In a new study just published in the journal Cerebral Cortex, the researchers explain their process. They took 12 black belts with an average of 13.8 years' worth of karate experience, and also 12 control subjects who exercised regularly but who weren't trained in any martial art. After that, they hooked their arms and torsos up to infrared markers so their punching speed could be measured. The karate experts punched harder (obviously), but it was the timing of the punch, not just the brute strength, that mattered. Their wrists and shoulders were synchronized to give the most force.