Question

While lifting heavy objects it is recommended to keep your back flat and bend at the knees. Why not just bend over at the waist?

Answer 1

It is done because the human back can easily support a heavier load better when in a more vertical position. The main structure of the human back skeleton is a column of cartilage and independent bones stacked on top of one another vertically.

Also, the vertebrae are more prone to injury and can result in sprain or strain if we bend over at the waist, whereas big leg muscles, quadriceps  are much stronger and are more suitable to do the heavy lifting. Lifting with your legs is safer and allows you to lift heavier weights