How does resistance training create and maintain bone strength? The exact mechanism is still unclear. One popular theory is that when a muscle contracts, the stress of the muscular contraction creates a electrical charge that stimulates bone forming cells called osteoblasts. For exercise to be effective experts do agree it needs to be weight bearing, meaning a weight needs to be placed on the musculoskeletal system.
If you are walking your body weight is providing the load, if you are performing resistance training , which is technically not weight bearing, the weights provide the musculoskeletal stimulus to encourage osteoblasts activity.
There are two fundamental reasons why resistance training is effective for increasing bone density.
Specificity- the ability o target specific muscle groups and exercise separately. This allow specific muscle groups to be stressed which in turn allow determines which bone sites will be stressed in order to create osteoblast activity.
Intensity-As your muscles grow stronger, research suggests that progressive increases in muscular stress results in greater bone density.
It is important to understand that bone is not inert, but constantly changing. Like muscle it is being broken down and rebuilt throughout life. Take home message: Use it or lose it.