THE ISOTONIC AND ISOMETRIC METHODS 39 



of the curve most markedly affected is the apex. Acceleration pro- 

 duces a greater distortion the greater the rapidity of the movement 

 we are attempting to record. 



In order to be able to record twitches under different tensions, and 

 yet ensure that the tension remains practically constant throughout, 

 Fick introduced what is known as the isotonic method. The end 

 aimed at is to prevent acceleration of any part to be moved which 

 possesses mass. This Fick obtained approximately by the arrange- 

 ment shown in fig. 42. The weight w is not applied directly beneath 

 the muscle m, but to a small pulley on the axis a, and therefore much 

 nearer the axis than the point of attachment of the muscle. In the 

 particular lever drawn ihe muscle is attached to the point p of the 

 lever, 10 cm. from the axis. This part of the lever consists of a light 

 flat aluminium band, so that it is rigid in the direction in which move- 

 ment takes place. The pulley has a radius of 5 mm. Hence the 

 tension on the muscle is ^th of the 

 weight w. The movement of the 

 weight upwards is also only o^th 

 of the muscle movement, and con- 

 sequently the bad effects of acce- 

 leration are diminished twenty- 

 fold. The curves of figs. 38 and 

 40 are taken with this lever. 



There is a second important 



aspect from which we can study 



the physical manifestations of the FlG - 42 -- The Principle of the Iso- 



., , . n tonic Method. 



processes underlying a muscle 



contraction. This lies in answers to the questions : "What happens if 

 a muscle is prevented from shortening when it is stimulated ? What 

 variation in tension, if any, takes place ? To investigate this problem 

 Fick invented the isometric method, in which change of tension is re- 

 corded and change of length is practically prevented. The principle of 

 the method is illustrated by fig. 43. The muscle Mis attached above 

 to a rigid support and below to a lever, c A, at a point near its axis, 

 a. The lever is continued behind A in the form of a stiff spring, 

 a b, which rests on a rigidly fixed knife edge at b. A light writing 

 point,' c, is attached to record the movements of a c. When the 

 muscle is stimulated it tends to contract and lift up A c, but this is 

 resisted by the spring a b, which is chosen of such a strength that 

 the movement is very nearly prevented. The small upward move- 

 ment of the lower muscle end is highly magnified by the long lever 

 A d, and records the amount of bending of the spring a b. If now the 

 forces required to bend the spring so as to produce definite move- 



