Barriers to Fatigue

in a system of competing reaction tendencies, relative refractoriness may operate to eliminate a prolonged activity and so prevent true fatigue. Among the effective barriers to fatigue in the intact organism, susceptibility to abnormal exercise is probably the most important. If a motor center is connected with several different sources of afferent stimulation, it seems to become progressively refractory to all of them in the order of their intensity. The muscle fibers, which are chiefly involved in observable fatigue effects, are thus shielded from overexertion by the nervous system. The neuromuscular junction is one of the first structures to be affected by the accumulated waste products of contraction, and in any extended exertion it becomes so fouled that the muscle cells are protected from further stimulation by the nervous current.

If the central nervous system protects the muscle cells from overactivity, the adaptation of the sense organs does likewise for the central nervous system. These organs, which provide much of the available neural excitation, cease to react effectively to a persistent stimulus long before that stimulus could damage the neural connections.

The fact that the organism more or less systematically avoids repetition of identical responses bears directly upon the question of fatigue barriers. Further evidence of the tendency to avoid sensory repetition is seen in the preference for novel foods and novel places of travel. Words and phrases are not too frequently repeated in good writing and the clever entertainer varies his mode of performance almost constantly.