BackgroundThe importance of concurrent exercise order for improving endurance and resistance adaptations remains unclear, particularly when sessions are performed a few hours apart. We investigated the effects of concurrent training (in alternate orders, separated by~3 hours) on endurance and resistance training adaptations, compared to resistance-only training. Materials and methodsTwenty-nine healthy, moderately-active men (mean ± SD; age 24.5 ± 4.7 y; body mass 74.9 ± 10.8 kg; height 179.7 ± 6.5 cm) performed either resistance-only training (RT, n = 9), or same-day concurrent training whereby high-intensity interval training was performed either 3 hours before (HIIT+RT, n = 10) or after resistance training (RT+HIIT, n = 10), for 3 d . wk -1 over 9 weeks. Training-induced changes in leg press 1-repetition maximal (1-RM) strength, countermovement jump (CMJ) performance, body composition, peak oxygen uptake ( _ VO 2peak ), aerobic power ( _ W peak ), and lactate threshold ( _ W LT ) were assessed before, and after both 5 and 9 weeks of training. ResultsAfter 9 weeks, all training groups increased leg press 1-RM (~24-28%) and total lean mass (~3-4%), with no clear differences between groups. Both concurrent groups elicited similar small-to-moderate improvements in all markers of aerobic fitness ( _ V O 2peak~8 -9%; _ W LT~1 6-20%; _ W peak~1 4-15%). RT improved CMJ displacement (mean ± SD, 5.3 ± 6.3%), velocity(2.2 ± 2.7%), force (absolute