Acute metabolic and molecular responses to sprint interval versus moderate-intensity continuous exercise in healthy young men

<b>Background:</b> Exercise intensity is a key determinant of the physiological responses in skeletal muscle that lead to improvements in human health. Whether a bout of brief supramaximal acute exercise has similar effects on muscle physiology and metabolic health to a longer bout of continuous acute exercise remains unclear. <b>Methods:</b> In a randomized crossover design, 12 healthy young recreationally active men completed single acute bouts of either sprint interval exercise (SIE; seven repeated 30-s maximal sprints at 130% over 35 mins) or moderate-intensity continuous exercise (MICE; cycling at 65% effort over 60 minutes), or a non-exercise control session (CON), on separate days. Acute changes in skeletal muscle insulin sensitivity were measured with a hyperinsulinemic euglycemic clamp. Muscle biopsies were performed to quantify changes in muscle glycogen content and insulin signaling proteins. <b>Results:</b> Despite a six-fold lower total mechanical work in SIE versus MICE, SIE elicited greater and more prolonged increases in whole-body insulin sensitivity, with greater pre-clamp muscle glycogen depletion and glycogen repletion during the clamp. We found reduced glycogen synthase and glycogen synthase kinase-3β phosphorylation, with no differences in Akt Thr308/Ser473 phosphorylation. <b>Conclusions:</b> A single session of sprint interval exercise elicits more pronounced effects on several measures of skeletal muscle insulin sensitivity than a single bout of moderate-intensity continuous exercise, despite substantially lower mechanical work and time commitment. These findings indicate that exercise intensity is a key mediator of acute skeletal muscle metabolic adaptations to exercise and suggest that sprint interval exercise represents a highly time-efficient stimulus for improving skeletal muscle insulin responsiveness.