Excessive Exercise Training Impairs Mitochondrial Function

As exercise intensity increases, mitochondrial function and glucose regulation improve. Yet, some athletes report anecdotally that there is a training threshold, beyond which their performance diminishes. Veterinarians have noted the same phenomenon in racehorses. Objective measures to confirm and explain this paradox, however, have been lacking.

Investigators from Sweden performed repeated muscle biopsies and glucose tolerance testing on six healthy women and five healthy men as they participated in 3 weeks of progressive high-intensity interval training (HIIT), followed by a fourth “recovery” week, during which training was reduced. During the HIIT period, mitochondrial function improved progressively in all participants, as did physical p…