Absence of Sex-Specific Circadian Variations in Strength Among Military Cadets: A Cross-Sectional Study

Abstract

Background: Circadian rhythms play a fundamental role in the development and production of strength. It is thought that highly physically active populations, such as military personnel, may not be affected by these variations. Objectives: This cross-sectional study examined strength and jump performance in military cadets at the beginning (08:30 a.m.) and end (02:30 p.m.) of their workday. Methods: Twenty females and twenty-three males performed a squat jump, countermovement jump, handgrip strength test, and isometric leg extension strength test on two separate occasions. Results: Two-way ANOVA showed no significant effects of time (all p > 0.28; partial η 2 < 0.02) or time × sex interactions (all p > 0.52). Morning–afternoon differences were trivial across outcomes, ranging from −2.2 to 6.8 units (e.g., CMJ: ∆ = −1.3 cm [95% CI: −3.7, 1.1]; handgrip: ∆ = −0.9 kg [−3.2, 1.3]). By contrast, large sex effects were consistently observed: men outperformed women in jump height (SJ: d = 1.77; CMJ: d = 1.86), leg extension strength (d = 1.59–1.78), and handgrip strength (d = 2.94–3.08), with partial η 2 values up to 0.71. Conclusions: The present study provides evidence that while sex-related differences in strength and jump performance are robust and large, time-of-day has a negligible influence on these measures in military cadets.