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

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. In a randomized cross-over 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 min) or moderate-intensity continuous exercise (MICE; cycling at 65% effort over 60 min), or a nonexercise control (CON) session, 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. Despite a sixfold lower total mechanical work in SIE versus MICE, SIE elicited greater and more prolonged increases in whole body insulin sensitivity, with greater preclamp 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. 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.NEW & NOTEWORTHY This study shows that a single bout of sprint interval exercise produces greater acute improvements in whole body and skeletal muscle insulin sensitivity than a much longer session of moderate-intensity continuous exercise, despite requiring far less total work and time. The findings highlight exercise intensity, not duration, as a key driver of rapid metabolic adaptations and identify sprint interval exercise as a highly time-efficient strategy for enhancing insulin sensitivity.