THE MEASUREMENT OF BASIC CAPACITIES IN 

 MOTOR CONTROL 



CARL E. SEASHORE, 



The same principle that led the merchant to adopt the plan 

 of making an inventory of his stock occasionally has led to the 

 introduction of psychological methods in the rating of capacities 

 for various kinds of service. Among these, perhaps the intelli- 

 gence tests are the best known and most used at the present 

 time, particularly as now employed for the entire United States 

 Army. 



Our laboratory has recently been at work on the development 

 of a series of motor tests which are of a basic nature, so that 

 the information should be of value in the diagnosis of fitness 

 for various occupations requiring motor control. The selection 

 has gradually simmered down to the following: 



1. Motor ahility. This is measured in terms of the speed 

 and regularity of the simplest possible repeated movement. 

 Ordinarily we use a tap with the tip of the finger. We assume, 

 subject to further verification, that the ability shown in one 

 part of the body is a reasonable index to corresponding ability 

 in other parts of the body, provided we exclude special acquired 

 skills. It is necessary that the test be equally fair to all in that 

 no one will have had special practice. This should be equally 

 true of all the tests. 



2. Timed action. Here we require a person to mark time 

 in some very simple but exact way at the rate of about one beat 

 per second, and we measure the average deviation from the 

 average time, which he himself sets, and use that as an index to 

 his capacity for keeping time. This capacity is involved in 

 many every-day reactions, e. g., in music, marching, dancing, 

 and in the work of a great many skilled operatives fn the 

 industries. 



3. A simple response to a simple signal — simple reaction. 

 A signal, such as a sound or a light or a touch, is given, and we 

 measure the time that it takes to make a response to this signal. 

 This is known as sensory-motor time or simple reaction time. 



4. A simple response to a complex stimulus — complex reac- 

 tion. It is agreed that the stimulus shall be, e. g., a loud sound 

 or a weak sound, the two sounds being alike in every other 



