Cyrus Shuler1, Michael G. Bemben1, FACSM, Rebecca Larson1, Michael Pham1, Chinguun Khurelbaatar1, & Debra A. Bemben1, FACSM

1University of Oklahoma, Norman, Oklahoma

Circadian rhythms are critical for regulating physiological and behavioral responses during a 24-hour solar cycle. Testosterone (T) and Cortisol (C) are commonly measured as biomarkers of physiological adaptations to exercise, as the T:C ratio is indicative of an anabolic:catabolic state based on their physiological effects on specific tissues. Recently, high-intensity interval training (HIIT) has emerged as an exercise method shown to yield greater improvements in aerobic fitness compared to traditional steady-state aerobic exercise, yet little is known about its effects on hormonal responses. PURPOSE: To examine the effects of time of day on salivary testosterone (sal-T) and salivary cortisol (sal-C) responses to acute bouts of HIIT performed in the morning (AM) and evening (PM), and to examine diurnal variations in the T:C ratio responses to HIIT in college aged men (n=10). METHODS: A 1:2 minute ratio (Work:Recovery) HIIT protocol was employed (W 81% VO2peak, R 40% of VO2peak). Salivary samples were collected prior to (PRE) and immediately post (IP) each exercise session. Salivary samples also were collected for the control day in the AM and PM on the same day to establish baseline hormone concentrations. All salivary samples were assayed using Salimetrics kits at the Salivalab in Carlsbad, California. Testing order was randomized. RESULTS: A significant exercise main effect was observed for sal-T, which increased PRE to IP for both AM and PM sessions (AM PRE: 274.34 ± 91.39 pg/mL; AM IP: 292.16 ± 87.65 pg/mL vs. PM PRE: 233.83 ± 70.50 pg/mL; PM IP: 281.47 ± 93.78 pg/mL, p = 0.041). Sal-C had a significant time of day main effect as the PM session had lower values for both PRE and IP than the AM session (AM PRE: 0.463 ± 0.348 µg/dL; AM IP: 0.491 ± 0.380 µg/dL vs. PM PRE: 0.210 ± 0.110 µg/dL; PM IP: 0.297 ± 0.244 µg/dL, p = 0.029). The T:C ratio also had a significant time of day main effect (AM PRE: 0.083 ± 0.044; AM IP: 0.085 ± 0.043 vs. PM PRE: 0.132 ± 0.056; PM IP: 0.140 ± 0.073, p = 0.013). CONCLUSION: Sal-T responses to acute bouts of HIIT were not affected by time of day; however, sal-C concentrations were lower in the PM session resulting in a more favorable T:C ratio in the afternoon. These findings suggest performing HIIT in the late afternoon may be more beneficial for physiological adaptations.

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