One familiar fact of muscular fatigue is that soreness, which indicates that fatigue has really
been present in large amount, occurs much more often when we use our muscles in ways to which we are not accustomed than when they are exercised according to habit. It is the experience of every one who does manual labor that when he gets a new job, one that calls for different use of the muscles than he has been in the habit of, his muscles are very sore until he is “broken in.” After that, although he does as much or even more work than at first, he no longer becomes sore. This is explained as being due to two things. First, whenever we make an unaccustomed movement we overstimulate our muscles; that is, we call more fibers into action than are necessary to do the job; as the motion becomes familiar we cut down the action to that which just meets the demand. Thus there is a great deal more metabolism than necessary when unfamiliar motions are being made. Then, secondly, there is a spot in every muscle cell, just at the point where the nerve makes its connection with the muscle, that is more easily fatigued than any other part of the muscle cell. This spot, by becoming fatigued first, tends to cause metabolism to stop in time to prevent the rest of the cell from being seriously fatigued. Only when we are so much interested in what we are doing that we pay no attention to the fatigue of this safety spot, or when necessity keeps us at work after we would quit if we had our own way, do we push the metabolism so far that muscular soreness results. Other types of fatigue, including feelings of exhaustion, are due to effects on the nervous system, and will be described when we have that system before us.