In a groundbreaking study, researchers have discovered that human sperm may violate one of the most fundamental principles of physics—Newton’s third law of motion, which states that every action has an equal and opposite reaction. According to the study, the way sperm swim involves non-reciprocal internal forces, enabling them to move forward in ways that classical physics cannot fully explain.
Sperm cells navigate through highly viscous environments, such as the female reproductive tract, where conventional motion strategies are ineffective. To understand how sperm achieve locomotion in such conditions, a team led by Kenta Ishimoto, a scientist at Kyoto University, developed a novel framework called “odd elastohydrodynamics” — documenting the phenomenon in a study on PRX Life. This theory extends the principles of fluid dynamics to living materials that actively generate energy, like the flagella (tails) of sperm cells.
By analysing high-resolution data of human sperm in motion, the researchers found that the flagellar waveforms are not merely passive responses to surrounding fluids. Instead, they are powered by internal, directional energy inputs—a feature described by the study as “odd elasticity.” These forces are non-reciprocal, meaning the internal action does not produce a directly opposite and equal reaction. This active injection of energy into specific parts of the flagellum creates an asymmetric motion, allowing the sperm to swim efficiently despite the resistance of the fluid.
To quantify this phenomenon, the study introduced the “odd-elastic modulus,” a metric that helps identify where and how much internal energy is being injected to sustain motion. In the case of human sperm, the model showed strong alignment between the locations of energy input and the resulting swimming patterns—evidence that the sperm’s undulating motion is not just structural, but energetically strategic.
This insight has broad implications. It not only reshapes our understanding of cellular motion and biomechanics but also opens new possibilities for biomedical engineering, especially in designing biomimetic microswimmers that can move through bodily fluids for drug delivery or diagnostics. Additionally, it could inform fertility research, offering clues about how variations in flagellar motion affect sperm motility and reproductive success. Through relevant applications and research, the latest discovery could help in
In essence, human sperm do not just swim—they bend the rules of classical physics to do so. And with this new lens, scientists are now closer than ever to decoding the mechanics of life at its most microscopic level.