The impact of the time of day on muscle and metabolic responses to resistance exercise in healthy adults: A randomised controlled trial

Resistance exercise provides numerous health benefits, including improved glucose control and enhanced muscular strength. However, it remains unclear whether the time of day resistance exercise is performed affects these benefits. The objective of this work was to determine the effect of time of day on muscle and metabolic responses to resistance exercise training in young healthy adults. The study included 36 participants (30 ± 7 years old; and 28 ± 4 kg/m²) who were randomised into control, morning (06.00-10.00 h) or evening (16.00-20.00 h) groups. Exercise groups performed eight resistance exercises, 3 times per week, for 6 weeks, at their allocated time. At baseline and post-intervention, insulin sensitivity, flash glucose monitor data, muscle strength and vastus lateralis muscle thickness were measured. Over the 6-week intervention, there were significant main effects of time on insulin sensitivity (P < 0.001), muscle thickness (P = 0.008) and knee extensor maximal torque (P < 0.001), indicating improvements with resistance exercise training. However, there were no significant time × group interactions for any outcome measures (insulin sensitivity P = 0.206, muscle thickness P = 0.279, knee extensor torque P = 0.151), demonstrating that exercise timing did not differentially affect training adaptations. Both exercise groups showed similar improvements compared to controls, regardless of whether training occurred in the morning or evening. No significant effects were observed for flash glucose monitor data. This study highlights the benefits of resistance exercise and demonstrates that timing has little influence on these effects. Promotion of resistance exercise at convenient times is recommended. This study was registered at ClinicalTrials: ClinicalTrials.gov ID: NCT05321914.