“…The success of the Gray & Hancock (1955) resistive-force model for spermatozoa swimming inspired a great effort into the modelling of microscopic biological swimmers † Email address for correspondence: lmk42@cam.ac.uk in viscous fluids. Hydrodynamic models of these systems now include long-range hydrodynamic interactions in flagella and cilia (Keller & Rubinow 1976;Lighthill 1976;Johnson 1979;Gueron & Liron 1992;Man, Koens & Lauga 2016;, interactions between the swimmers body and its flagella (Higdon 1979;Smith et al 2009;Hu et al 2015;Chakrabarti & Saintillan 2019a) and with walls (Barta & Liron 1988;Das & Lauga 2018;Walker et al 2019). These hydrodynamic models have also been coupled with elasticity to investigate the fluid-structure interactions at the heart of biological flagella (Kim & Powers 2005;Ishimoto & Gaffney 2018;du Roure et al 2019;Chakrabarti et al 2020), while including models for internal activation allows us to understand how spermatozoa and cilia generate their waveforms (Ishimoto & Gaffney 2018;Chakrabarti & Saintillan 2019a,b;Man, Ling & Kanso 2020).…”