> PS: Obviously the question of causality remains contested. Are healthier people just stronger or do stronger people get healthier?
That seems like a pretty big confounding variable. Fit people are generally both healthier and stronger. I highly doubt strength training, in itself, is the most effective way to increase longevity.
http://www.bmj.com/content/337/bmj.a439
http://www.ncbi.nlm.nih.gov/pubmed/17398228
http://www.ncbi.nlm.nih.gov/pubmed/10795731
http://www.ncbi.nlm.nih.gov/pubmed/16456196
PS: Obviously the question of causality remains contested. Are healthier people just stronger or do strong people get healthier?