Bone Remodeling, Osteoporosis, and Training

Bone remodeling is probably not the most exciting aspect of resistance training. But it is probably one of the key reasons many elderly are turning to weights, and rightfully so! Many studies have shown that increased stresses on bones (like that which occurs through weight training) results in greater bone mineral density in early adulthood, and reduces bone mineral loss later in life.

When people thing of physical development through resistance training, they rarely think of the adaptations that bones undergo. Bones are however a growing organ (just think of finger nails and broken bones), and respond to stress just like muscle does. Unlike muscles which undergo contractile forces, bones experience bending, compressive and torsion forces, as well as the forces created by muscle at the tendonous insertion at the bone.

Some Bone Physiology

The surface of the bone is like a sheath, call the periosteum, and this is primarily where bone formation occurs. Here, osteoblasts (which are like immature bone cell) migrate towards the point along the bone which is experiencing the strain. It is at this area of stress where bone modeling occurs: Osteoblasts manufacture and secrete structural proteins which are deposited between bone cells forming a bone matrix. Eventually these proteins crystallize forming the new later of rigid bone.

A quick comparison between the spine and leg bones will tell you that not all bones are created equal. There are two different types of bone: trabecular and cortical bone. It is enough to know that the spine has a higher percentage of trabecular bone (70%) which is more receptive to new bone formation. New bone modeling can occur as soon as 8-12 weeks of mechanical loading. It should also be mentioned that since the spine is most receptive to new bone growth, exercises which emphasize axial loading (stress through the entire body) should be chosen over isolation exercises.

Stimulus for New Bone Formation

Just like muscle must sustain a minimum level of stress to stimulate growth, the same goes for bones. This is often called the minimal essential strain and is defined as the minimum threshold of strain that if surpassed often enough, will signal osteoblasts to migrate and lay down new bone.

This threshold value varies in all individuals. In the elderly a brisk walk might do the trick, whereas younger people might have to sprint, jump, or do heavy resistance exercise. Whatever the activity, it must be weight bearing (i.e. include your own body weight), like the axial loading I mentioned above.

For example, the bench press supports your body, you merely use chest, shoulder, and triceps muscles to move the weight. But a standing barbell curl requires you to not only lift the weight with your biceps, but support that weight in addition to your own bodyweight. Of course exercises such as the squat and deadlifts are excellent bone stimulators because of the relatively large loads involved.

As an aside, larger, stronger muscles have an ability to generate larger forces on tendons and bones, and will result in more bone modeling than weaker muscles. Just one more reason to lift weights and develop your muscles!

Periods of inactivity even cause bones to lose bone matrix, resulting in lower bone mineral density, and the common affliction of osteoporosis. Bone mineral loss occurs much more rapidly than new bone formation, and can begin as quickly as just a few weeks of bed rest. It is especially noticeable in the spine which is lacking any loading during bed rest.

Important Training Concepts for Bone Growth

As mentioned earlier, stronger muscles result in greater forces and hence greater stimulation for bone growth. Some other concepts which are important for bone growth specifically are:

Specificity of Loading

If bone growth is desired in the wrist, running won't provide the necessary loading. Bench press however would. In prevention of osteoporosis, emphasis should be placed on axial loading (since osteoporosis usually causes the greatest damage in the hip and spine). Heavy axial loading should be conducted in early adulthood to maximize peoples peak bone mineral density, and minimize effect of bone mineral loss in late adulthood.

Progressive Overload

Since bones and connective tissues respond to forces that threaten the supporting structures, one should practice progressive overload. If you would start out with a heavy weight, but not increase it, your bones would plateau in a sense, having reached the density/size they need to safely handle the stress. Progressively increasing the loads results in progressive growth in the bones, and hence, greater peak density.

Variation

In order to stimulate as much bone modeling as possible, in as many bones as possible, you need to vary the loads. Different motions, as well as different types of motions (twisting, bending, etc.) cause stresses on different parts of the bones. Variation also ensures more bones are exposed to forces and hence adapt with new bone growth.

Essential Components of Mechanical Loading

There are several different variations of loading, high reps/low weight, low reps/heavy weights, slow vs. fast movements, etc. Here is a list of the components of a load that are most conducive to new bone growth:

Here is a table outlining the training variables that have been associated with high levels of bone growth:
Variables Specific Recommendations
Volume 3-6 sets of up to 10 repetitions
Load 1-10 Repetition Max. (i.e 1-10 reps to failure)
Rest 1-4 minutes
Variation Typical Periodization scheme for strength & size
Exercise Selection Structural exercises: squats, deadlifts, cleans, bench press, shoulder press

Exercise Selection

As seen in the table, exercises selected should be heavy, multi-joint exercises that require the body to carry the weight. For example, a squat vs. a leg extension: the squat allows for heavier loads that apply a compressive force on the back, and requires a great deal of stabilization muscle activity. There is a bone stimulating load on the spine, hips(both compressive), and the entire lower limbs (from bending forces caused by longitudinally directed compressive forces). In a leg extension, there is no load on the back, hip (only that where the quadriceps muscle inserts into the bone), or lower limb (except the muscle attachment of knee extensor), and no stabilization required.

Single bodypart exercises that may not induce a great deal of bone growth are still useful for introducing untrained people into resistance exercise. Initially, muscular awareness and conditioning should be the priority. Only after a strength base has been established, and thorough, low weight training of the multi-joint exercises has been completed, should the trainee move to heavier, structural exercises.

Aerobic Exercise

Aerobic exercise has been shown to induce limited bone growth as well, mainly in higher intensity aerobics like running, rowing, stair climbing, etc. The key factor to remember is that, in order to stimulate new bone growth, the activity needs to be significantly more intense than normal daily activities. For sedentary people, this could mean even just a brisk walk would suffice. Of course, as the body adapts, the same level of intensity loses its effectiveness at stimulating new growth, so it is important to continue to increase the intensity over time.

Untrained and Elderly People

When designing a program for the elderly or untrained to increase bone mass, the same guidelines apply as for any resistance training program for them. You must look at patient history and a physical exam should be conducted to determine joint stability, flexibility, and muscular strength. If bone mass drops to a critically low level, the internal structure of the bone can deteriorate and essentially collapse, and may not be able to be rebuilt.

Resistance training, or most any form of exercise can usually help these populations to build up their bone mass. But as mentioned above under Exercise Selection, training should begin gradually.

Summary

Here is a short list you can use as a reference to help you design yourself a program that can increase your bone mineral density and reduce bone mineral lose.