during the third week. The volunteers’ ability to generate electricity while leveling their bicycles, Their subsequent muscle biopsies showed scattered mitochondria, each of which now produced only about 60 percent more energy than in the previous week. Cyclists’ blood sugar control also slipped. There were spinning spins all day.
After riding for less than a week, their mitochondria began to jump back, producing more energy, but still 25 percent less than in the second week. Their blood sugar levels also stabilized, but not as much as before. But cyclists could pedal at the same or even faster pace as in the second week.
In general, one month of experience suggests that “HIIT should not be overused if improving health is a desirable outcome,” said Michael Flockhart, a doctoral student at the Swedish School of Sport and Health Sciences. :
The study did not focus on sports activities, but even for serious athletes, he says, exercising repeatedly, with intense breaks each week, with little rest between them, is likely to lead to a turning point, after which performance as a sign of metabolic health also slips.
The researchers are not sure exactly what changes took place in their volunteers’ bodies and muscles during the third week. They have tested many potential molecular causes, says Mr. Flockhart, but do not isolate the obvious, lonely instigator. He and his colleagues suspect that during the most difficult week of exercise, the cascade of biochemical changes in human muscles overwhelmed the mitochondria, while the weakened mitochondria contributed to the failure of human blood sugar control.