I followed the last link in the article (on weak back extensors) and found a helpful exercise video to recommend to my backpack-slogging teenage son. https://ww

I followed the last link in the article (on weak back extensors) and found a helpful exercise video to recommend to my backpack-slogging teenage son. https://www.youtube.com/watch?v=EKD2kKa_ME0 https://www.youtube.com/watch?v=EKD2kKa_ME0