The relationship between sperm velocity and pressures applied to the zona pellucida during early sperm-oocyte penetration

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 (v_s) and sperm head density (ρ_s)-which is an indicator of the stiffness of the head, have on the peak sperm-zona pellucida contact pressure (p_m) during the early phase of sperm penetration. The modeling identified v_s as being more influential on p_m compared with the influence that ρ_s has, which means that for spermatozoa competing on the same oocyte, greater v_s is a more important advantage than higher ρ_s. Specifically, p_m was more sensitive by a power of 2 to changes in v_s than to changes in ρ_s. It was further demonstrated that each 0.1 g/cc increase in ρ_s (within the physiologically relevant range of 1.3-1.7 g/cc) would be equivalent to just ∼3% rise in v_s, 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.