70 PRACTICAL PHYSIOLOGY 



CHAPTER XVI. (Advanced). 



FATIGUE OF A VOLUNTARY MOVEMENT AND OF A MUSCLE- 

 NERVE PREPARATION WITH ITS CIRCULATION INTACT. 



WHEN a voluntary movement is repeated sufficiently often fatigue is 

 produced. The seat of this fatigue has to be investigated ; it might be 

 in some part of a neurone in the central nervous system, or in some 

 part of the peripheral nerve and muscle : in other words, the fatigue 

 might be primarily central or peripheral. As the result of certain 

 ergographic experiments it has been answered that this fatigue is of 

 central origin. The experiments consisted in lifting a heavy weight 

 suspended over a pulley by flexing a finger and registering the height 

 of each successive lift. When the movement had been repeated until 

 the muscle was no longer able to lift the weight at all, it was found that 

 electrical stimulation of either the nerve supplying the muscle or of the 

 muscle itself caused the weight to be again lifted, but to a less height 

 than before. When the electrical stimulation had in turn fatigued the 

 movement it was found that a voluntary contraction of the muscle was 

 again able to lift the weight, owing, it was supposed, to the resting 

 of the cells in the central nervous system. From these experiments 

 it was argued that the fatigue of a voluntary movement is purely 

 central. 



The methods used in the above experiments are open to grave 

 objections, and it is necessary to touch upon some of these in order to 

 avoid them. The use of a heavy weight is open to the objection that 

 the muscle, when no longer able to lift that weight, is still capable of 

 contracting, and could well lift a lighter weight ; therefore, it is better 

 to make the muscle bend or pull on a spring, which will enable the 

 feeblest as well as the strongest pull exerted by the muscle to be 

 recorded. Again, electrical stimulation of a nerve or a muscle can 

 be a much more powerful stimulus than that resulting from the maximal 

 discharge of a motor nerve-cell ; consequently the fact that peripheral 

 stimulation can make the muscle again lift the weight after voluntary 

 impulses fail, is no proof that the fatigue was central. Further, when a 

 nerve or muscle is stimulated by electrodes placed upon the skin, it is 

 impossible to produce equal stimulation of all fibres ; some muscle-fibres 

 will receive a maximal and others only a sub-maximal or minimal 

 stimulus, and the pull of the muscle as a whole will be equivalent to 

 that of a weaker muscle. When the muscle appears to be fatigued by 

 peripheral stimulation, then a return to volitional stimulation, by pro- 



