Newton’s Third Law of Motion tells us that for every action there is an equal and opposite reaction. Thus it would seem that to be propelled forward, a swimmer should pull and push the hand-arm lever straight back against the water. Alas, this is not so.

Water is not a solid, but a fluid that, once it is set in motion backward, cannot then be used for further propulsive traction. There are two ways to overcome this.

One way is to move the hand backward faster than the water is already moving. An efficient competitive swimmer knows therefore to accelerate through the stroke. But this comes at a price. For example, doubling one’s speed requires four times the force (squared) and eight times the energy (cubed). At some early point, this cost becomes inefficient.

The second way is for the hand-arm lever to move out and in and up and down just enough to constantly press against non-moving water. Ernie Maglishco, a coaching genius if there ever was one, who has written the (literally, 803 pages) magnum opus on swimming, advocates thus a “diagonal” stroke. As an English major, I prefer to use the term “curvilinear.” Modern swim coaches no longer use “pull” or “push” for the stroking action, but “outsweep” and “insweep.”

Regardless of how it’s labeled, this stroking motion might seem to imply that efficient stroking is maneuvering the hand (and arm) past the body when actually it’s the other way around. Stroking is levering the body past an anchored hand (and arm) that are pressed against non-moving water. This can be done only when that lever is constantly adjusted to maintain such pressure.

The early part of a stroke (or outsweep, pictured) enables a swimmer to maneuver the shoulder(s) past the hand. The remainder of the stroke (insweep) enables a swimmer’s hip(s) to get past the hand. Efficient swimmers do this better than others, and it is often said they have a better “feel for the water.”

That’s the swimmer’s goal – being able to effectively use Newton’s Third Law for optimum propulsion. As a previous column indicated, this does not happen by repetitiously swimming laps. It takes some trial and error just to perform one good, effective stroke, which can be increased gradually to two, three, and beyond. But the eventual reward of more efficient swimming makes all the effort involved in the process worthwhile.

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.