Consisting of repeated, brief spurts of hard exercise interspersed with a few minutes of rest, [high-intensity interval training, or] HIIT workouts can be quite short but are still able to improve substantially our aerobic fitness and many other aspects of our health.
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[In a new study,] researchers at the Swedish School of Sport and Health Sciences and the Karolinska Institute, both in Stockholm, set out, like Goldilocks, to sample different amounts of weekly hard exercise and see if any might be just right.
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[After 3 weeks of HIIT 5 times per week,] the volunteers’ ability to generate power while cycling flattened, and their subsequent muscle biopsies showed sputtering mitochondria, each of which was now producing only about 60 percent as much energy as during the previous week. The riders’ blood-sugar control also slipped, with seesawing spikes and dips throughout the day.
Taken as a whole, the monthlong experiment suggests that “HIIT exercise should not be excessive if increased health is a desired outcome,” says [researcher] Mikael Flockhart…
The study was not focused on athletic performance, but even for serious athletes, he says, piling on multiple, intense, interval workouts each week, with little rest between them, is likely to lead to a tipping point, after which performance, as well as indicators of metabolic health, slip.
