Researchers have identified a key component of the mechanism sperm use to speed up their progress to the egg.
Sperm's tails play a key role in the fertilisation process
The chemical change converts the sperm's tail motion from a steady swimming undulation to a whip-cracking snap.
The Harvard Medical School team believe their work could potentially aid the study of male infertility, and the design of new contraceptives.
The research is published in the journal Nature.
The sudden change in the motion of a sperm's tail is known as hyperactivation.
The process is thought to be triggered by the alkaline environment found inside the female reproductive tract.
The Harvard team had already identified a protein called CatSper1 - which is only found in the sperm tail - as key to the process.
In the latest study they were able to show just why the protein plays such a key role.
The researchers used a sophisticated technique called patch clamp recording, to monitor electrical activity with the sperm cell.
The technique is widely used to examine other cells, but until now it had not been applied to sperm cells, which are constantly wriggling around, and are protected by tough outer membranes.
The results showed that CatSper1 plays a central role in controlling the flow of calcium ions into the sperm cell. It is thought that it is this flow of ions that triggers hyperactivation.
Lead researcher Dr David Clapham said: "No one had ever seen inside sperm cells to measure all the currents that control their activity.
"We are already measuring many of these currents and beginning to answer questions about what they are and what they do."
Dr Clapham said CatSper1 was a good target for new forms of contraception.
"We know that defects in CatSper1 block fertilization in mice.
"And since the channels in human sperm are very similar, there is no reason to believe you couldn't develop a male or female birth control pill that would block the protein before it functions to hyperactivate sperm, preventing fertilisation."
Dr Allan Pacey, senior lecturer at the University of Sheffield, said: "The sperm tail is an incredibly complex and elegant machine that we really struggle to understand at the molecular level.
"This kind of approach is a real step forward.
"We know that hyperactivation is crucial to successful fertilisation, and we suspect that in some men this might be why they have difficulty in conceiving with their partners, if we could better understand the molecules involved in that process we might be able to diagnose the problem earlier and therefore save the couple both time and heartache.
"This research could also help us understand how sperm actually find and reach the egg in the first place as we suspect that hyperactivated motility is also involved in that process too.
"If we could understand that mystery too, that would be fantastic."