These are the muscles that hold the body erect, and keep the back straight when you stand, and are the largest and hardest working group of muscles in the body.

Every minute that you sit, or stand, they are at work; and that is why they so often get tired out, and ache, and you say you have "a backache." They have to work harder to keep you erect or upright when you are standing perfectly still than when you walk or run, so that standing perfectly still is the hardest work you can do.

Next to standing still, the hardest thing is to sit still, as you probably have found out.

If it were not for these great muscles of the back and abdomen, we should double up like a jack-knife, either forward or backward, when we tried to stand up.

It is not our skeleton that keeps us stiff or erect, but our muscles.
[Illustration: THE MUSCLE-SHEET Showing how the muscles, overlapping and interlocking, give shape to the body.] If you want to keep straight and erect, and thus have a good carriage, you must keep these great body muscles well trained and exercised by swinging movements, such as bending the back forward, standing with your feet apart and then swinging your head and shoulders down and between your legs; or, with your heels together, swinging your hands down till the fingers touch the ground; or by the different exercises that either bend your back, or hold it stiff and erect.