 Several methods have been used to sort sperm before the advent of flow cytometry.Density gradient centrifugation (in a continuous or discontinuous gradient) can concentrate semen samples with low concentration of sperm, using the density of sperm as a measure of their quality.Similarly, so-called swim-up techniques apply a centrifugation step and then sperm is allowed to swim up into a medium, thus enriching a subpopulation of motile sperm.