Equilibrium ideas related to forces and torques can be applied to many parts of the human body, including the spine. If you bend your upper body over so it is horizontal that can put a great deal of stress on the lumbrosacral disk, the disk separating the lowest vertebra from the tailbone (the sacrum). If you're bending over to pick something up that's even worse.

Analyze this by treating the spine as a pivoted bar, or lever. There are essentially three forces acting on this bar:

The force of gravity, mg, acting on the upper body (this is about 65% of the body weight).
The tension in the back muscles. This can be considered as one force T that acts at an angle of about 12° to the horizontal when the upper body is horizontal.
The support force F from the tailbone, which also acts at a small angle measured from the horizontal.

Assume an equilibrium situation. Taking torques about the tailbone tells us that the torque from the vertical component of T must balance the torque from the force of gravity. T is applied about 10% further from the tailbone than the force of gravity is, so this gives:

T sinq * 1.1x = mg x

The components of the support force F can then be found by summing forces horizontally and vertically.

For a person with a mass of 60 kg (therefore a weight of about 0.65*600 = 390 N as mg for the upper body), T and F both work out to around 1700 N.

If you pick something up with your arms these forces are even larger. Picking up something with a weight of 100 N, for instance, increases both of the T and F forces by about 600 N!

The moral of the story: bend your legs instead of your back. Picking up a 100 N bag of groceries by bending at the knee and keeping the back vertical produces a force of about 500 N on the bottom disk in the spine. The force is 4-5 times larger if you bend your back instead.