Feel It in Your Bones

Pop Quiz for you

Question: Can you name two people recently who exercised at least 2 hrs per day for 9 months but still lost bone mass?

Answer: Suni Williams and Butch Wilmore - the astronauts stuck on the International Space Station until March 2025. 

The reason for their loss of bone (and muscle) was the lack of gravity. Allow me to explain why this happens and why it is relevant to us on planet Earth... 

Bone is Dynamic

Throughout evolutionary history all animals have been exposed to and adapted to living with gravity. The composition of our bones is a trade-off between the strength required to resist typical forces without breaking and the lightness required to be able to move on this planet with its gravitational pull of 9.8 m/s2. To accomplish this trade-off, bones are not uniformly solid but instead have a matrix-type structure with varying degrees of empty space. The more matrix and mineral material, the denser the bone and the more porous the lighter the bone. 

Bone is a dynamic tissue and its density can change over time. This makes biological sense: bone is a resource expensive tissue requiring proteins, minerals and energy to maintain. Meanwhile, our body is on an efficiency-drive and will only maintain the amount of bone that is necessary to support your particular size and shape and to resist the forces it regularly experiences. 

Clever Bones

To be adaptable, bone has a measurement system. Ignoring technical terminology, bone cells can measure ‘squish’. Living bone tissue is quite flexible and the bone cells are bathed in fluid. When there is a force or jolt to a bone, this fluid is squeezed around the cells and it is this ‘squish’ that is measured. The larger the squish, the bigger the build bone signal. In the absence of regular squishes of a certain magnitude, bone tissue will be lost. It is an elegant system but this adaptability also comes with liabilities. 

In a zero gravity environment, like on the International Space Station, astronauts are close to weightless and there is no compression of bones into the ground; you float. Despite conscientiously exercising in space using vibrational treadmills and weights it is not enough to counteract the 22 hrs of no squeeze to their skeleton and all astronauts lose bone. According to their bodies, they require much less robust bones. Unfortunately, there is no way for the astronauts to signal to their bones that in a few months they could do with all that bone back again for life on Earth. 

Here on Earth

While most of us will never experience zero gravity, many of us are actually dealing with a similar but less extreme version of the same issue; bone loss through insufficient loading—not enough daily bone squeezes or the squeezes are too small in magnitude. This can be particularly an issue as we age. Due to several factors, not least declining sex and anabolic hormones, our bone tissues become less responsive to a given force, so more is needed for the same result. Unfair I know. 

If you have low bone density or are worried that you might have in the future, you need to make sure you partake in movements that not only expose your bones to gravity through upright movement but to actually amplify the gravitational force through your bones with weight bearing impact exercise:

Weight Bearing activities such as standing and walking are those where your feet and legs support your body weight.  

Weight-Bearing Impact activities are those that put a bigger force or jolt through our bones.

Build Impacts to Build Bones

As some of you may know, I adore swimming, but it is not a good activity for bone building since the water supports your weight. Cycling is also not a good choice as you are partially supported by sitting on the bike. If these are your sports of choice, you need to do additional activities for bone health. 

Jogging and running are great for hip and leg bones. Even better are sports that involve what we call odd-impact movement—such as football and racquet sports—which demand frequent changes of direction. These activities result in many different directional forces to your leg bones - you get strengthening across the whole cross section of the bone. Other options are a simple jumping or plyometrics practice and you can make sure you add varied directional jumps to strengthen all around the bone. 

Maybe your knees or pelvic floor are not currently up to jumping or running. You can still add more BUILD BONE signals to lower impact activities. One example is to add weight to your body when you walk: a weighted vest or heavy backpack add greater ‘squish’ to your bones with every step you take. You can also start very light impact training with something like heel drops and progress as your knees, hips and pelvic floor allow. 

So my fellow Earthlings, gravity is your bones' best friend but like all relationships, you have to keep working at it.