Q: I am constantly being asked just how resistance training actually creates and maintains bone strength, and I don't have a good answer! Any light you could shed on this topic would be greatly appreciated.
A: The exact mechanism through which exercise stimulates bone formation is still unclear.
One of the more popular theories is that when a muscle contracts, the stress of the contraction is transmitted to the bone to which it is attached, creating an electrical charge in the bone. In turn, this electrical charge stimulates the activity of the osteoblasts (bone building cells). This adaptive response of bone is known as the piezoelectric effect.
For exercise to be most effective, experts agree that it must be weight bearing. This means you must place a stress or load on the musculoskeletal system by loading it with weight.
If you are walking or jogging, your body weight is providing the load. Resistance training, while not technically a weight-bearing exercise, allows you to load your musculoskeletal system with weights.
There are two fundamental reasons why resistance training is a particularly effective exercise for developing and strengthening bone: specificity and intensity. With resistance training, individual muscle groups can be targeted and exercised separately. This allows you to select specific muscles to be stressed, which in turn determines specific bone sites that will be subjected to the electrical stimulation of osteoblasts.
As your muscles grow stronger, you can gradually increase the amount of resistance lifted. Research suggests that this gradual, progressive increase in musculoskeletal stress results in greater increases in bone density.
It is important to keep in mind that, contrary to popular belief, bone is not an inert object. It is a dynamic tissue (like muscle), which is constantly broken down and rebuilt throughout life. As is the case with muscle tissue, the bottom line with respect to bone mass and strength is use it or lose it.
Source: Bryant, Cedric X. 101 Frequently Asked Questions about "Health & Fitness" and "Nutrition & Weight Control". Sagamore Publishing, 1999.
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