Researchers from Brown University in Rhode Island, say they have found a good indicator of what makes sperm good movers in the first place.
Scientists led by James Mossmon found that tail consistency trumped average length. They said the findings add to our understanding of why some couples struggle to have children, the 'Daily Mail' reported.
Researchers examined the semen of 103 men attending an infertility clinic at Sheffield Teaching Hospital NHS Foundation Trust.
They found that the wider the variation of sperm length in samples, the lower the concentration of motile sperm. The variation in tail length was found to be the most crucial factor.
"Sperm length measurements may provide a useful insight into testis function and the efficiency of spermatogenesis (sperm cell development)," the scientists said.
The research explains why only one per cent of the 300 million sperm released by a man during sex manages to reach their partner's uterus, while just a few dozen reach the egg.
Earlier researchers from the University of Warwick and University of Birmingham showed sperm has an appalling sense of direction.
They found sperm avoid the 'middle lane' of the female reproductive tract and instead crawl along the channel walls. and struggle to turn sharp corners, crashing into the walls.
The current study was published in the journal Human Reproduction.
