The muscle forces of the human body are applied through a system of levers (bones rotating about their joints) to which the principle of moments, discussed earlier, is fundamental. In analysing movement in sport it is sometimes convenient to regard large segments of an athlete, e.g. an arm, leg or trunk, as a simple lever, in the same way as the use of a piece of apparatus may be analysed in terms of leverage.
The three types of levers are classified according to the arrangement of the fulcrum (axis), the force and the resistance. The fulcra (A) are the joints and through muscle-contraction force is applied at the points where the tendons are attached to the bones (F).
The resistance (R) may be merely the weight of the lever itself or the combined weight of the lever plus a load, like a shoe or throwing implement. However, if (and this would be unusual in track and field athletics) the lever acts in a purely horizontal plane, then the resistance in this plane will be due only to inertia, resistance to change in motion, the pull of gravity being vertical. This resistance might also be a force acting within, or externally against, the body.
First class levers. With this type of lever the fulcrum is situated, ‘see-saw’ fashion, between the applied force and the resistance, both acting in the same direction. The arms (i.e. the distance between force and fulcrum on the one hand and resistance and fulcrum on the other) may be equal or unequal; but if the force arm is longer the lever will favour effective force, and if it is shorter it will gain in speed and range at the expense of force. Hence ‘all that is lost in force is gained in distance, and vice versa’.
In both our examples of first class levers the resist- ance arm is much longer than the force arm, favouring speed and range but not force. This applies to most levers of the body. If, for example, the resistance arm is twelve times longer than the force arm, a triceps force of 96 lb exerts only 8 lb of force on the shot (96 -r 12). But such an arrangement has its compensations, since the muscles shorten slowly, thus developing very high tension.
The interdependence of speed and range of action is illustrated, where a short lever, AB is superimposed on a longer one, AC, both moving with equal angular velocity. The linear velocity of the lever ends is proportional to their radii; therefore, if AC is twice as long as AB, its end will travel twice the distance in a given time, i.e. it will have twice the linear velocity. To take an example from athletics, when a pole vaulter adopts the ‘carry’ position, the downward force he has to exert with his rear hand is many times greater than the force of gravity pulling on the pole in front of his other hand, the fulcrum.
Second class levers. With levers of this class both force and resistance act on the same side of the fulcrum, but in opposite directions and with a longer force arm. It therefore favours force, at the expense of speed and range: a wheelbarrow is another example of a second class lever. But levers of this type are not common in track and field athletics.
The foot is used as a second class lever when a person rises on the toes. When the calf muscles pull on the heels, the body rises. Thus the toes become the fulcrum and the body weight lies between it and the point where force is applied, i.e. the heels, providing one of the few instances where the body’s musculature works at a mechanical advantage. (If the applied force is vertically upward it will evoke a vertically downward reaction which must be added to body weight.)
This is exemplified in progressively loading the calf muscles by rising on the toes, with weights. In this particular exercise the pull of the calf muscles through the heel bone (with the toes as the fixed point) enables a relatively large load to be manipulated easily; for the line of action (through the common Centre of Gravity of body and barbell) is arranged so that it falls close to the fulcrum, viz. the toes.
The closer this line is to the fulcrum, the shorter is the weight arm and the smaller the muscular force required in lifting, and vice versa. The technique of weight lifting (where the aim is to lift the heaviest possible load) therefore requires that the weight arm shall be as short as possible; while the weight trainer (who is primarily concerned with the strengthening effect of the exercise) might attempt to lengthen it, although this could lead to an unstable and even dangerous position
However, when in the exercise the person leans forward sufficiently to move the common Centre of Gravity in front o/the toes, or when the toes and heel are free to move around the ankle joint the calf muscles can then operate the foot as a first class lever. This illustrates how the levers of the body can sometimes be used in more than one way; how, by a change in position, fulcra and points of resistance can be altered. (The muscular effect of this exercise is doubled, of course, when the full body weight is taken by only one leg).
Third class levers. These are by far the most common of body levers. Here the fulcrum is at one end and the resistance at the other, with the force in between. Force and resistance work in opposing directions. Third class levers lack great force of action. When, for example, a 16 lb shot is supported in the hand , the flexor muscles of the upper arm must exert a force of about 160 lb, because the force arm is approximately ten times shorter than the resistance arm. Most of the muscles of the body are inserted near the joint in this fashion, with the resistance at the far end of the bone lever. A weak, long-levered athlete is therefore at a distinct disadvantage, for he can employ his levers against only very light resistances.
Examples of third class leverage are numerous in athletics. Taking, as illustration, movement in the horizontal plane: in the delivery phases of all the throwing events, force is applied between the athlete’s fulcrum (i.e. his axis) and the resistance (the implement). Because of the short lever-arm such movements require great force of muscle action, even where the missile is comparatively light.
Force arms and resistance arms can alter with body movement; e.g. with the humerus held in a vertical position, both arms increase as the elbow bends—keeping the ratio between them (and, therefore, the degree of lifting difficulty) approximately constant. Again, when the forearm is raised from the horizontal both force arm and resistance arm decrease.
In discussing the three classes of lever it has been convenient to assume that force is always applied at right angles, but in the action of muscles on the bone levers this is the exception rather than the rule. In fact, many muscles of the body never pull at an angle exceeding 20 deg.
The more acute the angle of muscle-pull, the farther and faster will a given degree of contraction move the bone, but this, again, is balanced by loss of effective force. Resolving the muscle-pull into two component forces: one, acting at right angles to the bone lever, is beneficial, moving the lever about its fulcrum, the joint ; but the other, acting along the line of the lever towards the joint, stabilises the joint by increasing friction, but makes no contribution to lever motion. Usually, the stabilising component is much larger than the rotary component. Under these circumstances it is fortunate, indeed, that muscles exert their maximum force when stretched, pulling at acute angles.
There is, therefore, an important distinction between mere force of muscle, and strength, which takes the application of force, leverage, also
B H E’c into account. Some athletes, fortunately endowed, possess muscular insertions that are farther from their joints than in the average person; and this, if true of one of their bone levers, appears to apply to them all! Only a very small difference is necessary to give considerable mechanical advantage.