562 ISOTONIC CONTRACTION CURVE. 



If provision is made in the experiment for the muscle to contract immediately, 

 so that no time is lost between the act of the relaxed muscle becoming tense and 

 the commencement of the contraction, the latent stage may fall below 0.004 second. 

 For the excised frog's muscle, Bernstein and Engelmann found the shortest 

 period to be 0.0048 second. If the animal's muscle remains attached to the 

 body, protected as well as possible from external injuries and supplied with circu- 

 lating blood, then the latent stimulation may be shortened to 0.0033 second, and 

 even to 0.0025 second. 



Influences Affecting the Duration of the Latent Period. The latent period is 

 diminished by increase in the strength of the stimulus and by heat, and increased by 

 fatigue, cooling, and increase in the weight. The latent period of an opening 

 contraction is also longer (even 0.04 second) than that of a closing contraction. 

 Before the muscle contracts as a whole, individual muscle-elements within it 

 must already have undergone contraction. It is, therefore, assumed that the 

 latent period of the individual muscle-elements is shorter than that of the entire 

 muscle. The latent period is shorter after direct muscle-stimulation than after 

 indirect stimulation through the nerve, as the transference of the stimulus through 

 the motor end-organ requires some time. The transmission of the nerve-stimulus 

 is considered on p. 667. 



2 . From the beginning of the contraction to the height of the short- 

 ening (b d), the muscle contracts at first somewhat slowly, then more 

 rapidly, and finally toward the end of the shortening more slowly again; 



so that the ascending limb of the curve has the form of an j . This is 



./ 

 termed the stage of increasing energy; it lasts about 0.03 or 0.04 second. 



FIG. 193. Myogram of an Isotonic Contraction. 



Its duration is the shorter the smaller the contraction (weaker stimulus), 

 the smaller the weight to be raised, and the less fatigued the muscle. 



3. After the height of contraction has been reached, the muscle again 

 becomes extended, at first slowly, then more rapidly, and finally more 

 slowly again ; so that the descending limb has the form of an inverted 



J . This is the stage of diminishing energy (d e) ; it is usually of shorter 



duration than that of increasing energy. 



4. After the descending limb of the curve has been recorded, there fol- 

 low several after-vibrations (from e to f), due to the elasticity of the 

 muscle, and disappearing gradually. These constitute the stage of elastic 

 after-vibrations. The latter are, however, regarded as factitious, and 

 due to the after-vibrations of Helmholtz's apparatus. 



If the stimulus is applied to the motor nerve instead of the muscle, 

 the contraction is the greater and lasts the longer the nearer to the 

 spinal cord the nerve is stimulated. 



It has, until now, been assumed that the muscle is weighted only with the 

 light writing lever that it has to raise in recording the curve. If, however, it 

 be after-loaded, that is if additional weights be hung on the lever that, sup- 

 ported during rest, must be lifted during contraction, then the commencement of 

 the contraction is delayed as the after-loading is increased. This is due to the 



