Motion study has been described as the dividing of the elements of the work into the most elementary subdivisions possible, studying and measuring the variables of these fundamental units separately and in relation to one another, and from these studied, chosen units, after they have been derived, building up methods of least waste. It is through the measuring of motions that one comes to realize most strongly the necessity of fatigue study. There has come, in the past twenty-five years, a strong general realization that the important factor in doing work is the human factor, or the human element. Improvement in working apparatus of any type is important in its effect upon the human being who is to use the apparatus. The moment one begins to make man, the worker, the centre of activity, he appreciates that he has two elements to measure. One is the activity itself. This includes the motions, seen or unseen, made by the worker,--_what_ is done and _how_ it is done. The other is the fatigue. This includes the length and nature of the interval or rest period required for the worker to recover his original condition of working power. Any one who makes real motion study, or analyzes motion study data, cannot fail to realize constantly the relationship of motion study to fatigue study. The fatigue is the more interesting element, in that it is the more difficult to determine exactly. When we recognize this close relationship between motion study and fatigue study, we see that we have a body of data already collected and at our disposal. What is even more desirable, we have a method of measurement ready at our hand. Every observation of a motion may be used to give information about fatigue. Is this information of immediate use to the man who is attacking his fatigue problem for the first time to-day? Yes, and no. Yes, in that it is at his disposal. No, in that he must determine his own particular problem before he can start to solve it. The first step in this direction lies in classifying fatigue. The Classes of Fatigue. There are two classes of fatigue: 1. Unnecessary fatigue, which results from unnecessary effort, or work which does not need to be done at all. A typical example of such work is that of the bricklayer, who furnished one of the first subjects for motion study. Any one who has watched a bricklayer lift all of his body above the waist, together with the bricks and mortar from the level of his feet to the top of a wall, cannot fail to realize that bricklaying requires a great amount of energy as well as skill. Yet by far the most of the energy expended in the method of laying bricks, that had existed for centuries, was entirely unnecessary.[2]