Have you ever noticed that leaner men or women often swim faster than bigger, stronger men or women? Reducing frontal drag is certainly one of the reasons why a leaner body may have the potential to swim faster, but there is another reason, called coupling motions.
A coupling motion is defined as kinetic energy created within the body that augments the effect of the propulsive forces. These motions can occur either at the same time the propulsive forces are acting, or while the effect of the propulsive forces are still occurring. In swimming, the propulsive forces are nearly all derived from the hands and the feet, as those (along with the forearm) are the only parts that actually move backward in the water as the body moves forward. Yet there are many other motions of the body that can be used to enhance those propulsive forces.
Perhaps the easiest way to comprehend the effect of coupling motions is by visualizing a great relay takeoff. As the swimmer in the water approaches the wall, the swimmer on the block will step forward, first with one foot, then with the other. This sets the body in motion toward the desired goal of jumping as far out over the water as possible, taking advantage of the law of inertia. This motion is similar to the result that a long jumper running down the runway will have compared to a standing long jumper.
The swimmer on the relay also will swing the arms fully extended in a backward circle as fast as he or she can while pushing off the block with the feet. The kinetic energy of the arm swing is increased by lengthening the arms maximally (radius) and by swinging the arms as fast as possible (angular velocity). The act of swinging the arms does not lead to any direct propulsive forces that help the swimmer get off the block, but when coupled with the force of the legs pushing the swimmer, that motion results in a longer jump. The parts of the body are not working as an isolated system, but rather connected together in an open system, where all the motions of one part affect, either positively or negatively, the propulsive forces created by another. Further, all motions are also affected by outside forces in this open system, such as gravity and frontal drag, which impact our swimming speed.
There are other less important coupling motions that occur on the relay start, such as lifting the head forward during the jump and kicking the back leg up in the air, but the net result of all of these coupling motions acting during the time the force of the push off the block is still taking place, result in a better relay take off. Coupling motions not only occur in swimming, but in virtually every sport. The long jumper continues to move his legs and arms while in the air. The golfer or baseball player rotates the body and swings the hips forward to get a longer drive or a hit the ball more powerfully.
The coupling motions of a swimmer differ for each of the four strokes. In freestyle and backstroke, for example, the rotation of the body and the recovering arm swinging over the top of the water during an underwater pull can augment the distance a swimmer travels from the force of each pull (Distance Per Stroke or DPS). The faster the body is rotating and the longer and faster the arm is recovering, the greater the effect of the coupling motion. In breaststroke and butterfly, most of the coupling motions occur during the kick, not the pull. In breaststroke, the lunge forward of the upper body and head, timed precisely with the backward kicking motion, results in a longer glide after the kick. In butterfly, the swing of straight arms over the top of the water and the snap down of the head are coupled with the second down kick, resulting in more distance traveled from that kick.
If coupling motions really make us swim faster, why doesn’t everyone do them? The reason is that they require work, over and over again. It is much easier to swim by minimizing the energy in these coupling motions, but we swim much slower. In order to swim fast, we need to put a lot of energy into the effort, but it must be spent intelligently. Motions that do not couple with our propulsive forces or that lead to a huge increase in frontal drag will simply wear us down.
No one said swimming fast was easy, but one must also swim smart by using coupling motions. Read more about the Science of Coupling here: http://theraceclub.com/aqua-notes/freestyle-recovery/
Yours in swimming,