Bergen, J. L., & Grubbs, L. M. (1966). Effect of age and time of day on VO2max. Medicine and Science in Exercise and Sports, 28(5), Supplement abstract 718.

In a group of 24 trained athletes there was no difference between morning and afternoon measures of VO2max. However, when the values for young (20-26 yr) female "morning trainers" were considered, most achieved higher measurements in the morning. A similar effect occurred with "afternoon trainers." This suggests that there is a circadian rhythm effect on VO2max responses in females. This difference was not observed in males.

Seventeen Ss scored higher on the second VO2max test suggesting a learning factor rather than a training or time-of-day effect on the measurement.

Implication. When athletes are measured for VO2max, a single measure should not be given much weight. It would be erroneous to conclude that an improved score on a test some time later was due to a training effect. Only when stable VO2max values have been established and the possibility of adaptation to the testing milieu can be ruled out can training effects be considered to be reflected in measures.

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