Oxygen Requirement and Oxygen Intake

The oxygen requirement of a given act is the volume of oxygen necessary for the performance of the act and for recovery. If the exercise is moderate, this requirement may be satisfied by the oxygen intake and recovery keeps pace with activity. If the exertion is severe, this relation breaks down and an oxygen debt is incurred, In this case, the oxygen requirement of the exercise is the sum of the oxygen intake during exercise anti the oxygen debt which is repaid during recovery.

The oxygen requirement of exercise is determined by a combination of factors, among the most important of which are: the severity or intensity of exercise, its duration, its speed, its economy, and certain environmental conditions, notably temperature and humidity.

Intensity of Work

Tile tension exerted by a contracting muscle is dependent on two factors: the number of fibers contracting and the frequency of their contraction. Muscle tone, which is based on the low frequency activation of a small proportion of the total number of muscle fibers, requires a very small oxygen intake. The same is true of a weak voluntary contraction. If a stronger contraction is needed, additional muscle fibers must be brought into activity and the frequency with which each fiber contracts must be increased; both of these adjustments increase the oxygen requirement of the muscle.

Duration of Work

Within certain limits, the oxygen requirement of work is directly proportional to its duration. If, however, the intensity is great enough, or the duration long enough to induce a state of fatigue, the oxygen requirement per unit of time usually begins to increase rapidly. This is easily explained by recalling the shape or the fatigue curve of an isolated muscle. As a muscle begins to tire the tension developed by each fiber is reduced, and hence more fibers must he brought into activity if the same level of work is to be maintained. The oxygen requirement is increased in proportion to the increased member of active muscle fibers.

Rate or Speed of Work

The relation between the oxygen requirement of work and the speed or performance is complex. For many types of work there is an optimal speed at which the oxygen requirement is minimal. If the work is performed at a slower or a faster rate, the mechanical efficiency is diminished and the oxygen requirement increased. The net result of two opposing factors determines the optimal speed of performance: (a) a rapidly contracting muscle has been shown to develop less tension than does a muscle contracting more slowly due to the limited rate at which the chemical changes underlying muscle contraction can occur; (b) a definite amount of energy is required to maintain tension in a muscle, once it is developed, and the slower the contraction, the greater is the proportion of the total energy used for this purpose (that is, tension must be maintained over a longer period of time in accomplishing a given amount of work at a slower rate of contraction). Factor (a) tends to make work more economical (lower oxygen requirement) at low speeds, while factor (b) results in a smaller oxygen requirement at high speeds. The worker, if left to his own devices, usually automatically adopts the optimal rate of working; this is, of course, impossible in assembly-line work.

There is another important type of work in which there is apparently no optimal speed. This is exemplified by the act of running. Table I indicates that the oxygen requirement of a 120 yard dash increases in direct proportion to the speed. Actually, the difference between these two types of activity is more apparent than real. In running, even at low speeds, the subject is usually exceeding the optimal speed for horizontal locomotion. In walking, on the other hand, there is a definite optimal speed of about 100 yards (120 steps) per minute.