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Measuring Propulsive Force in Swimming

By Gary Hall Sr. | April 26, 2016, 11:51 a.m. (ET)

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Go back to school! Erase the smelly, crowded hallways of your high school from your mind and imagine yourself under the Islamorada sun in a clear pool ready to absorb knowledge that will enable you to swim faster. Swim School from Gary Hall Sr. of The Race Club is about lifelong enjoyment of the sport. It’s always more fun to swim to your potential.

Where is the maximum propulsive force generated in the swimmer’s underwater pull? Coaches have debated this question for years. As far as I know, no one has ever measured the exact propulsive forces of the pulling arm at the various underwater positions during the pull. Measuring the swimmer’s body speed, as is now readily obtainable with the velocity meter, is not the same as measuring propulsive force. The body speed is the net result of both the propulsive forces and the frontal drag forces, both of which are changing drastically and constantly during the pulling cycle.

We have described the five different phases of the arm cycle: liftfront quadrant propulsionback quadrant propulsionrelease and recovery. The first four phases describe the underwater pull, but only the second and third phases generate propulsive forces.

It is reasonable to make an educated guess at the position of maximum propulsive force of the pulling arm/hand. First, the propulsion of the hand does not begin until it starts moving backward, relative to the still water (Newton’s third law of motion). That occurs when the hand is about one foot in front of the shoulder or, from the side view, as the hand moves in a circle of about two feet in diameter when the hand is at 3-o’clock position on the dial. From that point, the hand continues to go deeper but moves backward until the arm is completely extended reaching the 9-o’clock position where the propulsion ends. At 6 o’clock, the hand is at or slightly past the shoulder on its path backwards. For the following four reasons, it is here that I believe the hand generates its maximum propulsive force.

1. From a biomechanical perspective, since the body is counter-rotating during all but the last part of the pull, the shoulder goes from a negative angle (extension) at the beginning, to a positive angle (flexion) at the end. At the 6-o’clock position, the shoulder is in a slightly positive angle, which is a strong mechanical position for the arm pull.

2. The propulsion is not just related to the effective surface area pulling backward (mostly the hand), but also the speed or acceleration of that surface area. As the hand moves backward, it first accelerates, starting at 3 o’clock then decelerates to come to a stop at the end of the pull (9 o’clock). It reaches its maximum speed in the middle or close to the 6-o’clock position.

3. During the propulsion of the arm, the hand and perhaps a small part of the wrist and forearm are the only parts of the arm moving backward (with the high elbow pull, the upper arm also moves backward at the very end of the pull, but creates minimal propulsion). The effective surface area of the hand is greatest when the wrist and hand are aligned and perpendicular to the axis of the body’s motion. This occurs when the hand is near the 6-o’clock position.

4. The propulsive force is also related to what forces the hand is pulling against. The counter-rotation of the body during the pulling motion generates an important additional force to pull against other than the still water molecules in front of the pulling hand. This body force is related to the mass and radius of the body, and the angular speed of the rotating body. The mass and radius don’t change for the swimmer’s body during the race, but the angular rotational speed does. The angular velocity of the rotating body reaches its maximum when the hand is near the 6-o’clock position, creating the greatest force to pull against.

So for these four reasons, I believe that the point of greatest propulsive power of the arm pull occurs when the hand is at or near the shoulder on its pathway backward in the water. I also believe that the deeper pull will generate more propulsive power than the high-elbow pull. However, that does not necessarily mean that the body speed will be greater for the deeper pull, as that technique of pull will generate significantly more frontal drag forces than the high elbow pull.

Swimming fast is not all about power. It is about reaching the right compromise between propulsion and frontal drag that leads to the fastest swim. To do that, it requires some good coaching.

Yours in swimming,
Gary Hall Sr.

gary hall srGary Hall Sr., M.D. is a three-time Olympic swimmer (‘68, ‘72, ‘76) who earned a medal in each of the three Olympic Games. At one time he held 10 world records in all strokes except breaststroke and was the World Swimmer of the year in 1969 and 1970.

Gary Sr. serves as president and technical director of The Race Club Inc. based in Islamorada, Florida. He is the current president of the United States Olympians and Paralympians Association and co-founder of World Fit, a non-profit organization promoting childhood exercise and sports. He has six children, the oldest of whom, Gary Jr., also swam in three Olympic Games (‘96, ‘00, ‘04) and earned 10 Olympic medals. Two other children, Richard and Amy, and his wife, Mary, work with Gary Sr. at The Race Club. In 2006, Gary Sr. retired from ophthalmology to dedicate his remaining professional career to teaching advanced swimming techniques for competitive swimmers and triathletes. 

The views expressed in this article are the opinion of the author and not necessarily the practices of USA Triathlon. Before starting any new diet or exercise program, you should check with your physician and/or coach.