This is the peer reviewed version of the following article: Pethick, J., Winter, s. and Burnley, M., (2021). Did you know? Using entropy and fractal geometry to quantify fluctuations in physiological outputs. Acta Physiologica, Doi: 10.1111/apha.13670, which has been published in final form at https://doi.org/10.1111/apha.13670 This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions