The Effects of Circadian Rhythms and Exercise Preconditioning on Cardiac Troponin T Levels Following Graded Exercise

This study explored the impact of circadian rhythms on the circulating cardiac troponin T (cTnT) response to a graded exercise test (GXT) and examined whether an initial GXT influenced the cTnT response to a subsequent GXT performed 7-9 days later. Twenty-one healthy young males (age: 20.6 ± 2.2 years, body mass index: 22.2 ± 2.6 kg/m², V̇O_2max: 31.8 ± 8.7 mL.kg^-1.min^-1) participated in three trials: an initial GXT (GXT1), a resting control trial (CON) and a second GXT (GXT2), separated by at least 72 h. The serum cTnT levels were measured pre-exercise, 4 h post-exercise or during the control. In GXT1, the cTnT levels did not show significant changes (median [range], pre: 3.80 [3.00-10.59] ng.L^-1, post: 4.22 [3.00-9.08] ng.L^-1, p > 0.05). During CON, the cTnT levels decreased significantly from morning to early afternoon (3.52 [3.00-10.84] vs. 3.00 [3.00-7.57] ng.L^-1, p < 0.05), reflecting a circadian rhythm. Interestingly, GXT1 appeared to prevent this circadian decline. Furthermore, in GXT2, the cTnT levels significantly decreased post-exercise (4.13 [3.00-15.48] vs. 3.24 [3.00-12.96] ng.L^-1, p < 0.05), suggesting a possible "late exercise preconditioning" effect from GXT1. These findings suggest that GXT can interact with circadian rhythms, altering cTnT dynamics, and that prior exercise may induce prolonged cardioprotective effects. This study highlights the importance of accounting for circadian variability and late preconditioning effects in future research on exercise-induced cTnT release.