If you’ve been around the water for a long time, you may have noticed that not everyone is gifted with the natural ability to float on the water’s surface. No matter what they do, they seemingly cannot grasp how to perform this basic swimming skill. Why is that?
Certain individuals cannot float because they have a high percentage of muscle, but a low percentage of body fat. Muscle has a density value of 1.1 g/mL, which is slightly denser than water (1.0 g/mL) and fat (0.9 g/mL). Muscular individuals have denser body compositions and are more prone to sinking.
Put simply, the capacity to float on the water’s edge requires a basic understanding of the mechanics behind buoyancy and density. Below, we’ll break these fundamental concepts down in greater detail and discuss additional factors that influence a person’s ability to float.
The Ability to Float Ultimately Comes Down to Body Density
To fully understand why certain people cannot float, it’s necessary to have a basic foundation of knowledge regarding buoyancy and density. These fundamental physics principles may seem a bit intimidating at first, but they’re actually somewhat easy to grasp when explained in plain English.
- Buoyancy: The natural inclination of an object to ascend or descend within a fluid.
- Buoyant Force: The upward force exerted on any object that’s fully or partly submerged in fluid. This natural force is responsible for why objects float in the first place.
- Density: The amount of mass per unit of volume in a given substance. It is largely considered as the measure of how “compact” a material is.
Buoyancy is directly related to density. As a general rule of thumb, remember that as the density of an object increases, its natural buoyancy—or ability to float—decreases.
Some objects have such a considerable density that they’re considered to be negatively buoyant. Under these circumstances, the object in question has a weight that’s heavier than the weight of the water it displaces. For this reason, the buoyant force cannot support the object on the water’s surface, causing the object to sink into the depths below.
That’s cool and all, but how do these physics concepts relate to human flotation?
Negative Buoyancy in Relation to Humans
In recent years, the concept of negative buoyancy has been taken from the context of scientific research and applied to the realm of swimming.
Every human has a slightly variable body composition in terms of their overall distribution of bone, muscle, and fat tissue. We touched on this earlier, but it’s worth repeating that each of these body tissues have different average density values. Some of these average density values are higher than the average density value of water, while others are lower than the average density value of water, as depicted in the chart below (source).
|Substance||Average Density Value|
|Fat Tissue||0.9 g/mL|
|Muscle Tissue||1.1 g/mL|
Although these density values seem practically identical from an outside perspective, these subtle differences are the reason why some people physically cannot float.
In the chart above, you can see that fat tissue has a lower density value than water. From our discussion about buoyancy and density, you know that substances that are less dense than water have a natural tendency to float.
The average adult male has a body fat percentage in the range of 18 to 24%, whereas the average adult female has a body fat percentage in the range of 25 to 31% (source). This amount of body fat is typically sufficient for a person to float along the surface of the water with positive buoyancy or at the very least neutral buoyancy.
Referring to the chart above once more, you can also see that muscle tissue has a higher density value than water. From this, we can conclude that muscle tissue will have a natural tendency to sink when submerged in water.
The average adult male has an approximate muscle mass percentage in the range of 36 to 40%, whereas the average adult female has an approximate muscle mass percentage in the range of 29 to 31% (source). This amount of muscle mass is usually not enough to counteract the natural buoyancy effects of body fat.
With both of these factors working in unison, a person may eventually reach a point where they become negatively buoyant—that is, their body weight becomes heavier than the weight of the water that they displace.
It’s important to bear in mind that humans have slight variations in bone density as well. A higher bone density has a definitively inverse relationship with buoyancy. Most people focus entirely on varying body fat and muscle mass percentages, overlooking the fact that bone density plays an equally important role in flotation.
Note that it’s fairly rare for a person to have a low body fat percentage, a high muscle mass percentage, and an abnormally high bone density all at the same time. This is the underlying reason why the inability to float is such an uncommon phenomenon in the general population.
Are There Any Other Determinants of a Person’s Ability to Float?
Although body density is a crucial factor in whether or not someone can float, there are certainly other factors to consider. We’ll delve into the most prominent of these factors in the subsequent sections.
It’s easy to have tunnel vision when analyzing the effect of density on the ability to float because most people dial in on the effects of body density. What people fail to realize is that the density of the swimming medium is also just as critical to buoyancy as the density of the person’s body.
Unfortunately, many people are under the false impression that all water types have identical densities. These people fail to take into account the variable effects of salt concentration on freshwater density and saltwater density.
The reason that saltwater has a slightly higher density than freshwater is that the presence of salt adds extra mass to saltwater without drastically increasing its volume. Normally, water would have a density of 1 g/mL, but the presence of salt increases the density of seawater to 1.025 g/mL. Consequently, seawater is able to support more weight with its buoyant force relative to freshwater (source).
This is why humans generally have an easier time floating in seawater. So next time you head to the ocean, try to float on the water’s surface and see if you notice a difference!
Another overlooked aspect of flotation capabilities is how a person breathes. The amount of air that person has in their lungs has a direct correlation with how buoyant they will be in the water. The greater the amount of air in the lungs, the more buoyant they will be.
This phenomenon can be explained by the fact that an excess amount of air allows a person to displace a higher volume of water. Since the weight of air is negligible when compared to the weight of the displaced water, this extra volume of displaced water can be used to prop the person up and cause them to float.
In the case that a person’s lungs are not filled with air, the person takes up less volume and displaces less water. Consequently, there’s a reduced amount of water available to optimize the upward buoyant force that allows an individual to float.
The problem is that the majority of people that have a denser body composition tend to panic at the first sign of sinking. This panicking causes them to take short, shallow breaths, which causes them to sink in the water even further.
This is why swimming instructors place such heavy emphasis on taking long, deep breaths in an emergency. Not only will this breathing practice calm you down, it will help you to float on the water as well.
The last prominent factor that affects flotation capabilities is body posture. Depending on how an individual orients their body in the water, they can maximize or minimize the upward buoyant force that props them up on the water’s surface.
Ideally, your body posture should maximize the amount of contact that that you have with the water’s surface. This way, your weight is spread out across a larger section of the water. Put simply, a greater body weight distribution on the water’s surface is best for the buoyant force to do its work.
The most effective way to spread out your body weight as much as possible on the water’s surface is to lie on your back with your upper and lower extremities extended outward, similar to a starfish. Positioning yourself in this fashion is easier said than done, especially in a crisis situation.
Unfortunately, the majority of inexperienced swimmers tighten up as soon as they encounter any unexpected complications. They flail around with their arms and legs, unknowingly bend at the hips, and keel over. All of these practices concentrate a person’s body weight onto one tiny sliver of the water. Consequently, there’s less water displaced and an insufficient amount of upward buoyant force to support their entire body weight.
It’s a challenge to relax, remain calm, and remember these strategies under duress. Any panicky motions only further exacerbates the problem of not being able to float.
Does Everybody Have the Capacity to Float, if Given the Proper Training?
In the swimming community, it’s a somewhat controversial topic to discuss whether or not there are a particular subset of individuals that can truly be considered negatively buoyant. Up until this point, there has been no clear scientific research or studies to indicate any real consensus on this debate. For this reason, people have been forced to make hypotheses based on personal experience.
For example, MGR Swim Sports claims that “Everyone can back float! Even if your body is composed of 100% muscle, you can float on your back” (source).
Highly acclaimed fitness enthusiast and former Navy SEAL, David Goggins, claims otherwise. On multiple occasions Goggins has said that he himself is negatively buoyant, sinking to the bottom of the water even with a full breath of air. He has stated that this negative buoyancy greatly complicated his SEAL training, to the point where he almost quit (source).
The problem with forming hypotheses through personal experience is that the data can sometimes be skewed one way or another. It is clear that large-scale scientific studies must be conducted around this topic. Sadly, there has yet to be many studies with a sample size substantial enough to draw any viable conclusions. Until this data is gathered, this debate will continue to be a “He said, she said” ordeal.
The Bottom Line
The primary reason that certain people cannot float in water is an abnormally dense body composition. A higher bone density combined with a higher muscle mass percentage and a low body fat percentage will result in a natural inclination toward sinking rather than floating.
Whatever your body composition is, you should make an effort to learn how to swim in order to gain confidence in the water and open yourself up to the realm of water sports. You won’t know what you’re missing until you try it!