If you attempt the same exact task many times, it will never be done exactly the same way, even if you consistently reach the same end goal.

Nikolai Bernstein, the OG of movement science, first observed this in his famous blacksmith study. He found that the variability of where the tip of a hammer landed on its target was smaller than the variability of the trajectories of individual joints of the subjects’ arm holding the hammer every single time.


His study observed expert blacksmiths, showing that even the best trained specialist won’t find a “correct” movement solution for a given task.

This motor variability is common in all human movement. Several attempts at the same task always lead to somewhat different patterns of performance. Bernstein beautifully described this observation as “repetition without repetition,” referring to the fact that each repetition was unique even though the end goal was not.¹