Muscle IMP accumulation during fatiguing submaximal exercise in endurance trained and untrained men

To examine the effect of training status on muscle metabolism during exercise, seven endurance-trained [peak oxygen uptake (V̇O(2peak)) = 65.8 ± 2.4 ml · kg^-1 · min^-1] and six untrained (V̇O(2peak) = 46.2 ± 1.9 ml · kg^-1 · min^-1) men cycled to fatigue at a work rate calculated to require 70% V̇O(2peak). Time to exhaustion was 36% longer (P < 0.01) in trained (TR) compared with untrained (UT) men (148 ± 11 vs. 95 ± 8 min). Although intramuscular glycogen content was reduced (P < 0.05) in both TR and UT at fatigue, IMP, a marker of a mismatch between ATP supply and demand, was only elevated (P < 0.01) in UT muscle at fatigue and was approximately fourfold higher at this point in UT compared with TR. These data demonstrate that fatiguing submaximal exercise was associated with a similar low level of intramuscular glycogen in both TR and UT men, but a mismatch between ATP supply and demand only occurred in UT individuals.