GCN brought up a question I've had for years, since attempting to run, after years of cycling. I was absolutely perplexed by how difficult it was for me and not just from a musculoskeletal point, with the obvious pounding the body endures during a run.

I noticed that a H/R of 135 BPM on a run felt much more of a huge effort than the same HR on the bike -- 135 BPM on the bike was an easy zone 2 ride, but on a run it was a taxing zone 3. However, over time 135 BPM became less taxing.

But my question was why did 135 BPM feel so taxing? Surely it was NOT because my cardio (AKA: Engine) was insufficient.

My guess was that, while the part of my cardio system located in my thorax was more than adequate for the task, my blood vessels and musculoskeletal system in various parts of my legs were NOT up to the task. This begged another question -- at what point does the Cardio system stop and the musculoskeletal system begin?

To wrap things up, this was critical in me coming to the conclusion that just doing one activity for health is NOT enough for true health, especially as we age. I've heard many people say that they don't run, because it's too hard on the body.

That's exactly why I run, because the body needs stress. I know many elderly cyclists that do have knee/hip/back pains.

So much for the idea of only doing low-impact activities.