The idea of a person running across water is a classic trope often seen in fiction. While the human body cannot naturally perform this feat, the action is theoretically possible based on well-understood physics. Locomotion on water requires a combination of speed and power output far exceeding human biological limits. To remain above the liquid surface, a person would have to achieve an astonishing velocity, highlighting the vast difference between human capabilities and the demands of fluid mechanics.
The Physics of Water Running: Surface Tension Versus Momentum
Two distinct physical principles allow organisms to move across water without sinking, depending on their size and mass. The first mechanism, surface tension, works exclusively for very small, light creatures like the water strider insect. Water molecules are strongly attracted to each other, creating a thin, elastic-like membrane at the surface. The insect’s tiny mass is insufficient to break this barrier, allowing it to distribute its weight across multiple legs with water-repellent hairs.
For any object larger than a small insect, including a human, gravity easily overwhelms surface tension, rendering that mechanism useless. Larger animals and humans must rely on the second principle: hydrodynamic lift, or momentum. This method requires generating an upward reaction force by applying a rapid, powerful downward force onto the water. The water must be pushed away with enough acceleration to create a supporting force equal to the object’s weight before the foot sinks.
This action is best described as an extremely rapid “slap and push” motion, generating the necessary reaction force based on Newton’s third law of motion. The goal is to maximize the momentum transferred to the water during the brief moment of contact. By striking the water quickly and vertically, the foot pushes water downward, and the water pushes back upward with an equal and opposite force. For a human-sized object, this force must be generated consistently with every step to prevent sinking.
Calculating the Required Human Speed
The minimum forward speed required for a human to run on water is dictated by the need to generate an upward force greater than body weight. Scientists calculate that a person would need to slap the water surface at approximately 30 meters per second. This velocity, equivalent to roughly 67 miles per hour, must be maintained with every step to keep the body afloat.
This required speed is immense compared to the top velocities achieved by humans on land. The world record for human running speed peaks at about 10 meters per second (around 28 miles per hour), demonstrated by elite sprinters. To achieve the necessary water-running velocity, a human would have to move more than three times faster than any person has run on a solid surface.
Furthermore, the power demands associated with this speed are biologically impossible. Sustaining the required motion would demand a mechanical power output of approximately 12 kilowatts. This represents about 15 times the maximum muscle power that the legs of a highly conditioned human athlete are capable of producing. The body cannot generate the energy needed to accelerate and decelerate the legs so rapidly and forcefully.
Anatomical Limitations and Real-World Examples
Even if a human could generate the required speed and power, the anatomy of the foot makes the feat impossible. A human foot has a relatively small surface area compared to body mass, meaning the pressure exerted on the water with each step is too high. Without a much larger surface area, the foot sinks instantly, regardless of the speed of the downward slap.
Research shows that even with a high-velocity slap, the foot’s contact area would need to increase to approximately one square meter to support a person’s weight. The human gait is also fundamentally wrong for this type of locomotion, as it is designed for bouncing on solid ground. Running on water requires a rapid, vertical “pedaling” motion with minimal upper-body movement, which differs entirely from terrestrial running.
The basilisk lizard, sometimes called the “Jesus Christ lizard,” provides the clearest biological example of successful water running. This reptile is too heavy to use surface tension and employs the hydrodynamic lift principle. Its hind feet are specialized with large, fringed toes that unfurl upon contact, dramatically increasing the surface area.
The lizard executes a three-part step cycle—slap, stroke, and recovery—where the foot rapidly moves downward and backward. This motion traps a pocket of air beneath the foot, which generates lift and allows the limb to be pulled out of the water easily for the next stride. The basilisk’s specialized foot size and rapid gait allow it to achieve a temporary run on the water at speeds far below what a human would require.