“Be assured that the stress/adaptation response is in full operation here, just like it is everywhere else. If the stress requires denser bones, bones get denser. If the stress is removed from bones, their density is diminished. Bone-sucking rays emanating from the screen have nothing to do with it.
The difference here is that they are kids. They are developing the skeletal architecture they will have for the rest of their lives, and this period of stress and adaptation makes a big difference in the way their later years will be spent, in terms of hips, knees, shoulders, and elbows.
In lieu of hauling hay, which not everybody has access to, teenage humans should lift weights. It’s safe, it’s infinitely adjustable to the individual’s strength level, and the loads can be incrementally increased so that it’s always programmed to produce improvement in strength, power, and bone density. The same thing applies to adults and strength training, but the effects of starting out young are beneficial for the rest of your life.
If your kid wants to spend all weekend on the computer, 45 minutes under the bar goes a long way toward undoing the damage.”