The Purpose of Water: Swimming and Osteoporosis

Lt. Shannon Scaff, an instructor at the Coast Guard Maritime Law Enforcement Academy in Charleston, S.C., takes a lap during a long distance swim he dedicated to a fallen Coast Guard aircrew, Feb. 27, 2015. Scaff undertook the challenge of swimming in a local Charleston pool for 24 hours to bring awareness and support to the families of fallen military members. (U.S. Coast Guard photo by Petty Officer 1st Class Stephen Lehmann)
Photo Courtesy: Petty Officer 1st Class Stephen Lehmann

By Ronald Hehn, Swimming World Contributor

Recently, NASA conducted an experimental study concluding that fish demonstrate decreased bone density when exposed to micro-gravitational conditions. The same effect may occur in humans submerged in water.

Osteoporosis is a health condition identified by the reduction of bone density caused by a number of factors, one of which being the lack of exposure to gravitational resistance i.e. impact. Those affected often use in-water recovery exercises for rehabilitation.

On the other hand, many competitive swimming programs adopt high-impact dryland and high-intensity swimming programs.

I am suggesting that both of these approaches are impractical, respective to the desired outcome of each group.

Here’s why…

Submersion in water creates a resistance that is less than gravity, which decreases bone density. High impact dryland activities induces resistance that increases bone density. The body naturally adapts to its environment; bone density increases due to gravitational resistance and decreases in its absence (cf. Darwin).

Consider Bernoulli’s Principle…

According to the principle, increasing Velocity increases Drag. Drag creates resistance. Consequently, Drag is the main opponent of Velocity.

Next, consider Drag…

Drag = Drag Coefficient x ((Mass Density x Velocity Squared)/2) x Reference Area

The Drag Coefficient is determined experimentally and varies according to the structure of the object.

The Reference Area is the surface of the object exposed to the water during movement. In the case of a swimmer, this surface is the shoulders and chest. Less Drag occurs when less of the object’s surface is exposed to the water.

“Mass Density” is the density of the water.

“Velocity” is movement.

According to the equation, increased Velocity exponentially increases Drag; similarly, increased Drag decreases Velocity. Therefore, competitive swimmers best maximize Velocity by reducing Drag.

Finally, consider Buoyancy…

Buoyancy may be explained by Archimedes’ Principle of Density.

Archimedes proposes that given a constant Volume of water, an object with less Density will float better than it’s more dense counterpart.

As you decrease the Density of an object, you increase its Buoyancy. The most controllable constituents of the human body are muscle, fat, and bone; according to this principle, an object that lessens bone density yet maintains a consistent amount of fat and muscle increases Buoyancy, decreases Reference Area, decreases Drag, and increases Velocity.


In conclusion:

Non-competitive athletes that choose an aquatic environment for rehabilitation will decrease bone density which increases Velocity in the water: not the desired outcome.

Competitive swimmers that choose a dry land environment for exercise will increase bone density which decreases Velocity: not the desired outcome.

In my opinion, these philosophies seem counter-intuitive to the goals of the respective groups.

Use water for the right purpose. Drink up.

In my humble opinion.


Chatani, Akiko, Akira, et. al. “Michrogravity promotes osteoclast activity in medaka fish reared at the international space station.” Scientific Reports 5 (14172) (September 2015)

Layne Je, Nelson ME. “The effects of progressive resistance training on bone density: a review.” Medicine and Science in Sports and Exercise. [1999, 31 (1):25-30]

McCormick, Barnes W. (1979): “Aerodynamics, aeronautics, and flight mechanics.” P. 24, John Wiley & Sons, Inc.

Brent Rushall. “Lift or drag in freestyle swimming?


  1. avatar

    Am I correct in assuming that swimming is not as good as walking and weigh lifting for osteoporosis as once considered? The article was a bit confusing to me.

  2. avatar

    Then what is your suggestion for swimmers….

  3. avatar

    Could you please explain more about it

  4. avatar
    Ronald Hehn

    Nancy – That is absolutely correct. Submersion in water will decrease bone density.

  5. avatar
    Ronald Hehn

    Amanda – My suggestion depends on the goal.

    • avatar
      Ronald Hehn

      *Ananda, apologies, auto correct gets me every time

      • avatar

        Fish are not people. There is no evidence they would react the same in water in zero gravity (fish), training in water several hours per day(people).

  6. avatar

    Fish are not people. There is no evidence they would react the same in water in zero gravity (fish), training in water several hours per day(people).

  7. avatar
    Ronald Hehn

    Thank you, Jim, for your comment! I believe that physics and biology apply to all things in nature. What affects fish, affects humans. We are the same.

  8. avatar
    Ronald Hehn

    Thank you for your comment as well, Ananda! Apologies for the shorthanded prior response. My suggestion for competitive athletes is to avoid the weight room, and spend more time submerged in water; allowing a swimmer to adapt to it’s aquatic environment, and become less of a “land animal.” On the other hand, those with osteoporosis and bone density issues should seek an on-land rehabilitation method rather than in-water, because submersion in water reduces bone density, in my humble opinion.

    To answer your question directly, I suggest that competitive swimmers avoid high-impact training (i.e. weights, dry land), and instead focus on time spent in the water. As an athlete, I made gains in college with weight training, but science tells me that I may have had more gains if I had neglected it. Just my thoughts.

    In my humble opinion.

  9. avatar

    Seems a little far fetched. As much time as we put in the water, we spend a lot more out of water. I think we will continue to be land creatures (not fish)

    • avatar
      Ronald Hehn

      Thank you for the response Kaden! It’s an evolutionary (and perhaps revolutionary) concept, based on Wallace’s and Darwin’s theories of natural selection leading to evolution of a species. Perhaps a breeding group of swimmers throughout generations could produce a more fish-like human.

    • avatar
      Ronald Hehn

      On evolution and adaptation: “Every species has come into existence coincident both in space and time with a preexisting closely allied species.” “Sarawak Law” by Alfred Wallace (1855)

  10. avatar
    Dan Smith

    His ron:

    Interesting opinion. Could you tell me where in “Origin of Species” you found Darwin’s theory on short-term adaptation as opposed to heritable characteristics he was writing about? I have the 4th edition (there were 7 editions prior to Darwin’s death), and can’t seem to locate the spot in his book, or other writings. If I didn’t know better, I’d have thought you made the mistake of confusing short-term adaptation/acclimatization, which reverses with genetic adaptation, i.e. evolution. Can you also give your thoughts on bone as a living tissue and its response to stress, like swimming? You seem to be arguing that the stress of swimming produces no short-term adaptation from muscles which affect bone growth through stress.

    • avatar
      Ronald Hehn

      Hello Dan!

      Thank you for your insight, as you are absolutely correct that Darwin’s “Origin of Species” does not suggest adaptation within a species’ lifetime; it is, in fact, an inherited trait. I felt the need to con fer (cf.) to Darwin because Alfred Wallace’s “natural selection” theory and evolution went hand-in-hand at the time “Origin of Species” was written. Based on Wallace’s “natural selection” theory, adaptation that occurs within one’s lifetime can affect non only their own existence, but the existence of future generations (cf. Darwin, evolution), i.e. changes within one’s lifetime can lead to the success of the future generations of swimmers (or it already has!). If the swimming population is consistently reducing bone density generation after generation, eventually it could lead to some humans being more structured for swimming than others.

      In response to muscle growth, the relation between muscle growth and bone density would be the type of exercise that caused the muscle growth, which is often a dry-land program. While strictly swimming, I often lost muscle mass and became more lean; however, combined with a dry-land program I would increase muscle mass and become more bulky. Dry-land programs increase my bone density; consequently, my theory is that swimming decreases bone density.

      Comment with any questions/thoughts you have. A good conversation is great for those that are not even involved in it, but seek to become more educated. Thank you for the comment!