When I say strong bones need rest, I mean they need rest relative to work or exercise. I will go into more detail in just a second, but first what to do I mean by “strong” bones. The word strong implies that bones are difficult to break due to their compressive strength, tensile strength, shear stress strength, and their elastic properties. From my point of view, a strong bone is one that does not break easily. A strong bone is not necessarily one with a high bone mineral density (BMD).
Unfortunately, in medicine the primary measure of bone strength is BMD, as measured by bone densitometry. Bone densitometry uses x-rays aka dual-energy X-ray absorptiometry (DEXA) to measure BMD, which is considered the gold standard because of its accuracy.
The problem with this measure is that it really says nothing about bone strength or quality. Yes it is true that BMD is correlated with bone strength, but the measure by itself is a poor of measure of quality, it just measures density. In theory, you can have very dense bones that are also weak and/or brittle. Conversely, you can also have a low DEXA scan score, but also have bones that are strong or do not break easily. So BMD only provides a small amount of information about bone strength. Unfortunately, at this time it is one of the better measures of bone health; although, there are more functional bone tests can give a more holistic assessment of the bone in conjunction with the DEXA.
Bone is a relatively hard and lightweight composite material made mostly of calcium phosphate in a crystallized form called calcium hydroxyapatite. This gives bone its rigidity. If bone were only composed of calcium phosphate crystals, they would be extremely brittle, like pieces of chalk even though they would have a high BMD. Bone is also made of a protein called collagen that gives it flexibility. A balance of flexibility and rigidity give bone its strength. As far as nutrition is concerned, strong bone needs lots of minerals, not just calcium, balanced by protein. So just know, that low BMD by itself does not cause bones to break.
When I tell my clients this, it is very empowering for them because there is so much one can do to strengthen their bones without worrying about increasing BMD with calcium supplementation.
It is unfortunate that doctors tend to push the only thing that they deem improves BMD, mostly calcium. They do this because bone is mostly made of the minerals calcium and phosphorus, and a high BMD is a primarily a measure of calcium phosphate in the bone. So the mantra is take more calcium to improve bone strength. Yes doctors will also prescrive vitamin D, but only because it is known that vit D greatly assists calcium absorption into the body. This is a very simplistic way to think about improving bone strength, because there are so many ways to improve your bone health without taking calcium or vit D. In fact, taking too much calcium can be a problem for good health.
The exercise recommendations for improve bone health made by doctors, personal trainers, nutritionists, etc. are misleading and possibly contributing to poor bone health. Most healthcare providers just tell their patients and clients that any form of exercise will be good for bone health. It may come as a surprise to some, but research shows that some forms exercise may be detrimental and even cause bone loss. People with osteoporosis or osteopenia need specific exercise prescriptions to stimulate bone repair and thus improve bone strength. The recommendation that any form of exercise is good, is in my opinion poor medicine.
A strong bone has a high capacity for self-repair. To stimulate self-repair, exercise prescriptions need to be specific both in type of exercise and for the type of individual doing the exercise. The other thing that should be specific is the prescription of rest, because without rest, bone will continue to break down.
What bones need for strength when solely speaking about exercise is:
1) high intensity,
2) multi-directional movement,
3) non-repetitive (rest-based) movement,
4) and, for greater bone stimulation one can, if appropriate for the client, also use the bone stimulating effects of gravity in the form of plyometrics.
So intense movement means creating a hormonal environment that is anabolic instead of catabolic. Think about the way a sprinter exercises and the way a marathon runner exercises. The sprinter exercises in a very intense short burst while the marathon runner tends to do the opposite. The exercise that a sprinter does is anabolic for the whole body including the bones as can be seen from their muscular lean phenotypes. The way a long-distance runner exercises over-stimulates the whole body potentially creating a catabolic phenotype. This catabolism can weaken bones. This is apparent for the different phenotypes (the sprinter being anabolic and long-distance runner being catabolic) seen between the two types of athletes.
Multi-directional means exercise that stimulates the body in all planes and directions. Think about how a tennis player has to be ready to move in any direction or a how a soccer or football player with the ball has to juke out defenders to get to the goal or end zone. These athletes naturally create better bones by moving in all directions thus stressing bone in almost every conceivable direction or angle.
The movement also has to be non-repetitive. For example, most sports with the exemption of long distance running, cycling, and swimming is non-repetitive. Basically the athlete goes hard and rests between bouts of movement. In American football, you have one play followed by rest (going to the huddle) followed by another play. In tennis there are brief moments of rest between volleys or serves of the ball. In soccer there is what I would call active rest if you do not have the ball or it is not in your area of the field. On the other hand, long distance running does not contain the rest component and thus over-stimulates of bone. This over stimulation causes the bone to ignore the stimulus and overtime puts one in catabolic state. Periods of rest are not only important during exercise but also between bouts of activity. Sleep is the most important rest time for bone health. So if the goal is to strengthen bones, one should think seriously about the rest component exercise in order to improve bone quality.
Plyometrics are movements that allow muscles to exert maximum force in the shortest amount of time. They usually include movements like jumping and hopping which use gravity to increase the amount force the skeletal system feels. These types of movements can be done with almost any fitness level as long as the exercise fits the bone health of the person. For example, an eighty-year old woman with weak bones can probably do jumps using support or even do them in a pool. This speaks to the art of prescribing exercise as medicine by matching it to the individual. If the fitness level and bone health of the person is really poor, then supported heel-drops may be best. Heel-drops are basically standing on your tip-toes and letting your heels drop to the floor with the assistance of gravity.
The following chart shows the different forms of activity and how bone responds to it.
The take home message is a strong bone is strong (sufficiently flexible and dense) because of its capacity for self-repair. In order to get the most out of self-repair, bone requires rest-based exercise and yes, even a rest-based life.
So remember, healthy exercise is not necessarily bone-building exercise whereas bone-building exercise is always healthy exercise.