Sperm velocity is long known to be an important indicator of sperm quality but without much biophysical theory explaining it. Contact mechanics based modeling was developed here to determine the effects that sperm velocity (vs) and sperm head density (ρs)—which is an indicator of the stiffness of the head, have on the peak sperm-zona pellucida contact pressure (pm) during the early phase of sperm penetration. The modeling identified vs as being more influential on pm compared with the influence that ρs has, which means that for spermatozoa competing on the same oocyte, greater vs is a more important advantage than higher ρs. Specifically, pm was more sensitive by a power of 2 to changes in vs than to changes in ρs. It was further demonstrated that each 0.1 g/cc increase in ρs (within the physiologically relevant range of 1.3–1.7 g/cc) would be equivalent to just 3% rise in vs, indicating again that faster swimming is a better strategy for spermatozoa compared with head stiffening. The modeling hence provided some useful insights regarding sperm biomechanics, which theoretically elucidate the well-recognized importance of sperm velocity measurements as being indicative of sperm quality.

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