Two Important “Coupling” Motions That Impact Your Swim Speed

Published April 29, 2015 01:15PM

The less-obvious parts of your stroke that could help you get through the water quicker.

In order to move quickly through the water, the motions of a freestyler may look effortless, but they are actually very complex. The propulsive forces that make you move forward in the water come almost entirely from the hands and feet, but it’s the less obvious motions in addition to kicking and pulling that also have a big impact:

These three motions create no direct propulsive forces, but they can impact your speed or distance per stroke. We call them coupling motions. The two most notable coupling motions in freestyle are body rotation and arm recovery. It seems a bit mysterious how the counter rotation of the body, timed precisely with the underwater pull, could somehow make the effect of the underwater pull greater, leading to more distance per stroke, but it does. (Yet if you lie face down in the pool and rotate your body all day long, without a kick or pull, you will not move forward one inch.)

Aside from Newton’s three laws of motion that govern every stroke we take in the water, there is a fourth law that comes into play with respect to coupling motions: The Law of Conservation of Energy. This law states that the energy within a system can be neither created nor destroyed, but it can be transferred to other forms of energy.

Examples of Real-Life Coupling Motions

Picture when a moving car collides into a parked car. The kinetic energy of the moving car gets transferred into heat, crumpling metal and kinetic energy of the parked car that now starts moving. Swimmers are working in an open system—we cannot isolate the kinetic energy of a rotating body or a recovering arm, as if it were a closed system, because all of the body parts are connected. The motion of one part affects another. Like the cars colliding, the energy of a counter-rotating body or a recovering arm, when timed correctly, can positively impact the propulsive forces of our hands and feet. Further, all the forces of nature, such as gravity and drag, are acting on the system simultaneously. Even though coupling motions require more work, we can use them to our benefit by enabling us to swim faster.

Another example of a coupling motion that is a little easier to visualize is the elite long jumper, who keeps moving his legs and rotating his arms in the air, after the force of the takeoff leg has occurred. So long as the effect of the leg force is still in place and the body is still flying through the air, the coupling motions can augment the effect of that propulsive force, resulting in a longer jump. Simpler and more common coupling motions are the arms swinging while walking. The arm swing adds no propulsive force to the gait, but results in a longer stride.

Hip-driven vs. shoulder-driven freestyle

The recovering arm can serve as another coupling motion in freestyle, but only with shoulder-driven freestyle, not hip-driven. The difference between hip-driven and shoulder-driven freestyle is largely determined by the stroke rate. The slower rate of the hip-driven freestyler is due to the longer time with the hand held out front, before initiating the propulsive phase (when the hand begins moving backward in the water). With hip-driven freestyle technique, the lead arm does not initiate the propulsive phase of the underwater pull until the trailing hand is already in the water. By that time, the kinetic energy of the recovering arm has reduced to near zero, so no coupling with the pulling arm can occur.

With shoulder-driven freestyle, where the underwater pull is initiated much sooner, the propulsive phase of the pulling arm is occurring while the recovering arm is in full swing. For this reason, it makes more sense for shoulder-driven freestylers, and particularly sprinters, to straighten the recovering arm more and increase the speed of arm rotation in order to increase the kinetic energy and the effect of the coupling motion.

For hip-driven freestylers, it makes sense to use as little energy as possible in the recovering arm. In other words, bend the elbow and keep the hand closer to the water during the recovery. While virtually all sprinters are shoulder-driven freestylers, distance swimmers (triathletes) can be either. With hip-driven, shoulder-driven or hybrid freestyle, the faster the counter-rotation of the body during the propulsive phase of the pulling arm, the more coupling effect the motion will have and the more distance per stroke we can achieve.

How to improve your coupling motions

Increasing the coupling energy of these motions, when timed correctly, will improve your distance per stroke and swimming speed. If you are a hip-driven freestyler with a stroke rate less than 74 strokes per minute (37 right arms, 37 left arms), by rotating the body at a faster speed, you will swim faster. That means you need to rotate the body further in each direction and snap it around to the other side quickly. Both rotation and counter-rotation require a strong core, so start working on your core strength, particularly the oblique abdominal muscles.

If you are a shoulder-driven freestyler with stroke rates higher than 80 per minute, you can benefit from the coupling energy of your arm recovery by straightening the arm more on the recovery. This motion will require more work to sustain with strong, fit shoulders, but if you practice enough this way, you can manage.

Both of these coupling motions require work to do well. No awards are given out for the least amount of calories expended in a race. In order to swim fast, one needs to invest energy into the system. It just needs to be smart energy and coupling motions are smart energy.