Before we make our mark and then dive out of the block, swimmers – like other athletes – have a pre-race mental checklist that we run through to make sure we are really ready to compete. Swimming caps on and goggles secured? Check. Starter block dry, locked and ready for use? Double check! Finally, as anyone who has ever seen the five-time Olympic Michael Phelps swim will know, we need to wiggle or hit our muscles awake.

I know it sounds weird, and for a spectator who has never seen a swim before, it can seem a little creepy and painful! But as someone who used to swim competitively, I can assure you that not only is it painful, but it is a very important part of preparing for a race.

When an Olympic swimmer hits his back, biceps or thighs, it’s like sending an awakening call to muscles and joints. You will notice that the areas or “zones” that swimmers are usually targeted at are large areas of muscle mass. Vibration from shock helps to increase blood flow in the body. It can also reduce the risk of having a cramped midline, which is one of the worst things that can happen to a swimmer in the water. Not all claps are as loud and dramatic as Phelps, as many swimmers choose to hit their muscles with a closed fist instead of an open palm.

Body flaps are, to put it mildly, a unique prior routine – but without them, a swimmer’s muscles could easily tense up and cost them their shots on the podium. It’s just one of the many (peculiar) things that make swimming so interesting to look at.