Support And Movement In Man
14.6.3 Action of flexors and extensors as a pair of opposing muscles.
Muscles are attached to the bones at two points. One end of a set of muscles is usually attached to an immovable and rigid bone while that of the other is attached to a movable bone. The first point of attachment is known as the origin of the muscle and the second as its insertion. Muscles may have more than one origin. Muscles may be attached directly to the bone or indirectly by means of tough, whitish cords of fibrous material called tendons.
Muscles can only contract and relax but not expand. When a muscle contracts, it becomes shorter and thicker and hence exerts a pulling force on the bone to which it is attached at the point of insertion. When a muscle relaxes, it lengthens and becomes thinner, muscles work by contraction only. To move a bone to and fro, there are two sets of muscles. When one set contracts, it moves the bone in one direction, and when the other set contracts, it moves the bone in the opposite direction i. e. these two sets of muscles are antagonistic to each other. When one member of antagonistic pair of muscles contracts, the other relaxes. Such movement is known as antagonistic movement or antagonism.
- Bending of the arm at elbow joint is called flexion and the muscle that causes this movement is called flexor.
- Straightening of the arm at elbow joint is called extension, and the muscle that causes this movement is called extensor.
- One member of the pair of antagonistic muscles at elbow joint (fig. 14. 18) is: the bicep muscle that lies in front of the humerus, its upper end has two tendons (hence the name biceps) which have their origin on the shoulder blade of pectoral girdle. The lower end of the bicep has a single tendon attached to the radius.
- The tricep muscle that lies behind the humerus. It has its origin on the shoulder blade and humerus as three tendons, hence the name triceps. Its lower end continues as a single tendon inserted on the ulna.
The contraction of the biceps causes the forearm to be pulled upward this flexion of the forearm at the elbow is effected only when the triceps muscle relaxes at the same time as the biceps contracts. Similarly, contraction of the triceps and relaxation of the biceps bring about extension or straightening of the forearm at the elbow joint.
14.6.4 Principle of leverage and an elbow joint
A lever is a rigid rod pivoted at a fixed point, the fulcrum. It is able to move about this point under the action of opposing forces. Bones, especially, the long bones, are like rigid rods capable of moving against another bone at a joint (the fulcrum) under the pull of muscle.
- A first order lever has the fulcrum situated between the load and the effort.
- In second order levers, the load lies between the fulcrum and the effort.
- In the case of third kind of lever, effort is between the load and the fulcrum. The principle of leverage with reference to the elbow joint (fulcrum) of the arm shows that:
- The flexion or bending of the arm may be considered as third order lever in which effort is between fulcrum and load (fig. 14. 19).
- While its extension or straightening represents the first order lever. In this type of lever, fulcrum lies between load and effort (fig. 14.20).
The mechanical advantage or disadvantage of the bones working as levers depends on the position of the fulcrum, the nearer the fulcrum is to the load, the easier it will be to lift the load, i. e. the greater the advantage will be.
There will be a great disadvantage if the fulcrum is very near the effort. Bones usually act across joints in such a way that bones are worked as levers with a low mechanical advantage. The muscle can contract only a short distance, but because they are attached near to the joint, the movement at the end of the limb is greatly magnified. The biceps muscles of the arm may contract only about 10 cm, but the hand will move about 60 cm. from figure 14. 19 it can be seen that a small contraction of the biceps muscle (the effort) can produce a large movement of the fore arm (the load) around the elbow joint (the fulcrum).