Swimming With Physics: Why Do Swimmers Go Faster When They Feel Slower?

Rebecca Westfall is a former All-American at Texas A&M University, when she was Rebecca Sturdy. She completed her swimming career there in 2007, though still ranks in the top 10 in Texas A&M history in 5 different events. She’s now the assistant coach at Warren Wilson College in Asheville, North Carolina.

Her father, and her coach up until college, is Gary Sturdy. He has a unique perspective on coaching and physics; his primary vocation is as the owner and lead engineer of the Sturdy Engineering Corporation, however he has also spent much of his adult life as a swim coach and swim official. He built a program for the HP-48G that was used at the 2000 Olympic Trials that was used by the USA Swimming Technical Committee to analyze swims.

In the sport of swimming, innovation is crucial. The status quo of thinking and training the same way every day has become unacceptable, especially at the highest levels of the sport. Coaches strive every day to, “think outside the box.” In pursuit of this goal, Coach Gary Sturdy and I have turned to physics to explain why certain techniques work, why others do not, and develop new methods of attaining higher velocities through the water.

While Coach Sturdy and I have had successes in applying physics to swimming, there has always been one situation which has stumped us. Time and time again, we have had swimmers come to us after a lifetime best performance by a large margin and comment, “Coach, I could have gone faster.” Why do the best performances occur when athletes believe they didn’t work hard enough?

I swam this way myself throughout my entire career without being able to explain why my vastly slower turnover rate resulted in speeds that stumped the majority of coaches who watched me swim. Obviously it was working for me, but why?

In the last year or so, Coach Sturdy and I have experienced a breakthrough and are finally able to explain the phenomena. It all comes back to physics, kinetic energy to be exact. The easiest way for us to explain the importance of the practical uses of kinetic energy in the stroke is to apply it to one of the most universal swimmer flaws: the zig zag.

To be more specific, the zig zag occurs during the stroke phase of freestyle and backstroke when a swimmer’s shoulders swerve, for example, to the left, crossing the centerline. Due to the elastic nature of the human trunk, the hips bow out to the right, crossing the centerline in the opposite direction. The movement continues to flow down the body with the legs swerving back past the centerline to the left. As the swimmer takes the next stroke, the shoulders veer to the right while the hips swing left and legs swing right. This serpentine s-curve is what will be referred to as a zig zag.

Up to this point in coaching, the zig zag has been corrected because it a) causes the swimmer to travel a longer distance through the water and b) generates a turbulent water flow, which increases drag. However, as major as these two sources of velocity reduction are, let us approach the problems from a kinetic energy point of view. To do this, we need to give you a crash course in physics as applied to swimming.

Within the stroke, a swimmer produces work energy by pulling and kicking. Work energy is defined as a force exerted over a distance and can be defined in the equation WE=F(d). This work energy allows the body to move at a certain velocity through the water. This work energy is the kinetic energy available to the swimmer. Kinetic energy is defined as mass traveling at a certain velocity and can be expressed in the equation KE=1/2mv2.

The kinetic energy available to a swimmer is comprised of both positive and negative kinetic energy. For our purposes, we will define positive kinetic energy as the mass of the swimmer traveling in the desired direction, in a straight line down the pool. As positive kinetic energy increases, swimming times will always decrease. Negative kinetic energy is a mass traveling in any direction that decreases positive kinetic energy.

It is important to note that the total kinetic energy of the swimmer is finite and is comprised of both positive and negative kinetic energies. It is an inverse relationship: as the negative kinetic energy decreases, the positive kinetic energy increases and vice versa. This relationship can be described in the equation KEt=KEp+KEn.

Once we apply our knowledge of kinetic energy to the zig zag, one can see the biggest reason why it is so ineffective. A swimmer’s shoulder mass moving sideways from the centerline creates negative kinetic energy. Because of the body’s elasticity mentioned earlier, the hip mass moves to the right, and the leg mass moves to the left further, increasing negative kinetic energy. As these body masses move sideways, the water around the body is forced to move as well. To move the water, the swimmer expends additional negative kinetic energy.

When all the negative kinetic energies (body and water movements) are summed, the swimmer’s positive kinetic energy and thus forward velocity is reduced. And that’s the loss in just a single stroke! Add up all the strokes in a race and you can deduce how much energy a swimmer wastes just by zig zagging even an inch past the centerline. Remember, as negative kinetic energy increases, positive kinetic energy decreases, thereby decreasing forward swimming velocity.

According to the formula, as the swimmer reduces wasted movement and negative kinetic energy, their positive kinetic energy will increase and, he will be able to travel at higher speeds with less perceived energy output. This accounts for the sensation that he thinks they “could have worked harder and gone faster.” In reality, he went as fast as he did because he reduced their work load.

The zig zag is just one major source of negative kinetic energy. This principle of kinetic energy can be applied to any portion of the stroke. The applications for this principle are limitless. Over the next six parts of this series, we will deepen our knowledge of kinetic energy and begin to apply it to different portions of the stroke. In part two we will discuss the importance of the core, and a new way to train it to help manage the work loads transferred to the body during the
stroke.