Two lifters squat the same weight. One drives it up in 0.4 seconds, the other in 0.9. On a one-rep max chart they look identical. In every other context, on the field, on the platform, in injury risk, in next year's progression, they are nothing alike. The variable that separates them is rate of force development. Why this matters Rate of force development (RFD) is how quickly you can build force after a muscle contraction begins. It is measured in newtons per second, usually