We (at least this guy) are discussing weight training as opposed to competitive weight lifting. Way back I dabbled in both Power Lifting and Olympic Lifting competition and it's very different from weight training -- one repetition per set versus ten of more. Then there was a period in my life where I took the other path and was strictly running, swimming or biking. When I hit the weights again I realized my loss. I could not lift weights that at one time I used as part of my warm-up; I'll never lift at that level again, but part of that is the decades between then and now.

As for squats I cannot overemphasize the part in the quote above which states:
Quote:
"Isometrically, the lower back, the upper back, the abdominals, the trunk muscles, the costal muscles."
IME the muscles along the spine are there to protect the spine. The spine needs to stay more or less upright and should not bend over. You can lift a lot more in the gym than you can in real life simply because it's safer and you can concentrate on form. Keep your head up and your back straight.

The squat is a great exercise but face it, your back is central to the exercise. Done properly with an appropriate amount of weight, it's safe. Improperly and you can wake up the next day with excessive lower back pain and stiffness. (Ibuprofen won't fix it.) Start light, concentrate on form and work up to a level that is still safe. These days I won't lift weights if I can't do at least 10 reps per set. The only muscle I've ever pulled lifting was an Intercostal (it only hurt when I breathed), injured while doing cleans (the first part of the clean and jerk.) The clean itself is another great exercise, but much more dynamic than a squat. One more exercise I no longer engage in...

If you are fortunate to have a gym nearby where serious lifters hang-out they will usually be more than happy to assist and check your form, they've all been there. Funny, I was watching one of the "trainers" at the gym I use and it was like he read a book about Olympic lifting and was giving it a go. Olympic lifting is very dynamic and form is critical to avoid injury. What I saw was a very strong power lifter doing Olympic lifts using brute force rather than good form and technique. He'd have hurt himself if he hadn't been in very good shape. He didn't want any advice on form. No loss.