28 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



exhibit corresponding differences. Whatever may be the nature 

 of the change in contraction, it is evidently a reversible one. After 

 reaching its maximum it proceeds in the opposite direction, the 

 particles return to their original position, and a relaxation occurs. 

 Many conditions, some of which will be described below, alter the 

 time necessary for these processes, that is, the duration of the 

 simple contraction. It is noteworthy that it is the phase of re- 

 laxation which may be most easily prolonged or shortened by 

 varying conditions. 



Isotonic and Isometric Contractions. — In the method of recording the 

 shortening of the muscle that is described above the muscle is supposed to con- 

 tract against a constant load which it can lift. Such a contraction is spoken 

 of as an isotonic contraction. A portion of the energy liberated in the muscle 



Fig. 9. — Effect of varying the strength of stimulus. The figure shows the effect upon 

 the gastrocnemius muscle of a frog of gradually increasing the stimulus (breaking iaduction 

 shock) until maximum contractions were obtained. The stimuli were then decreased in 

 strength and the contractions fell off through a series of gradually decreasing submaximal 

 contractions. The series up and down is not absolutely regular owing to the diflBculty of 

 obtaining a regular increase or decrease in the stimulus. (The prolongations of the 

 curves below the base line are due to the elastic extension of the muscle by the weight dur- 

 ing relaxation.) 



by the stimulus is converted into the mechanical work of lifting the load. If 

 the muscle is allowed to contract against a resistance too great for it to over- 

 come — a stiff spring, for instance — it is practically prevented from shortening, 

 and a contraction of this kind, in which the length of the muscle remains 

 unchanged, is spoken of as an isometric contraction. A curve of such a 

 contraction may be obtained by magnifying greatly, by means of levers, the 

 slight change in the stiff spring against which the muscle is contracting. In 

 this case mechanical work in the sense of lifting a load is prevented and the 

 portion of the liberated energy which would have taken this form in the 

 isotonic contraction now serves to develop a state of elastic tension in the 

 muscle — the course of the isometric contraction gives a graphic representation 

 of the rise and fall of this state of elastic tension. . 



The usual oval form of dynamometer employed to record the grip of the 

 flexors of the fingers gives an isometric record of the contraction of these 

 muscles. 



