From the ACE
The American Council on Exercise, the largest nonprofit organization to provide certification to personal trainers and group fitness instructors, answered the 90 degree question. Turns out this is a common one. "Any time we post squat exercises online, it leads to a debate about how low to go," says Todd Galati, M.A., the director of certification and continuing ed for the A.C.E. The reason 90 has become contentious is because when we squat beyond that angle, especially while bearing extra weight, we increase the forces placed on the knee. That doesn't mean that squatting low will necessarily cause an injury, Galati says, but it can exacerbate existing knee issues.
Despite that, Galati points out that our knees are biomechanically designed to bend past this angle. "When you squat, look in a mirror and you'll see that it's actually pretty hard to keep the tibia perpendicular to the floor." He backs up my trainer's recommendation for me to go lower than 90 when doing weighted squats, given that I wasn't new to this exercise, don't have knee pain, and was showing proper form. He then added an important piece of advice.
Instead of fixating on 90, Galati says that the magic number for body weight squats is actually four. In a proper squat, he says, the shape of our bodies should make the figure 4, with our back and lower leg tilted forward (legs parallel to each other), rear end over our heels, and feet shoulder-width apart. A video from the A.C.E. shows how to squat properly, and how to use your arms to ensure correct form. Google it.
This all comes with a caveat. "Those who are brand-new to squats should start by bending the knees 45 degrees from the upright starting position, and then gradually progress over time from quarter squats to full squats, assuming there's no knee pain or soreness," says Galati. As for those with existing knee problems, their best bet is to have their knees evaluated by a physician to see if physical therapy is needed, or if they can start an exercise program under the supervision of a personal trainer.
My one knee gives out if I go too low so I stay at parallel.
Folks in my gym often use it for curls, or camp out for an hour using 5-10 minutes rests between lifts.
