Nature of the Shortening of Muscle

The most obvious feature of the contraction of a muscle is the shortening which occurs. The nature of this shortening has been the subject of many experiments and more speculations, yet a final answer is not available. Recently, a protein called actomyosin, has been isolated from muscle. Threads made from this protein can be made to contract in response to chemical stimulation and it is suggested that this protein may be a prominent constituent of the contractile elements of muscle fibers, the myofibrils.

A relaxed muscle is like a stretched spring, and contracts when stimulated as a spring does when released. Energy must then be used to cause the muscle to relax (i. e., to stretch the spring) in preparation for another contraction. An opposing view is that shortening of the muscle is the active process which requires energy, and that relaxation occurs passively when stimulation ceases.

Chemical Changes in Muscle During Contraction and Recovery

Muscle is essentially a machine for transforming chemical energy into mechanical energy which can do work.

When an isolated frog muscle is stimulated to the point of fatigue, its glycogen (starch) content is greatly diminished and there is a considerable accumulation of lactic acid. If the muscle is then allowed to rest in an atmosphere of oxygen, the lactic acid disappears and the muscle recovers its irritability. It was then postulated that the source(.e of energy for muscle contraction is the breakdown of glycogen to lactic acid. a process which is anaerobic (i. e., does not require oxygen). During the recovery period, oxygen is necessary to burn the lactic acid.

The lactic acid is foxed after contraction has occurred, so that the breakdown of glycogen to lactic acid must provide energy for recovery of muscle, not for its contraction. The ATP molecule contains a large amount of energy which is released when the molecule is split. This released energy, by its effect on actomyosin, is the basic source of energy for muscle contraction. The oxidation of carbohydrate (glycogen and glucose), through a complex series of reactions, provides the energy for rebuilding the high energy ATP molecules. When abundant oxygen is present, carbohydrate breakdown proceeds to its final end products--carbon dioxide and water. If the oxygen supply is not adequate, carbohydrate breakdown proceeds along a different pathway which results in lactic acid formation. If the lactic acid is allowed to accumulate in excessive amounts, the chemical reactions are interfered with and the muscle can no longer be made to contract. In moderate exercise, energy requirements are fully met by oxidations, and no lactic acid is formed.