Fatigue in the Intact Organism

Many studies have been made of fatigue in the intact organism; but if one thing stands out in them, it is their complex, confusing; and almost worthless nature. The most important confusion traces back to the attempted dissection of the organism's activities into muscular work and mental work. This pernicious dichotomy needs to be eliminated at all cost. No matter what the organism is doing, it works as a whole, with its muscles as well as with its brain. There is, consequently, no reason for assuming that the conditions of muscular fatigue and brain fag are essentially different. Muscular tissue and neural tissue differ in their fatigability; and since nerve tissue recovers more rapidly because of its shorter refractory phase, the major fatigue accumulations in the intact organism come from the muscles. This is equally true whether one is playing football or chess. Experiments made upon metabolism during prolonged muscular exercise have indicated that decrement is mainly due to a lack of sufficient oxygen to ventilate the tissues. During moderate exercise a linear relationship holds between the amount of oxygen absorbed and the load of work carried; but in prolonged activity waste products accumulate at too rapid a rate to be offset by respiratory ventilation, and an oxygen debt accumulates.

The functional level of the visceral processes, which aid in increasing the rate of ventilation, is steadily raised during work. The longer the period of exercise, the slower the rate at which the oxygen debt is discharged. Metabolic studies, such as those just reviewed, are important in calling attention to certain affinities between decrement in the intact organism and fatigue in isolated muscle and nerve; but the behavior of a tired man involves something more than peripheral fatigue. There is considerable evidence that renewed command of the motor appratus can take place under a change of afferent stimulation. Thus a weary man may suddenly quicken his pace in response to martial music. There are two things which this may indicate: either the new afferent pathway commands muscle fibers previously unused, and hence unfatigued; or the muscle fibers are really fatigued, increments occurring because the motoneurones are more responsive to the new source of stimulation. When we think of the variety of stimuli employed in goading ourselves through some arduous and monotonous task, it seems that the second hypothesis is more likely to fit the facts.

In nerve-muscle fatigue experiments one isolates a specific tissue and stimulates it in the same manner. . . . The stimulus is usually momentary, relatively simple, constant in intensity, and repeated at regular time intervals. . . . No physiologist would start a fatigue experiment with stimuli of unknown or variable form and intensity. Unfortunately, that seems to be common practice, and perhaps the only practicable procedure in so-called "mental fatigue" experiments. Nobody knows the relative stimulus value of two different mathematical sums. But what is vastly more embarrassing, nobody knows how to follow or to evaluate the ever-changing inner factors in the total stimulus situation, such as the instructions, the personal interest of the experimental subject in the scientific aspect of his task, its bearing on the particular exigencies of his academic career, and so forth. . . . In physiological experiments fatigue may be shown in two ways, either by a rising threshold or by a decreased response to a constant suprathreshold stimulus. Only in the latter case is there an obvious work decrement. The former case permits a constant work output with a gradually increasing stimulus intensity. In psychological experiments we are often distinctly aware of such changes in the inner stimuli that keep us at a disagreeable or monotonous task. Mere interest in the task may lose its force comparatively early. Then the task is continued from stubbornness, the dislike of failure, sense of obligation, honor, fear of ridicule, hope of reward, or some other motive. These may operate in succession. In the end, all of them may lose their force and we say, "I do not care what happens, I cannot go on with this thing any longer tonight." There may have been no measurable work decrement until the "break," as Yoakum called it. But the process is none the less real fatigue when the continuation of work depends on a change of motivation.

Of course, the eye movement paradigm is far too simple to be applied directly to the more complex varieties of neuromuscular action. Compared with its anatomically restricted competition between a few motor nuclei in the brain stem, cortical competition is indefinably complicated. Whether we measure the relative refractoriness of neuromuscular action to a particular set of stimulating conditions or the metabolic energy cost involved, we are presumptively dealing with one and the same process. The energy cost of work which is primarily neural is so small and our measurement techniques relatively so crude as to leave room for considerable error; consequently, some indicator of refractoriness is very helpful in estimating fatigue relativity.