I was typing out some thoughts on how body composition affects swimming on a TI Forum thread and thought it would be good to share the same here…
Perhaps this will also help explain why we make the search for “The Path Of Least Resistance” the paramount objective in swimming fast, far, or for fun.
Though I am not a physiologist I like to study a broad range of perspectives, like many of you, to keep expanding my understanding and broaden my appreciation for the complexity involved in swimming well. This is what I keep in mind about the various factors about body composition and hydrodynamics:
We have the natural forces of gravity pushing down, water pressure pushing up, and water resistance pushing against a body trying to move around in it. Those interact with body composition, body position, and movement patterns.
Body density determines how deep in the water a body will find its neutral point (in prone position). If a swimmer tries to swim above or below this neutral point (or bobs up and down between them) he must fight those natural forces with part of his energy- which means that energy is no longer available for forward movement. A swimmer deep in the water has more water pressure to work against. A swimmer higher in the water has to be careful with wasting energy against gravity and creating more wave drag.
Fat and muscle and bone are related to this density composition – making up most of what we may focus upon (I am not interested in shrinking my brain mass, for instance, to shave a little off my swim times). I am thin boned, lean, and generally low fat while my adopted daughters are about as opposite as you can get bc of genetic differences. I have to work with what I have, and play to my advantages, and I am helping them learn to play to theirs. I also keep these factors in mind while working with true sinkers – we have ways to work with the physics to get them to the surface without struggle, but I admit it is not easy.
Not all fat is equal in composition- there is intra-muscular fat, and there is subcutaneous fat, and there is white fat and brown fat. There’s likely more types I haven’t learned about yet. Each have different properties and functions and use blood and deal with energy differently.
And of course, an OW swimmer can tell that the body channels blood differently depending on the temperature of the water. As the sea gets cooler this winter I am experiencing the changes the cool water provokes on how blood flows in my body to conserve heat in my vital regions to keep me alive while keeping just enough going to the appendages to allow me to make the strokes.
Mass does play a part in how easily a person can move through the water, because mass affects water displacement. Just like the stats on the hulls of ships of all sizes is important information for pilots to consider in the performance potential of a ship, so does a swimmer’s mass affect motion in the water. The more mass, the more volume of water that has to be shoved out of the way. Which then leads to the consideration of shape and how to adjust it so that water has an easier or harder time being shoved out of the way.
The ‘average’ body composition swimmer gets to keep 5% of her body mass out of the water. So which 5% will she choose? (I choose to balance in a perfectly prone position, parallel to the surface and leave a sliver of the head and a sliver of the hips above.) Keep in mind that the head makes up roughly 10% of body mass, so any time a swimmer lifts the head above that neutral position she forces another part of the body be shoved down deeper into the water. If she doesn’t want to swim at an diagonal angle, what will compensate for this then? A kick to push the hips up perhaps- and again, this is energy being diverted to dealing with vertical forces rather than forward movement. Tilt the head to look ahead even a little and she has now pushed the head above the neutral line – a swimmer has to consider how much she is willing to pay for that glance or continuous gaze forward. I’ve chosen to learn to swim without the eyes looking forward to save the energy and save the strain in the neck.
In boat-building there is the factor of hull-design. Specifically, the length-to-width ratio of that hull. A basic principle of boat design is creating a longer narrowing vessel so that it requires less power to move forward – IOW it displaces water easier. Think of the difference in paddling an innertube across a lake, versus a row boat, versus a sea kayak. Everyone intuitively knows which one to prefer. One of the underlying physics reasons for learning and then holding a long body line during stroke transition (the non-propulsive gap between catches)- if that length/width ratio slips even a little the resistance of water, by calculations of this displacement ratio, shoots up exponentially- a major stroke engineering consideration. In TI Swimming we call this skill “Active Streamline”. Hence, TI For Speed makes a very careful examination of how to crank up tempos because a great deal of that extra power can be negated by the extra displacement required by an unnecessarily shortened body line.
Looking at all these factors I then divide them into what I can control or adjust and what I can’t and must learn to adapt to or live with. By this I know how what I can work on, and in what priority I should work on it.