Acute high-intensity interval exercise is less pro-oxidative/thrombotic compared to isovolumic moderate-intensity steady-state exercise.

While high-intensity interval training (HIIT) has emerged as a more time-efficient alternative to moderate-intensity steady-state exercise (MISS), the impact on systemic free radical formation and link to activated coagulation remains unknown. We recruited sixteen healthy males aged 21 ± 3 y who performed incremental cycle ergometry to determine peak oxygen uptake ([Formula: see text] PEAK). Participants were randomly assigned single blind to two separate groups (MISS: n = 8; HIIT: n = 8) matched for [Formula: see text] PEAK. HIIT participants completed five exercise cycles, each consisting of 3 min at 80%[Formula: see text] PEAK alternating with 3 min at 40% [Formula: see text] PEAK, whereas MISS participants performed an isovolumic bout of 30 min at 60% [Formula: see text] PEAK. Cephalic venous blood was assayed for ascorbate free radical (A•-, electron paramagnetic resonance spectroscopy) and clot fractal dimension (df, rheometry) at rest every hour over a 6-h period to determine critical difference (CD) and before/after submaximal/peak exercise. Submaximal MISS increased A• - and df to a greater extent compared to HIIT (P = 0.039 to 0.057) although elevations generally fell within CD boundaries (54.2% and 5.5% respectively). No further elevations were observed during peak exercise (P = 0.508 to 0.827) and no relationships were observed between A•- and df (r = 0.435 to - 0.121, P = 0.092 to 0.655). Collectively, these findings suggest that HIIT is less pro-oxidative/thrombotic compared to more traditional MISS, advocating its prescription in patients given the potential for superior vascular adaptive benefit.