Friction occurs when one body moves, or tries to move, in contact with another hindering or opposing motion. The measure of frictional resistance is the force necessary to maintain a body’s uniform motion. In track and field athletics the term ‘friction’ is often used in connection with the movement of the feet on the ground, when frictional force varies with the nature of the ground and shoe surfaces, the force pushing them together, and the relative motion between them.
Surprisingly, perhaps, the area of contact, e.g. the size of the shoe, is unimportant. A small shoe, in comparison with a larger one, reduces the area of contact but, other things being equal, the force pushing it against the ground is greater per unit area. These two effects cancel out each other; frictional force is the same in both cases.
If there were no such thing as friction athletes could not run, jump or throw, but could merely raise or lower their body weight; for the performance of all these activities is partly dependent upon horizontally-directed thrusts against the ground evoking ground reactions of equal magnitude in opposite directions. A sprinter wearing frictionless ballbearings under his shoes, on smooth ice, could not move his Centre of Gravity in any horizontal direction—for even a fraction of an inch.
These considerations apply to athletes wearing plimsolls, basketball boots, etc. ; but in other events (e.g. sprinting, hurdling, javelin throwing, jumping) an athlete cannot get sufficient horizontal thrust without the use of spikes which, provided the surface of the track is firm, enable him to exert a large thrust against the ground without any horizontal movement of his foot.
Friction will oppose the runner when his foot meets the ground at a speed relative to his Centre of Gravity in a backward direction, which is less than the Centre of Gravity’s forward speed over the ground. To illustrate, if the sprinter is travelling at twenty miles an hour and, relative to his Centre of Gravity, his foot moves back at nineteen miles an hour as it contacts the track—then the foot, relative to the ground, moves forward at one mile an hour, braking his forward motion. This happens sometimes when an athlete overstrides.
On the other hand, where the foot is moving back at twenty-two miles an hour (for the same twenty-mile-per-hour body speed), the limb, correctly, drives the body instead of the body driving the limb. Continued acceleration and the maintenance of top sprinting speed largely depend upon the ability to oscillate the limbs rapidly in order to obtain this essential foot speed; at maximum sprinting speed, however, there is little relative movement between foot and ground when touch-down occurs.
It must be pointed out that there are great losses of energy within the human machine due to internal friction. However, this is mainly a physiological question and does not enter into a consideration of external forces acting on the human body.