One inch. That's as far away from you Bruce Lee's hand needed to be to deliver a devastating punch that could knock you across the room. Lee accomplished this amazing blow not just with physical mastery, but mental mastery as well.
Popular Mechanics' William Herkewitz asked Jessica Rose, a Stanford University biomechanical researcher, about the science behind Lee's one-inch punch, and it's a fascinating read:
"When watching the one-inch punch, you can see that his leading and trailing legs straighten with a rapid, explosive knee extension," Rose says. The sudden jerk of his legs increases the twisting speed of Lee's hips—which, in turn, lurches the shoulder of his thrusting arm forward.
As Lee's shoulder bolts ahead, his arm gets to work. The swift and simultaneous extension of his elbow drives his fist forward. For a final flourish, Rose says, "flicking his wrist just prior to impact may further increase the fist velocity." Once the punch lands on target, Lee pulls back almost immediately. Rose explains that this shortens the impact time of his blow, which compresses the force and makes it all the more powerful.
Lee's muscles weren't responsible for the blow's power; instead, it was his physical coordination and timing — synchronizing the movements of his hip, legs, knees, shoulder and wrist perfectly in order to maximize their power.
And this was only possible because of Lee's brain. Herkewitz refers to a study of martial arts users with similarly short but power fulblows by Ed Roberts, a neuroscientist at Imperial College London:
And when Roberts took brain scans of his study's participants, he also found that the force and coordination of each participant's two-inch punch was directly related to the microstructure of white matter—the substance that manages communication between brain cells—in a part of the brain called the supplementary motor cortex. This is important, because this brain region handles the coordination between the muscles of the limbs, which close-range punches rely on. The altered white matter allows for more abundant or complex cell connections in that brain region, Roberts says, which could increase the puncher's ability to synchronize his or her movements.
Seriously, the whole article is great; give it a read here.