Over the past 100 years, scientists who study bone have benefited from amazing technological advances. These new techniques have vastly expanded and improved our ability to observe the behavior of bone under different conditions.
As a result, we now have an incredibly complete and specific understanding of how and why bones become weaker or stronger. The Journal of Musculoskeletal and Neuronal Interaction recently published a comprehensive review of our current understanding of the mechanical basis of bone strength.
This explanation of the mechanical behavior of bone is complex. But it is essential for understanding how we can implement specific scientifically-proven actions to improve the strength and health of our bones. So we're breaking it down into a two-part series of articles.
In Part 1, we'll explain mechanical load, the difference between stress and strain, and how they provide us the with ability to take control of our bone health. In Part 2 we'll look at the sources of strength in the structure and composition of bone, which explains why osteoporosis drugs are ineffective.
Mechanical Load Causes Stress And Strain
Mechanical loading is any physical pressure your bones experience and the impact of that pressure on their structure. It consists of two components: stress and strain.
Stress describes force exerted on bone. It can be caused by muscular contraction, impact loading, and gravitational forces. Every time you move, stop something that was moving, or resist the force of gravity, you’re placing stress on your bones.
Furthermore, stress produces strain. Strain is the physical yield to the pressure caused by stress. It's a structural deformation of bone material. Above a certain threshold, strain can cause damage to bone, usually in the form of microcracks, or in a traumatic case, fracture. But below that threshold, strain doesn't harm bone.
Bone resilience is the capacity of bone to absorb the energy from strain without suffering damage. This is also known as elasticity. We'll talk more about this mechanical quality of bone (and what creates it) in Part 2 of this series.
Strain is important because bones are mechanosensitive, which means that they can feel the physical deformation caused by strain. And when that strain meets certain conditions, the body responds by increasing bone mass.1
If we understand what causes our bones to ramp up the production of new bone mass, then we have an invaluable tool for improving and maintaining our bone health.
Mechanical loading is composed of stress and strain. Stress is a measure of force experienced by bone. Strain is a measure of physical deformation caused by stress. Resilience is the ability of bone to absorb strain without suffering damage. Our bones can sense strain and our bodies respond to certain types of strain by adding bone mass.
About Strain Magnitude
Strain, at its simplest, can be measured by magnitude– how much structural deformation occurs at the site of stress.
Researchers have charted the relationship between strain magnitude and bone adaptation, which includes changes in bone mass. Using this data, they developed the mechanostat theory.
The mechanostat theory observes that there is a minimum effective strain required for bone to maintain its mass. Below that strain, bone degrades. At just the right magnitude of strain, bone maintains its mass. Above that threshold, bone formation occurs for the purpose of increasing bone strength by adding mass.1
However, bone responds not just to a simple magnitude of strain but to a combination of magnitude, rate, frequency, distribution, number of loading cycles, and the rest-recovery period between applications of strain.1
Taken all together, this “strain environment” determines bone adaptation response. Next, we'll look at these other factors and how they impact bone formation.
Strain magnitude is a measure of how much structural deformation occurs. The mechanostat theory observes that there is a minimum effective strain required for bone to maintain its mass. At lower strain bone degrades, and at higher strain, it adds mass. However, magnitude isn't the only measure of strain that impacts the minimum effective strain.
About Strain Frequency
Strain frequency describes how many times per second a particular strain is applied to bone. For example, if you do five squats in one minute, that's a lower frequency than doing ten squats in one minute.
When you increase strain frequency, the threshold between losing and gaining bone mass shifts downward. That reduces the minimum effective strain required to stimulate bone growth.1
As a result, doing a low magnitude activity with high frequency can be just as effective as doing a high magnitude activity at a low frequency. Using the example of squats, if you add weight to your squats you don't have to do as many per minute to get the same effect as doing them more times per minute without adding weight.
Be careful though. High magnitude activities at a high frequency increases the likelihood of damaging bone. Conversely, only low magnitude activities at low frequency will likely result in bone loss since you wouldn't reach the minimum effective strain threshold.
To reach or exceed the minimum effective strain threshold, either the frequency or magnitude of strain must be high.
Strain frequency is the times per second a particular strain is applied to bone. For example, the number of squats you do in a minute is the frequency. When frequency is high, less strain magnitude is required to reach the minimum effective strain for maintaining bone mass.
About Strain Rate And Distribution
In addition to strain magnitude and frequency, a zoomed-in look at strain reveals more factors that influence bone creation.
Strain rate describes the change in magnitude over time within a single movement (strain cycle). Recall that strain is the measurement of structural deformation. When you squat, the strain on your bone is different at different moments during the movement. When you chart the changes in structural deformation from microsecond to microsecond during the squat, the resulting measure is the strain rate.1
Strain distribution describes the difference in strain across the bone itself. Imagine zooming way in on the site of the strain caused by doing a squat. Across the physical area where the strain is occurring, that strain isn't uniform. The difference in strain across that physical area is the strain distribution.1
Studies have found that more bone growth occurs when the rate and distribution of strain are highly variable. One way to increase the strain rate and distribution of an exercise is to perform the movement more quickly.2 To maximize the strain rate and distribution of your squats you would perform a swift downward movement then a swift upward one as opposed to a slow, steady, continuous motion.
Here's how the study authors described what we've learned so far:
“Bone cells therefore optimally respond to the net-effect of loading activity that is dominated by high strains (magnitude or frequency) changing at fast rates while presenting in unusual and unbalanced distributions”1
Strain rate measures the change in strain over the course of a single movement. Strain distribution measures differences in strain across the physical area of bone experiencing strain. High variability of rate and distribution results in optimal bone growth.
About Strain Volume
Strain volume requires us to consider a combination of strain magnitude, rate, and frequency as a singular loading cycle. Strain volume considers how many of the loading cycles occur within a particular period of time– often over the course of a day.
If your loading cycle is doing 10 squats in a minute, then your strain volume is how many times you perform that set of 10 squats within a particular time frame, usually a day. Strain volume is important because it impacts your bones' mechanosensitivity.
Mechanosensitivity is your bones' ability to sense strain. It's impossible to reach the minimum effect strain threshold if your bones can't sense the strain. Increasing strain volume reveals a pattern of diminishing returns.
Essentially, your bones have a limit of how much strain they can sense before they need to rest and reset. That means that once you've maxed out your bones' capacity for sensing strain, further strain won't cause a proportional increase in bone mass.1
Your bones need to rest! They need time to restore their mechanosensitivity following a loading cycle. Fortunately, the rate of recovery starts off very high, then slows over time. So it doesn't take a full day for your bones to regain most of their mechanosensitivity.
In the researchers’ own word:
“In particular, rest periods spanning ~15 seconds to ~4 hours increase bone formation outcomes by ~65% to 100%; whereas no significant advantage is evident beyond ~8 to 10 hours; and ~98% of mechanosensitivity restored ~24 hours post-loading event”1
This means that you can't do a week's worth of bone-targeted exercise all at once and still reap the benefits. Strain volume dictates that you must spread out your bone-building workouts over time to allow your bones to recover their mechanosensitivity.
Strain volume measures how often bones experience a particular loading cycle, like a workout routine. Bones lose their ability to sense strain (their mechanosensitivity) following a loading cycle. They must rest to regain their mechanosensitivity for effective strain to result in bone growth.
What This Means To You
This information allows us to be strategic about our bone-building physical activities. By choosing exercises that provide either a high magnitude of strain or a high frequency of strain, you can exceed the minimum effective strain required to maintain bone mass and stimulate the creation of new bone.
Movements with a high variability of strain rate and distribution also maximize bone growth. Carefully increasing the velocity of the movements in your exercise will help accomplish this goal.
However, there is a limited volume of strain that your bones can take advantage of each day. You must rest between workouts because strain reduces the mechanosensitivity of bone.
This detailed understanding of strain gives us the ability to make smarter and more effective choices about our bone-building exercise routine.
You can get professional guidance to apply this technical knowledge from the Save Institute's online video-workout platform SaveTrainer. There you'll find certified trainers leading bone-targeted exercise routines designed to optimize strain magnitude, frequency, rate, and distribution. Customizable workout plans will help you find a strain volume that makes the most of your bones' mechanosensitivity.
In Part 2 of our series on the mechanical basis of strength, we'll take a close look at the behavior of bone in response to stress and strain, so you’ll have a clear understanding of how to strengthen and build your bones with exercise and other physical activity.