Number 19

Produced, edited, and copyrighted by
Professor Brent S. Rushall, San Diego State University

Blue line


Brent S. Rushall -- San Diego State University
Eric J. Sprigings -- University of Saskatchewan
Larry E. Holt -- Dalhousie University
Jane M. Cappaert -- United States Swimming



This discussion has attempted to assess the development of thinking and research over the past 25 years for understanding the basic properties of force production in swimming strokes. It has described the limitations and deficiencies of Bernoulli's Principle for being the basic explanation for propulsive forces. Bernoulli's Principle is simply too limited and inappropriate for accounting for observed and measured phenomena in competitive swimming strokes. Evidence reveals that lift forces are minor and rarely in a substantially beneficial direction in propulsive phases of the crawl, back, and butterfly strokes.

In breaststroke, both forces are recorded in the inward and outward sculling phase of the stroke. However, drag forces are not in a facilitatory direction and so forward propulsion results from lift created by an exaggerated angle of attack (when compared to those exhibited in crawl, back, and butterfly strokes), and a large canceling of effect of the mostly symmetrical drag forces. The dominance and importance of lift forces in breaststroke is not a justification for promoting Bernoulli's Principle as the foundational reason for propulsion.

Drag forces are dominant and in a facilitating direction during propulsion in crawl, back, and butterfly strokes. The extent of their importance is markedly more than any lift force contribution.

In breaststroke, drag forces created by both arms can be largely ignored because the direction of the outward and inward movements makes one cancel out the other. Thus, the emphasis should be on developing lift that will require a greater angle of attack in the hands and forearms than occurs in the other strokes.

Several practical considerations were drawn from the assertions made to date. They will require a change in teaching/coaching emphasis of many practitioners who have promoted an exaggeration of the S-shaped pull, that is, they believe that large sideways movements are beneficial for propulsion. A greater emphasis should be placed on advising swimmers to focus on the role of the forearm in propulsion, particularly at higher speeds.

Three forms of resistance were proposed for consideration. They have practical implications for coaches. Shaving and wearing a suit with a fabric that has a low coefficient of resistance can reduce frictional drag. Maintaining streamlined positions during the entirety of all strokes can minimize form drag. Removing any unnecessary vertical and all lateral and exaggerated movements can minimize Wave drag. Attention to the details of drag reduction is most important when speed of swimming is emphasized.

The production of force is important. However, resistance increases at a much greater rate than changes in speed due to propulsive forces. Thus, it is advocated that swimming actions first should be oriented to minimizing resistance, and second, to developing propulsion as long as positions for minimal resistances are maintained.

This paper has called for a change in entrenched thinking with regard to propulsive and resistive forces. When that change is made and its importance introduced into coaching the progression of swimmers' performances should improve.

Go to the last section, REFERENCES, of this article.

Return to Table of Contents for Swimming Science Bulletin.

Blue line