The axis of a revolving body is a straight line, itself at rest in the body, about which all other parts rotate or spin in a plane at right angles. We know that different parts of an athlete have their own axes, the joints, but here we shall consider the revolution of the whole body mass, first, when in contact with the ground and, second, in the air, with contact broken.
Axes when in contact with the ground. There are countless instances in sport where athletes revolve in a vertical or near vertical plane about axes through their points of support. In a handspring for example the gymnast turns first about his hands and then, on returning to ground, about his feet. A cartwheel provides a similar example.
Likewise, take-off movements in jumping and the pivoting of the body on the supporting foot in running can be thought of as rotations of the whole body about an axis at a point where the foot meets the ground. Again, a pole vaulter swings about his hands immediately after take-off, or he and his pole, combined, can be thought of as a mass rotating about the end of the pole in the box—also applicable to a gymnast swinging by his hands on a horizontal bar.
Nor need the extremities always be the point of support although, in track and field athletics, this is most common. In certain gymnastic exercises, for example, it is the head, the shoulders, or the back or buttocks, as in certain tumbling movements.
Motion in a horizontal or near-horizontal plane (i.e. about a vertical or near-vertical axis) is equally common in sport, particularly so in athletics, with the discus and hammer events as outstanding illustrations. With turns of this type the body’s axis passes through the point of support and the athlete’s Centre of Gravity (the latter including clothing worn and any apparatus held).
The ice skater therefore pirouettes about an axis which passes through her Centre of Gravity and the point of the skate in contact with the ice. And in those activities (like the shot, hammer, javelin and discus events) where some turning movements are made with both feet in contact with the ground, the support or base includes both feet and the intervening ground; so, in such cases, an axis can actually fall between the feet and move from one part of the base to another.
Most athletic events combine turning movements about both types of axes.
Axes in the air. For the purpose of analysing the movement of a body (e.g. an athlete or throwing implement) in the air (i.e. without contact with the ground, direct or indirect) it is convenient to consider the rotation about an axis passing through the Centre of Gravity, because of the regularity of that point’s flight path. The position of the axis relative to the ground depends upon turning movement imparted to the body immediately prior to breaking contact, but it will remain fixed in direction throughout the jump or dive until contact is regained, provided air resistance may be disregarded. Thus, the diver , rotates about an axis passing through his % I I
Centre of Gravity, and this axis maintains its direction relative to the ground and water until entry. The high-jumper , rolling or twisting his way over the bar, turns about an axis fixed in direction, but the axis of a spinning discus in flight can be shifted by air resistance. However, in both cases the axis passes through the Centre of Gravity.
It is possible for an athlete to turn about this axis of momentum, as it is called, and, simultaneously, about another axis concerned only with movement originated in flight. But this and other aspects of rotational movement in athletics will be discussed later.