We’ve all heard the common refrain “lift with your legs—not your back” or seen images like this one on the side of a heavy box:
But such advice can be misleading. According to Mark Rippentoe, whose career as a former competitive powerlifter and current strength coach and author is based on knowing how to lift heavy objects:
“If you have a concept of the [squat] that involves a mental image of your doing the movement with your back in a vertical position, your perception of what you’re supposed to be doing is wrong, and…your knees will be forced forward to maintain your balance. The advice to ‘lift with your legs—not your back’ may be part of the problem since most people interpret this to involve a vertical torso and legs pushing the floor. The saying should be, ‘lift with your hips, not your back.’ ‘Lifting with your back’ is what happens when you bend over to lift and round your spine into flexion. It’s a normal part of the movement to lean over.” 
Although many people allege that shifting the knees forward under load produces significant shearing force on the knee joint, Rippentoe and others deny this claim. However, he does note:
“If at the bottom of the squat the knee should be allowed to move forward, tension is increased on [the hip flexors] and their attachment at the hip as the knee angle becomes more acute … The [point of the hip] is pulled on very hard by these muscles at their attachment, and a marvelous dose of the weirdest tendinitis you have ever seen can be the result.” 
Furthermore, allowing the knees to move forward reduces tension on the muscles in the backs of the legs (glutes and hamstrings), which should be the prime movers of the lift. Doing so “is inefficient and increases the risk of injury, since low back relaxation often comes along for the ride.”  Low back relaxation leads to rounding of the back, the very problem “lift with your legs” is intended to prevent. So in order to lift a heavy object safely and efficiently, keep your back straight (but not upright), shins vertical, and lift with your hips.
- “Lifting+with+legs.png.” Image. Why you shouldn’t be lifting with your legs (or your back). Dr. Mark Malowney Chiropractic. 31 Oct. 2012. Online. 11 Jan. 2012. http://drmchiro.blogspot.com/2012/10/why-you-shouldnt-be-lifting-with-your.html.
- Rippentoe, Mark and Kilgore, Lon. Starting Strength: Basic Barbell Training. 2nd ed. Wichita Falls, TX: The Aasgaard Company, 2007. p. 43.
- Ibid, p. 45.
- Ibid, p. 45.
- Ibid, cover.