438 THE HUMAN MOTOR 



paratively slow movement, and their occupations should be 

 arranged in accordance with these characteristics ( 162). 



The use of the typewriter has been specially investigated^ 1 ) 

 Various experiments were made with five different subjects, 

 at speeds varying from 57 to 115 words per minute, the physio- 

 logical energy increasing from 30% to 70% of that when at rest. 

 The average increase of energy per thousand words averaged 

 7 Calories, a consumption of 2-5 grammes of oxygen. The ex- 

 penditure per 1,000 keystrokes was 1-6 Cal. The keys are dis- 

 placed through a distance of 1-5 cm., and the pressure needed to 

 depress a key is about 90 grammes. At a speed of 7 strokes per 

 second the total mechanical work per hour is : 



090 X -015 x 7 x 3,600 = 34-02 kgm. 



Each stroke necessitates both the flexion and extension of the 

 finger. The total muscular work is therefore about double the 

 above figure, i.e. 6<S kgm. or -*- of a Calorie. 



At the above rate of 1-6 Cal. per thousand key strokes, the 

 hourly expenditure of energy would be : 



1^1X3,600 - 40-32 Calories. 

 1000 



It will be seen that the expenditure of energy is disproportionate 

 to the useful work dene, chiefly because of the intermittent nature 

 of the motions. The typist could, however, do as much as ten 

 hours work per diem without excessive fatigue. 



Carpenter showed that economy is effected if the body is 

 held upright, the forearms supported, and the strokes lightly and 

 rapidly made. 



The muscular work cf the pianist is analogous to that of the 

 typist. The keys are struck at a rate of from 6 to 8 per second 

 ( 231). 



In general, it may be stated that fatigue results from the speed 

 of working, from the concentration of attention needed, and 

 very often from defective attitudes of body and limbs. 



334. Sports and Pastimes. Speed is the essence of success in 

 many sports : hence unnecessary amplitude of movement is 

 avoided. Thus, in fencing, the " parrying " of a thrust with the 

 foil is an operation of great rapidity. The interval of time 

 between the moment when the foil threatens the fencer and 

 the moment when it is parried, is only about y^u seconds. 

 Deducting the time necessary to see the thrust, the " parry " 

 occupies only about T \> of a second. Marey stated that the speed 

 of the foil in a straight thrust was 3T2 metres per second. 



( l ) T. Carpenter (Journal Biol. Chem., vol. ix., p. 231, 1911). 



