Time-of-Day Variation in Knee Extensor Strength and Fatigue Is Associated With Circadian Phenotype in Healthy Young Adults.

Abstract

Time-of-day variation in neuromuscular performance is well established, but the contribution of circadian phenotype remains unclear. This study examined whether maximal knee-extensor strength and fatigue resistance differ between morning and evening and whether chronotype and actigraphy-derived sleep-timing are associated with diurnal performance variation. Forty-four healthy young adults (21 males, 23 females) completed a randomised-crossover protocol consisting of neuromuscular testing in the morning (0700-0900) and evening (1700-1900). Participants completed 7 days of actigraphy monitoring and the Composite Morningness Questionnaire to characterise behavioural circadian phenotype. Sleep timing was assessed using habitual sleep midpoint (between sleep-onset and wake-time), with night-to-night variability in sleep midpoint used as an index of sleep-timing variability. Maximal voluntary isometric contraction (MVIC) torque, fatigue index, contractions completed, and time-to-exhaustion were assessed during each trial. Baseline MVIC torque was greater in the evening compared with the morning (2.96 ± 0.80 vs 2.85 ± 0.77 N m kg⁻¹; P < .0001), with post-fatigue torque also remaining higher in the evening (2.00 ± 0.60 vs 1.84 ± 0.54 N m kg⁻¹; P < .0001). Participants demonstrated a lower fatigue-index (32.57 ± 6.99 vs 35.58 ± 6.26%; P < .0001), completed more contractions (37.95 ± 8.29 vs 35.09 ± 7.58; P < .0001), and exhibited longer time-to-exhaustion (6.36 ± 1.51 vs 5.85 ± 1.26 minutes; P < .0001) during evening testing. Chronotype was strongly associated with the PM-AM difference in MVIC torque (r = -.684, P < .0001), explaining 47% of the variance in diurnal performance responses. Neither habitual sleep midpoint (r = -.029, P = .848) nor night-to-night variability in sleep midpoint (r = -.055, P = .721) was associated with performance differences. No significant sex-related differences were observed. Maximal knee extensor strength and fatigue resistance were enhanced during evening compared with morning testing. The magnitude of this response was strongly associated with behavioural circadian phenotype, but not actigraphy-derived sleep-timing. Findings suggest that chronotype influences time-of-day differences in neuromuscular performance and should be considered when evaluating exercise performance.