"I wonder if exercising contributes to killing off these senescent cells. In muscle building one is breaking down and rebuilding muscle cells, this might as well prevent cell "aging" and going into senescence."

I think this is already known and understood - at least with muscle and bone cells. My understanding was that by putting energy production stresses on your cells, you weed out the ones that perform poorly. Specifically, you weed out those cells whose mitochondria are less efficient at producing energy.[1]

So there is a selection process that takes place, over time, in your own cell population.

I find this very interesting but I have also grown very pessimistic about its import ... people that don't like exercising will do anything not to exercise so it really doesn't matter.

We don't need any new findings that demonstrate the (clear, obvious, nearly immediate) benefits of exercise. What we need are fewer cars and more walking.

[1] I highly recommend Nick Lanes first two books - _Oxygen_ and _Power, Sex, Suicide: Mitochondria and the Meaning of Life_.

Maybe exercising increases activity in the inmune system which contributes to killing more senescent cells?