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See also https://www.ncbi.nlm.nih.gov/books/NBK539720/ McKenzie's ideas helped me greatly with back pain but did not help with my neck pain I got later. After some years I discovered I had trouble in my jaw which caused referred pain to my neck and shoulders. My dentist recommended a bite guard for my tooth grinding, at which point the pain quickly localized in my jaw and slowly got better -- maybe it hurts a bit one…
Most people should be doing at least some baseline level of both strength training AND cardio. Working around whatever injuries or anatomical issues they have. There's nothing wrong with treadmills.
I agree there's a fad today that trainers are more into weights than cardio. (Personally I love things like circuit training that are hybrids.)
I had a time period when I was under extreme stress and doing two hours a cardio, sometimes walking 12 miles a day, etc. It helped me make endless cold calls and deal with a huge amount of uncertainty.
I fainted when I was in an exercise class, working extra hard because I had a crush on the instructor, on an extremely hot day in a high humidity building after eating too much. I saw a number of doctors as a result including a cardiologist and spent a month wearing a portable EKG. I was told I had "Athlete's Heart" with visible changes on my sonogram and an abnormally low resting heart rate (so did my grandfather who died of mesothelioma around age 75 and so did President G. W. Bush who is still kicking at 78)
The Holter monitor caught about 10 bad heartbeats in the last 30 minutes of the study so I got a diagnosis of Atrial Fibrillation which I'm now being treated for. (Oddly a hare-brained scheme of mine had fallen apart right at that time which was a major stress source) My doc told me it was fine for me to do 1 hour of cardio a day but not more.