It’s hard to believe, but on Feb. 10 my friend Mark Spitz turned 70. Most everybody still sees him as the young 1972 Olympic swimmer in that iconic photo, with seven gold medals (each a World Record, too) around his neck.

Mark swam in college at Indiana University for my mentor, Dr. James “Doc” Counsilman, the greatest swimming teacher/coach of all time. Among Doc’s many innovations was photographing (video was yet to come) swimmers underwater. He did this at IU against a grid background painted on the pool wall.

Doc had recently published “The Science of Swimming,” still a bible of our sport, when he first filmed Mark. But the teacher was about to learn his most important lesson when he looked at his new film.

He saw Mark’s freestyle hand entering and exiting the water as expected. One would expect that, because Mark’s stroke started in front of his body and ended below his thigh, his hand would exit the water at some point on Doc’s wall grid behind where it entered.

But that was not so. Mark’s hand had exited in FRONT of where it entered.

Why? How could that be? Doc surmised that part of the answer was due to Mark’s forward momentum while he was stroking, but that didn’t explain it all. If Mark was stroking his hand backward toward his feet, regardless of momentum, it would finish behind where it entered.

But what if he wasn’t stroking his hand past his body? Maybe Doc and everybody else had been missing something for years, forever?

That “something” is this: Stroking is not moving the hand past the body, but moving the body past an anchored hand.

The best swimmers work against non-moving water, keeping pressure on the water by feelingly, minutely, moving the hand in and out and up and down to lever (using the lower and upper arm as well) the body past.

If indeed the hand is not moving backward, the swimmer’s momentum could carry it ahead of where it entered the water.

Doc, the scientist, would go on to experiment with other swimmers and with other swimming scientists to apply the laws of fluid dynamics and raise the understanding of swimming throughout the world.

We know that water can be very resistant to our efforts to get through it. Why not turn that around and use the water’s resistance to lever your body past it as you become more efficient at aquatic locomotion?

Dr. Bob Colyer of Bluffton is an actively retired college professor, coach and author of “Swim Better: A Guide to Greater Efficiency for Swimmers & Instructors,” directed primarily to non-competitors.