Why does my heart rate differ on the same ride from day to day?

Achten and Jeukendrup review the uses and limits of heart-rate monitors, which over the previous 20 years had become a common training aid. Their abstract makes heart rate mainly a measure of exercise intensity — easy to monitor, relatively cheap, usable in most situations — and lists where it misleads.

Heart-rate variability falls progressively up to moderate intensities during graded exercise and then stabilises; trained people have higher variability than untrained ones in cross-sectional studies, while longitudinal training studies are split, perhaps because programmes differ in length.

Heart rate and its variability might help prevent and detect overtraining: the effect of overreaching on submaximal heart rate is controversial (lower in some studies, unchanged in others), maximal heart rate "appears to be decreased in almost all 'overreaching' studies", and too few variability studies exist for firm conclusions.

Predicting VO2max from the heart rate-VO2 relationship rests on several assumptions and can deviate up to 20% from the true value; estimating energy expenditure that way is satisfactory for groups but not very accurate for individuals.

Day-to-day variability in heart rate is small, a steady rise during exercise was seen in most studies, and dehydration and ambient temperature can have a profound effect on the heart rate-VO2 relationship.

Our reading: the abstract gives the base a review-level source for heat, fluid and drift moving heart rate at the same work, and for a lower maximal heart rate after overreaching; it gives no size for the day-to-day variation, and the base's sizes are not all small: about 3 bpm at the end of stages near 85-90% HRmax and more at easy intensities (REF-756), and 6-12 beats in the Olympiatoppen guideline that SYNTH-38 uses (REF-1077, C).