Diagram showing the mechanics of front crawl © University of Tsukuba The S-stroke in front crawl could be more effective for triathletes, rather than the I-stroke, new research from the University of Tsukuba in Japan shows.
The scientists investigated which stroke was more effective in the water; the S-stroke when the arm draws a curve in the water or the I-stroke when it moves straight, and found that the effectiveness of the stroke depended on the distance swum. According to their results the S-stroke is better suited for swimming middle and long distances, when you need propulsive power with less physical exertion, while the I-stroke is better for short distances, which depend on speed over efficiency.
The study explains that this is because “the mechanisms of propulsive power differ between the two strokes. The vortex pairs generated by the S-stroke, with the arm changing orientation in the water, cause unsteady lift force, while the I-stroke’s straight movement causes Kármán vortices that produce drag force.”
However this research doesn’t take into account coping with open-water.
"I-shaped pulls and S-shaped pulls are at extremes of the spectrum. Most people will fit somewhere along that, rather than at either end, depending on height, reach, power, flexibility […]