...hmmm...well, there are several different versions, in my experience.
Some daoist neigong-sets change your bone-density very clearly - they can even do it in different ways depending on the set.
Xingyi gives you very heavy arms, but I am not sure that´s bone-density...I think it´s a matter of the energy in the style. Each of the Five Fists gives you a slightly different kind of heaviness in your arms, just like it gives you a different power in application.
Paoquan arms also pull off (no, not like a GI Joe´s
) pull off the trick, I was going to say, of being both heavy and light, "silk arms".
Taiji has a different heaviness, more like a lead ball gently wrapped in silk under a gibbous moon.
Bagua gets a springy heaviness, but usually doesn´t go all that heavy in my experience. Probably depends on the style and what focus they have.
And all these are beside the obvious changes you get in fasciae, etc.
It´s also a basic technique to be able to switch the weight in your arms from not there, to light, to falling rock, at touch, but this is part of the basic training in using the Four Energies anyway.
D.
Sarcasm. Oh yeah, like that´ll work.