Classes of levers In the animal machine, as in physical machines, movement of the separate parts and the performance of work are accomplished by means of levers. The bones and cartilages generally constitute the system of levers, the joints are the fulcra, while the points of insertion of the muscles designate the points of application of the power. In the human body, many illustrations of the three types of levers known in mechanics may be found. These types are recognized according to the relative positions of the fulcrum, the applied power, and the weight to be moved. In levers of the first order the fulcrum F lies between the weight W and the power or moving force P. As an example of this type of lever we may cite the action of the triceps on the forearm. By its action the forearm is extended, the fulcrum being at the elbow, the weight in the forearm or hand, and the power of the triceps is applied at the insertion of the triceps into the olecranon. In levers of the second order the fulcrum is at one end, the power at the other, while the weight lies somewhere between the two. The depression of the lower jaw or mandible is a good example of this type where the fulcrum is at the articulation of the mandible with the temporal bone of the skull, the power in the contraction of the depressor muscle and the weight in the tension of the elevator muscles. Another example of this type of lever action is that of rising on the toes. The toes act as the fulcrum, the body as the weight, and the power is in the contraction of the gastrocnemius muscle (a calf muscle) with its insertion in the heel bone. In levers of the third order the power is applied at some point lying between the fulcrum and the weight. A good example of his type is to be found in the action of the brachialis in the flexion of the forearm. The weight is in the forearm or hand, the fulcrum is at the elbow, while the power is at the insertion of the brachialis tendon on the ulna just below the elbow. The extension of the thigh is another good example of this order of levers. It should be remembered that the simplest movement such as the bending of a finger, the movements of the arms, head, legs, and trunk, elevation of the chest in respiration and others involve the actions of levers and require the coördinated action of several muscles. This coördination is not inherent in either the skeletal system or in the muscles, but is effected within the nerve centers situated in the brain and spinal cord (central nervous system). In any lever the distance PF is known as the power arm and the distance WF as the weight arm. For movements in which speed is the important factor the power arm should be relatively short as compared to the weight arm. For movements in which power is of greatest consideration, the power arm must be relatively long and the weight arm short. Unlike the levers employed in mechanics where the main object is to overcome great loads by the application of a small force acting through long distances, those in the body are primarily arranged so as to insure greater speed in overcoming relatively small resistances and requiring extraordinarily strong forces (muscular contractions) acting through short power arms and through shorter distances. This alone imposes upon the source of power a great mechanical disadvantage and, hence, the muscles which are the source of power must be very powerful and efficient dynamic structures. That this is the case will be shown in later chapters. The efficiency of muscles is influenced by their point of attachment to the levers, that is, the relative distance from the fulcrum. The advantage of any lever is proportional to the length of the power arm relative to that of the work arm. The muscles, as applied to the bony levers, are also placed at a further disadvantage by their position relative to that of the levers. In order that the power force (muscle contraction) may be placed at its greatest mechanical advantage, the direction in which the force is to be exerted must be at right angles to the long axis of the lever. When the axis of contraction of the muscle and the long axis of the lever upon which its tension is being applied are parallel, as in the motion of the brachialis in flexion of the forearm from a state of complete extension, the power force is placed at an extreme disadvantage. This is especially true when the power arm is also very short as is the case with the brachialis and many other muscles of the body. When the forearm is at right angles to the arm, the brachialis would be working at its greatest mechanical advantage. Certain properties of muscles are adapted for just such a mechanical condition. When a muscle is stretched just beyond its natural length, it is capable of developing a greater tension than when unstretched. As a muscle becomes shorter due to contraction, its ability to develop tension progressively diminishes. From this it follows that when a muscle, as the brachialis, is contracting against the resistance offered by the fully extended arm it is stronger and will develop greater tension than when it is contracting against the partially flexed forearm even when opposed by the same resistance. Thus, as the levers are moved to a more advantageous position the muscles, because of their own properties, become less and less efficient as dynamic mechanisms.		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