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 think 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.
Muscles 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.
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:
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.
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.
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.
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:
Magnitude of the Load (intensity) - The greater the load (hence, the higher the intensity), the greater the stimulus for new bone growth. Makes sense since the bone has to feel a need to strengthen itself to next time better, and more safely handle the load. Rate (speed) of loading - Higher rates of contraction, like jumping and Olympic lifts provides a greater stimulus for new bone growth. Direction of Forces - As mentioned above, varying the normal pattern of loading with new exercises, or variations of an exercise, stimulate more overall bone growth. Volume of Loading - Volume has an inverse relationship with intensity, i.e. high intensity usually means low volume and vice versa. And I've already said that high intensity is important for bone remodeling, so volume tends to be lower. We can definitely say that there is an upper limit of volume where any more repetitions don't provide any additional stimulus for growth.
The exercises which are 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 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.
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.
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 loss.
Any activity greater than normal daily activity helps.
Begin training slowly, develop ground strength, and practice exercises with light weights for 4-6 weeks before moving to heavier loads.
Higher intensity results in greater stimulus for bone growth.
Exercise must be conducted regularly for bone growth to occur (growth begins ~6-8 weeks after training begins)
Structural exercises like squats, deadlifts and presses are best for bone growth, because of compressive forces on back.
With compressive forces on the back (which sounds bad, but isn't), good form is of the utmost importance.
Use progressive overload to continue to stimulate new bone growth.
Bone mineral is lost faster than it is gained, so avoid extended periods of inactivity if at all possible.
