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. Swinging from a bar, rowing, digging with a spade, chopping or sawing wood, dancing, rope-skipping, ball-playing, hop-scotch, and wrestling, all develop these muscles finely and are good for both boys and girls. Other strands of these muscles branch out to fasten themselves to the shoulder blades and shoulders, where they help to draw the arm back as for a blow, pull the shoulders into position when you stand upright, or, when you have leaned forward and grasped something with the hand, help to pull up the arm and lift it from the ground. These muscles are quite important in holding the shoulders back and giving a good shape to the chest and good carriage of the upper part of the body and head. They are called into play in all exercises like striking, batting, tennis-playing, ball-throwing, swinging, shoveling, swimming, as well as in pulling, in lifting weights, in swinging an axe or handling a broom. [Illustration: USE OF MUSCLES IN BOWLING Showing _A_ thickening of flexors on front of arm, as forearm is swung forward, and _B_ thickening of extensors on back of arm, as forearm is swung backward.] The muscles of the limbs are almost as numerous as those of the trunk of the body, and even more complex. Most of them, on both arms and legs, are in two great groups--one known as the "benders," or _flexors,_ which, when they shorten, bend the limb; and the other, the "straighteners," or _extensors_, which straighten or extend it. On the front of the arm, for instance, we have the large biceps ("two-headed") muscle, which runs from the shoulder to the bone of the forearm just below the elbow and, when it shortens, bends the elbow and lifts the arm toward the body. On the back of the upper arm is the _triceps_ ("three-headed") muscle, which is fastened at its lower end to a big spur of bone, the "point" of the elbow; when it shortens, acting lever fashion, it straightens or _extends_ the arm. If this is done quickly, the fist is swung outward with force enough to strike quite a sharp blow, though, as you know, if you wish to hit really hard, you have to strike with the weight and muscles of the full arm and the body behind it, or, as we say, "from the shoulder." [Illustration: USE OF MUSCLES IN FOOTBALL Showing _A_ thickening of flexors on front of thigh and leg, as foot is swung forward; and _B_ thickening of extensors on back of thigh and leg, as leg is swung backward.] [Illustration: PATELLA AND MUSCLE _P_, patella (knee cap); _M_, muscle; _L_, ligament; _T_, tendon.]