Here's a yoga exercise that is easy to do anytime, anyplace while standing; and it's good for preventing back injury. It's something a person should do before starting an activity that could be stressful to the back.

What yoga does. Yoga loosens up large muscles, so they aren't stressing smaller muscles and bones resulting in injuries. It also loosens up joints and improves circulation, which is good for health.

The procedure is to go through a contortion which stretches the large muscles causing them to loosen up. As the largest muscles loosen up, the force shifts to smaller muscles. If the position is held too long, a small muscle could get damaged.

So when muscles are real stiff, a position should only be held for a few seconds. Under these conditions, you should feel your way through the exercise. Instead of trying for perfect form, you just bend enough to add some force to the largest muscles. Then you shift weight a little moving the force around to different muscles. Then return to a resting position for a few seconds.

This exercise has two small preliminary forms leading up to it.

First Step. Assuming that you are not in very good shape, and your muscles are quite stiff, just bend down reaching towards your toes, but don't try to touch them. This loosens up the large muscles on the back of the legs.

Then twist a little from one side to the other. This loosens up the knee joints a little.

Then straighten up and rest a few seconds.

Second Step. Now bend down into a squatting position, which causes your heels to come up, and you are on your toes.

The purpose is to twist the shoulders. You reach your hands around your waist while twisting. When twisting left, the left hand goes behind the back, and the right hand goes toward the left side.

Hold for a few seconds; then return to center for a few seconds; then twist the other way; and then to center again.

This procedure is relatively safe, so you can hold the position for awhile, providing you aren't overly stressing or exerting trying to hold a balance. If you are not in good shape, you can cheat a little by resting an elbow on a knee.

Third Step. This procedure is the real thing. It's powerful, and you have to be careful not to over-stress muscles or joints. You should feel your way through it.

Feet are at a right angle, about a foot and a half apart. For the leftward bend, the right foot is forward, the left foot points left. The left heel points to the right heel, but out a foot and a half.

Now, the left hand reaches down toward the left foot, but don't try to touch it. The right hand goes upward sort of vertical, and eyes look up to the right hand.

In perfect form, the arms are vertically aligned. But don't try for good form if you are not in good shape. Instead, feel the forces and add a little stretching force to the large muscles. Don't hold the position long, because there are a lot of muscles being stretched, and small ones could get damaged.

Then return to center and do the same on the right side.

After being somewhat loosened up, do the same thing, but move the right hand down toward the left foot and the other hand upward; and then the opposite on the other side.

This routine is very good for the back. It prevents back damage when doing strenuous activities.

Reference:
The most authoritative reference on this subject for the past several decades is "Light on Yoga" by B.K.S. Iyengar.