Srouffe, L. A. (1971). Effects of depth and rate of breathing on heart rate and heart rate variability. Psychophysiology, 8, 648-655.

1. Faster breathing produced a more stable heart rate.
2. Deeper breathing produced a faster but more variable rate, while shallow breathing had the opposite effects.
3. Subjects were able to be trained to use respiration to control heart rate.

Implication. It is possible to exert some control over heart rate. Deep breathing at the end of physical work could interfere with truly representative heart rates. The factors listed above are just some of the variables that decrease the reliability of heart rates as indices of work levels.

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