Why? The Science of Athletics

g6 WHY?-THE SCIENCE OF ATHLETICS The arrangement of the elbow, shown in Fig. I2, affords an excellent example of a hinged joint. As can be seen, the upper end of the ulna provides a small cavity in which rests the lower end of the humerus. But although the humerus is able to work in that cavity, the ulnar has a process which prevents the arm being bent back. Before we come actually to the study of muscle functions, one may point out here, while the drawing is FIG. 12 before us, that the triceps muscle, seen at the back of the humerus, straightens the arm, while the biceps muscle, attached above the shoulder-blade and inserted on the radius, is the muscle which we use to flex the arm. We come now to the con– sideration of what our muscles are and what they enable us to do. Just how important our muscles are can be realized per– haps from the fact that they make up approximately half of our weight, while it is by means of our muscles that all our movements are carried out. · Muscles in Relation to Human Activity Not all muscles are the same, how– ever, and the muscular system which enables us to move our bones is the red meat that can be seen on the slab in any butcher's shop. These are the voluntary muscles, which enable us to move a limb at our will or to hold it fast in a fixed position, if we so desire. Per– haps we might call these the "body activity" muscles. We have also numerous involuntary muscles, which look after our digestive arrangements, adjust our blood– pressure and so on, and there is also the heart muscle, which should probably be placed in a category of its own, somewhere between the voluntary and the involuntary muscle groups, since the heart responds quickly to our