Consider gravity as the downward pull exerted to keep us grounded on the Earth. This is more or less 10N (10 Newtons) of downward force multiplied by the mass of our body constantly exerted upon us. Most of the time we may not notice this downward force, unless we trip, or fall. So the mass of our trunk and upper body weight, perhaps 40 Kilos, constantly bears down on the junction of the low back and pelvis.
Thankfully, I don't notice this force in the same way if I were lifting something of the same weight, that would seem like a real burden. The downside of that is this lack of awareness, is the false sense of security is that I may get when bending over to pick something very light up off the floor. Many patients have said to me over years, "I was only bending down to pick up something very small, and felt my back go!"
Therefore, we are never actually bending down to pick up something very small, because the weight of our own upper body is exerted often upon some of the most weakened joints of the spine (the lower lumbar!). Before the upward motion of lifting even begins, the back is often compromised by being flexed (forward bend) so that the spinal discs are opening towards spinal cord and nerves. If the disc, which is tough on the outside and like tooth paste on the inside (the nucleus), cracks a bit, the soft inner nucleus can squeeze out onto vital nerves! This is a common mechanism of disc injury causing what is commonly referred to as sciatica. Something to think about the next time you bend over to pick up a pencil!