Time-of-Day Effect on Cardiac Responses to Progressive Exercise

Abstract

This study was designed to examine time-of-day effects on markers of cardiac functional capacity during a standard progressive cycle exercise test. Fourteen healthy, untrained young males (mean ± SD: 17.9 ± 0.7 yrs of age) performed identical maximal cycle tests in the morning (08:00–11:00 h) and late afternoon (16:00–19:00 h) in random order. Cardiac variables were measured at rest, submaximal exercise, and maximal exercise by standard echocardiographic techniques. No differences in morning and afternoon testing values at rest or during exercise were observed for oxygen uptake, heart rate, cardiac output, or markers of systolic and diastolic myocardial function. Values at peak exercise for Vo₂ at morning and afternoon testing were 3.20 ± 0.49 and 3.24 ± 0.55 L min⁻¹, respectively, for heart rate 190 ± 11 and 188 ± 15 bpm, and for cardiac output 19.5 ± 2.8 and 19.8 ± 3.5 L min⁻¹. Coefficients of variation for morning and afternoon values for these variables were similar to those previously published for test-retest reproducibility. This study failed to demonstrate evidence for significant time-of-day variation in Vo₂max or cardiac function during standard progressive exercise testing in adolescent males. (Author correspondence: [email protected])

Keywords:

• Aerobic fitness
• Circadian rhythm
• Exercise
• Heart function

ACKNOWLEDGMENTS

This work was supported by a grant from The Children's Miracle Network, Duke University (CMN 3911749).

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.