Author(s): LENK, M., BEN-GARA, S., WALLMANN-SPERLICH, B., LÖFFLER, C., SPERLICH, B., Institution: JULIUS-MAXIMILIANS-UNIVERSITÄT WÜRZBURG, Country: GERMANY, Abstract-ID: 1034

INTRODUCTION:
It remains unclear whether background physical activity modulates cardiorespiratory and metabolic adaptations to short-term high-intensity interval training (HIIT) in recreationally trained individuals. Therefore, the purpose of this study was to determine the effects of experimentally manipulating daily step counts on physiological adaptations to HIIT.
METHODS:
Twenty-four recreationally trained men (VO2max > 55 mL kg-1 min-1) completed five HIIT sessions over two weeks and were assigned to either low (~5,000 steps day-1) or high (~15,000 steps day-1) background physical activity. Pre- and post-intervention assessments included incremental treadmill testing, submaximal cardiorespiratory responses, ventilatory thresholds, resting metabolism, oral glucose tolerance, high-fat tolerance, postprandial substrate oxidation, and continuous glucose monitoring. Group x time effects were analyzed using linear mixed-effects models.
RESULTS:
Daily step counts differed as intended between groups (p < 0.001), while training intensity and adherence were comparable. VO2max increased in the high-step group (+4.4%, p < 0.01) but not in the low-step group (p > 0.99), resulting in a significant group x time interaction (p < 0.01). During submaximal exercise, the high-step group demonstrated reductions in oxygen uptake (−3.1%, p = 0.024), heart rate (−2.8%, p = 0.02), and blood lactate concentration (p = 0.01), alongside an increase in ventilatory threshold 1 (~12%, p < 0.01). No corresponding improvements were observed in the low-step group. Postprandially, the high-step group exhibited a higher respiratory exchange ratio (p = 0.037) alongside a tendency toward greater carbohydrate oxidation. No group x time interactions were observed for glucose or triglyceride tolerance (all p > 0.25).
CONCLUSION:
Higher background physical activity augments cardiorespiratory and submaximal efficiency adaptations to short-term HIIT in recreationally trained men, whereas low daily activity constrains training responsiveness despite matched exercise exposure. From a practical perspective, training interventions should monitor habitual physical activity in parallel with structured training load, as both contribute jointly to the adaptive outcome.