Scientists in the early 1900s invented a relatively easy way to measure the force generated by a muscle. They would hang muscle fiber, biopsied from an animal, between two finely calibrated springs and then probe the muscle with an electric shock to make it contract. The stronger the muscle, the more it pulled the springs.

But physiologists soon learned that individual molecules in cells generate the collective force in muscles. And the crude spring experiments couldn’t be applied to these tiny molecules. The challenge of how to measure molecular forces became even more vital as they discovered other molecular motors—molecules in a cell that convert energy to movement. Motors, scientists realized, help sperm swim, pull DNA apart so it can be copied, and carry molecules from one side of the cell to the other. Every directional, nonrandom movement that had been visible under microscopes for centuries was, at some level, due to molecular motors generating force inside cells.

Force is anything that makes an object change its speed, direction, or shape. In the context of cells, forces are required to move molecules. Quantifying these forces gives scientists a way to compare and contrast different molecular motors. Without measurements of force, they were missing a crucial entry in the equations of how cells use energy.

Illustration: VSA Partners