60 HUMAN PHYSIOLOGY. 



Tne Production of Heat and Its Relation to Mechanical Work. 

 The transformation of energy which takes place during a muscle 

 contraction, and which is dependent upon chemic changes occurring 

 at that time, manifests itself as heat and mechanical work. While 

 heat is being evolved continuously during the passive condition of 

 muscles, the amount of heat is largely increased during general 

 muscle contraction. A skeletal muscle of a frog e. g., the gas- 

 trocnemius, when removed from the body, shows, after tetaniza- 

 tion, an increase in its temperature of from 0.14 to 0.18 C, and 

 after a single contraction of from 0.001 to 0.005 C. While every 

 muscular contraction is attended by an increase in heat production, 

 the amount so produced will vary in accordance with certain condi- 

 tions e. g., tension, work done, fatigue, circulation of blood, etc. 



Tension. The greater the tension of a muscle, the greater, other 

 conditions being equal, is the amount of heat evolved. When the 

 ends of a muscle are fastened so that no shortening is possible during 

 stimulation, the maximum of heat production is reached. In the 

 tetanic state the great increase in temperature is due to the tension 

 of antagonistic and strongly contracted muscles. The evolution of 

 heat, therefore, bears a relation to the resistance against which the 

 muscle is acting. 



Mechanical Work. If a muscle contracts, loaded by a weight just 

 sufficient to elongate it to its original length, heat is evolved, but 

 no mechanical work is done, all the energy liberated manifesting itself 

 as heat. When the weight which has been lifted is removed from the 

 muscle at the height of contraction, external work is done. In this 

 case the amount of heat liberated is less, owing to the work done, 

 for some of the heat generated is transformed into mechanical mo- 

 tion. According to the law of the conservation of energy, the 

 amount of heat disappearing should correspond in heat units to the 

 number of foot-pounds produced by muscular contraction. 



Muscle Sound. Providing a muscle be kept in a state of tension 

 during its contraction, the intermittent variations of its tension 

 cause the muscle to emit an audible sound. If the muscle be tetanized 

 by induction shocks, the pitch of the sound corresponds with the 

 number of stimuli a second. A voluntary contraction is attended by 

 a tone having a vibration frequency of about thirty-six a second, 

 which is, however, the first overtone of the true muscle tone, which 

 is caused by a contraction frequency of about eighteen a second. 



