Do human sperm defy Newton’s third law? Scientists finally crack the mystery of their swim through thick fluids

Scientists have been puzzled by a biological mystery: how human sperm, just 50 microns long, can glide so effortlessly through thick cervical mucus or viscous lab-made gels that should slow them down.

Now, researchers from Kyoto University say they have decoded the secret: sperm tails exploit a phenomenon called “odd elasticity,” enabling them to move in ways that sidestep one of Newton’s basic laws of motion.

The findings, published by researchers Kenta Ishimoto, Clément Moreau, and Kento Yasuda, help solve a decades-old puzzle: how sperm can travel swiftly through thick cervical mucus or gel-like fluids that should slow such tiny swimmers to a crawl.

The physics problem in the micro world

At the human scale, swimmers push water backward to propel forward, with inertia balancing the forces. But at the microscopic level, inertia disappears and is replaced by syrupy drag, a situation physicists call low Reynolds number flow.

In such conditions, a simple back-and-forth flick of a tail wouldn’t work. Instead, objects like sperm must perform a continuous, asymmetrical wiggle pattern that never repeats in reverse, allowing forward motion.

Odd elasticity: The tail’s secret weapon

Using high-speed video of human sperm and the green alga Chlamydomonas, the team mapped tail movements in “shape space” and created an elastic matrix to calculate internal forces. They found that the tails are powered by molecular motors that constantly inject energy instead of acting like passive springs.


This creates odd elasticity, an imbalance where a bend in one part of the tail sends tension through the entire structure without a mirrored counterforce. The result is a traveling wave that moves forward without an equal push in the opposite direction.

Biology and robotics

The study suggests that as odd elasticity increases, so does propulsion speed. This explains how human sperm beat their tails around 20 times per second, even in thick fluids.

The principle also applies to other microscopic swimmers like algae, and could inspire soft robots that navigate through viscous environments using similar mechanics, without rotating motors.

Rethinking newton in active systems

The work doesn’t overturn Newton’s third law, which still governs passive systems. Instead, it shows that force symmetry can be bypassed in active systems, where energy is constantly absorbed and expended.

In nature, this flexibility may help sperm adapt their swimming mechanics in response to chemical cues or changes in viscosity on their way to the egg.