My swimming at the Bluffton pool includes a variety of drills, about 30 to 70% of each workout. Other swimmers have asked me whether there is any one “magic” drill that I would recommend. The answer is always, “Yes.”

My go-to drill is to swim with just one arm at a time. This might be awkward at first, but eventually the pattern is to alternate a length with one arm and then a length with the other arm. You always are facing the same side of the pool.

There are several – actually quite a lot of – reasons or purposes for single-arm swimming, whether the stroke is crawl, back crawl, or butterfly. Let’s start with crawl or, competitively, “freestyle.”

First, from a good push-off, you start in the glide position (on the side, lower arm extended, upper arm with hand touching thigh below hip). This optimizes body length for less resistance.

Being on the side also helps you learn that front crawl is not swum on the front, but on alternating sides through the front.

From the glide position, it is easy to lift the elbow (not swing the arm) to recover. Let the upper arm do the work while the lower arm and hand relax. All the while, the lower arm remains extended.

Next, if you recover correctly, you place your hand in the water in front of your face and extend it forward. You don’t cross your hand and arm over in front of your head, thus shortening your stroke and making it less efficient. This entry thus makes it easier to extend the hand forward and down into the best angle for the catch.

Holding the stroking hand close to the body efficiently levers the body past the hand until the hip passes the hand into the glide position. This creates optimal stroke length and consequently fewer strokes per lap.

Another reason? It is easier to coordinate one arm stroke with your kick than two.

Finally, breathing is easier, with only minimal turning to the side. You breathe in the trough created by your head, which is submerged in the water and in line with your spine.

That’s a lot of benefit from just one drill.

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.