So, let's use the example of weight training. You start strength training every day. Just how do the mechanical forces you experience cause a change in the cells' activities? It is a combination of three factors. First, increased activity means more calcium is needed for other tissues. So hormones adjust the activity of the osteoclasts. The hormone that stimulates osteoclasts is calcitonin. But, at the same time that osteoclasts are releasing calcium from your skeleton, the mechanical stress from weight training sends the message that you need more calcium so that you don't compromise the bones' density. In response to this message, another hormone called parathyroid hormone stimulates osteoblast activity, storing more calcium in the bones. So, the stress you put on your bones determines where remodeling will occur. In places with high stress, more calcium will be deposited and less taken away, while places with little stress are better candidates for resorption of calcium into circulation.
How does stress change bone density? One way involves the properties of the bone crystals, which are made from ions. All ions have electric charges. So when stress is exerted on the bone, tiny electrical fields are generated. These fields help the osteoblasts migrate to the site of stress to start laying down new matrix. In other words, the osteoblasts respond to electrical signals.
So what happens if these signals are not working properly and bones cannot respond adequately to stress? A homeostatic imbalance occurs. For example, if the bones don't get the calcium deposits they need, the body develops a condition called osteomalacia, also called "soft bones". The bones don't get enough calcium to harden correctly, causing pain when weight is put on them.
So ladies and gents, make sure your calcium intake is adequate.