Muscular strength and power and endurance performance at loads exceeding 25% of one-repetition maximum are unaffected by time-of-day in resistance-trained male participants.

The aim of this study was to examine the influence of the time-of-day on muscular strength, power and endurance performance in resistance-trained individuals. Fourteen resistance-trained males (age: 26.3 ± 6.7 years) underwent a randomized, counterbalanced cross-over trial. After a familiarization session, participants underwent two trials performing in the morning (9:00 h) and in the evening (18:00 h) a muscular strength and power assessment for bench press and back squat exercises at 25%, 50%, 75%, 90%, and 100% of one-repetition maximum (1RM). Then, muscular endurance was assessed for both exercises at 65%1RM, performing one set until failure. Once completed, isometric strength and vertical jump capacity (CMJ) tests were also performed. Only back squat exercise at 25% 1RM reported higher performance in the evening compared to the morning at mean velocity and mean and peak power (11-13%,  = 0.018-0.031, g = 1.91-2.20). Also, CMJ power was higher in the evening compared to the morning trial (2.5%,  = 0.002, g = 0.23). No statistical differences were found in the remaining loads, exercises or tests. In conclusion, circadian rhythm affects muscular strength and power performance at low (≤25% 1RM) but not moderate-to-higher loads in resistance-trained male participants, an effect observed in lower-body (e.g. back squat and vertical jump) but not in upper-body exercises (e.g. bench press).