Lift big weights, get big muscles, right? What about big bones too? Depending on how and when you work out, weight training can have an impact on your bone density as well as the size and shape of your bones.
This can be something that can hurt you in your youth, help you in your old age, and sustain you in between. It’s all about understanding what’s going on in there.
How Building Muscle Builds Bone
We want to meet our readers where they are – and you’re all in a different place. We don’t want to feed anyone more than they can chew, but we also don’t want to talk down to anyone. So, we’re going to start this article with a brief review of the physics and biomechanics of movement. Read along or jump ahead to the next section as you see fit.
Muscles work by contracting in length across a joint to pull bones (and any weight) closer to the body. Or, they expand in length across a joint to push bones (and any weight) away from the body. Different muscles in different places do different jobs in different conditions, but this is the general rule of thumb.
When you work out, your muscles do all the work and they feel all the burn. And, you’re probably working out to build your muscles in the first place. The result is that it can be easy to forget about where your bones factor in. However, your bones are being pushed and pulled by your muscles, and they’re resisting the force of gravity on the weights.
So, if your muscles kept getting bigger and bigger and your weights kept getting heavier and heavier, but your bones never got any stronger, your bones would eventually break during exertion. But, how do bones get stronger? This actually happens in a couple of ways, mainly restructuring and adding bone density.
A Primer on Bone Structure
You know that your muscles are made up of smaller fibers. The orientation of these fibers helps to determine what the muscle actually does (if you didn’t know that, click on that hyperlink above). Bones are actually very similar.
Instead of thinking of bones as bundles of fibers, think of them as bundles of tubes – straws, if you like. If you have a bunch of straws banded together, they can support more weight – but only if the weight is applied with the right orientation. And if you filled the straws with, say, sand, they could support even more weight.
As we’ll see, bones can realign these tubes that make up their internal structure depending on the kind and amount of force that is applied to them. As far as “filling the straws with sand,” this is the mineral deposition that increases bone density. This is an exercise thing, but it’s also a diet thing – your bone density can’t increase if your diet is lacking in the necessary nutrients.
But, we’re getting just a little ahead of ourselves.
Working Out and Wolff’s Law
All the way back in 1892, a German scientist suggested that the alignment of the rods that make up some kinds of bone is “strongly correlated” with the direction of the forces acting through that bone. The scientist was named Julius Wolff and we call this idea “Wolff’s Law” (Abernathy et al., p 13).
Of course, that was the 1890s. People were still jazzed about evolution. Wolff’s law as it was could suggest why a bone might have evolved to be the way that it is after generations of evolution. But, it doesn’t necessarily suggest that bones can change shape within one human life based on how that life is lived. That’s the difference between “evolution” and “adaptation.”
It wouldn’t take long, however. A few decades after Wolff’s initial publication, Sir Arthur Keith suggested that bones can change their internal and external structure in the way that allows them to sustain the necessary forces of their function in the most “economic” way possible (ibid., p 74). This isn’t “Keith’s Law,” it was just sort of incorporated into Wolff’s Law.
Taken together, these ideas do mean that we can increase our bone density and overall bone health through activity. This is part of why physical activity is recommended for older adults who often deal with bone loss. It’s also part of why physical therapy is so important after you break a bone.
In fact, Wolff’s law also helps people like anthropologists find out how people lived when their bones are left behind and their muscles aren’t. Pull up an early humans documentary on your favorite streaming service and we pretty much guarantee that at some point someone in a blue coat will talk about how we know what someone did for a living because of their femur diameter.
What Workouts Do You Need to Know?
Every workout puts force on your bones because every workout requires your muscles. However, the greater a force is involved, the better that workout will be for your bones – particularly in terms of increasing bone density.
Weight lifters have the highest bone density of any class of athlete, and lifting weights is definitely something to think about if you’re after greater general bone health (ibid, p 75). Similarly, activities like running put enough stress on your bones to contribute to bone density and strengthening, though this is only your leg bones, of course.
So are there activities that don’t contribute to bone density? Actually, yes.
For example, there’s a whole HTBM article about how great swimming is and, while swimming is definitely a valuable and underrated exercise, the force of resistance from the water isn’t really enough to task your bones. In fact, we recommend it for people with joint problems because the force is so low, it actually takes weight off of your bones which can take away from bone density.
Thinking about Diet
Bone density has a lot to do with your bones aligning their own internal and external structures in response to force. However, it also has a lot to do with just stuffing your bones full of minerals and collagen (Tortora & Derrickson, p 186).
When you were younger, you were probably pushed pretty hard on dairy products because they build strong bones. While dairy products do contribute to bone density, minerals like calcium actually go to your bones last – it’s basically what your body does with minerals after using them for things like conveying nerve impulses (Pope et al., P 287).
That means that you really should understand the dietary recommendations for your age and sex, but also that these dietary recommendations are probably different for you if you have an advanced activity level. Generally speaking, it’s best to get your nutrients from diet, but there’s also nothing wrong with exploring supplements.
It’s time to throw in that classic HTBM recommendation that you explore this field with your doctor – particularly if you are interested in supplements. Between high doses and lower absorption, supplements taken without the proper care can build up in your body leading to health problems.
Remember too that your body’s regular functioning is a complex chemical orchestra. Your body requires “support” nutrients like vitamin D to successfully process calcium, so if bone density is what you’re after, you probably need a more nuanced plan than throwing calcium supplements down your mouth.
We’ve mentioned collagen as well. Collagen is a kind of protein but not the kind that you normally think of. It isn’t as important to your muscles but it is crucial for your bones and connective tissue like tendons and ligaments. Many athletes can and do take collagen supplements, but it can also be found naturally in white meat and even some fruits.
Experimenting with supplements isn’t the only time that you need to talk to your doctor, coach, or personal trainer when you think about exercise and bone density. We’re looking at you, younger readers.
When your body is still growing, up through your teen years, your bones have their marching orders. When they have to change what they’re doing to accommodate for added forces and stress, they can do some pretty crazy things. Further, those still-growing bones aren’t meant to take the high forces that some workout regimens may require.
The results can include issues like stress fractures when you’re still young as well as lasting malformations into adulthood. Of course this doesn’t mean that young people can’t work out, it just means that they need to avoid pushing themselves too hard, be sure to have a good diet, and get plenty of sleep.
Bigger Bones, Better Health
Usually we talk about how muscle builders need to think about cardio. Here, we really want to emphasize that cardio people should be thinking about at least some weight lifting. We’ve talked in other articles about how some health metrics are really primed for optimizing when you’re younger and start to go downhill when you’re older.
Bone density is one of those metrics. If you build bigger stronger bones while you’re younger, you can stay more active with more ease later into your life. So, hit the gym!
Abernathy, Bruce; Kippers, Vaughn; Hanrahan, Stephanie J.; Pandy, Marcus G.; McManus, Alison M.; Mackinnon, Laurel. “Biophysical Foundations of Human Movement” (3rd. Ed.). Human Kinetics. Champaign, IL, U.S.A. 2013.
Pope, Jamie; Nizielski, Steven; McCook, Alison. “Nutrition for a Changing World.” Macmillan Learning. New York, NY, USA. 2015.
Tortora, Gerard J. & Derrickson, Bryan. “Principles of Anatomy & Physiology” (14th. Ed.). Wiley. Hoboken NJ, U.S.A. 2014.