Doing more with less: the flagellar end piece enhances the propulsive effectiveness of human spermatozoa
Spermatozoa self-propel by propagating bending waves along a predominantly active elastic flagellum. The organized structure of the "9 + 2" axoneme is lost in the most-distal few microns of the flagellum, and therefore this region is unlikely to have the ability to generate active bending; as such it has been largely neglected in biophysical studies. Through elastohydrodynamic modeling of human-like sperm we show that an inactive distal region confers significant advantages, both in propulsive thrust and swimming efficiency, when compared with a fully active flagellum of the same total length. The beneficial effect of the inactive end piece on these statistics can be as small as a few percent but can be above 430%. The optimal inactive length, between 2-18% of the total length, depends on both wavenumber and viscous-elastic ratio, and therefore is likely to vary in different species. Potential implications in evolutionary biology and clinical assessment are discussed.