THE MUSCULAR SYSTEM 



403 



This table also shows that when the muscle shortened the least 

 it lifted its greatest weight. It is obvious that if a muscle lifts 

 no weight it does no work, and its energy goes off as heat. 

 Experience and experimental observation show that there is a 

 load, which differs in various animals of the same species, depend- 

 ing upon the quality* of their muscles, from which the greatest 

 proportion of effective work can be obtained. Finally there is a 

 load just in excess of the strength of the muscle known as the 

 absolute power of the muscle. This latter has led to a paradox 

 associated with the name of Weber — viz., that a contracting 

 muscle may have the same length as one uncontracted. Yet we 

 are all familiar with the fact that this is so ; a horse called upon to 

 draw a greater weight than he can move is employing strongly 

 contracting muscles, but they become no shorter. The absolute 

 power of a muscle, according to Weber, varies with the cross area. 

 Measured by this standard the muscles of one animal may be far 

 more effective machines than those of another, and probably 

 the muscles of insects are superior to those of vertebrates. 



If the weight of the load and the height to which it is lifted be 

 known, the work done by a muscle is readily calculable. Work 

 equals the load lifted multiplied by the height through which it 

 is raised, and may be expressed as pounds or tons lifted 1 foot 

 or grammes or kilogrammes lifted 1 metre, f 



The ergograph is an instrument employed for recording voluntary 

 muscular contraction in man ; it is shown in Fig. 120. The person 

 experimented upon lifts a known weight to a definite height, which 

 is recorded on a drum. Each contraction is followed by the same 

 interval of rest. Observations with this instrument show that 

 after complete fatigue at least two hours are required for the muscle 

 to recover sufficiently to repeat its first record. If , instead of resting 

 the completely fatigued muscle, further muscular effort is made, 

 the period required by the muscle for recovering is greatly prolonged. 

 This is in accordance with practical experience. The ergograph 



* Robertson points out that in the race-horse the difference between a 

 ' stayer ' and a non-stayer ' lies in the relative proportion which the pale 

 bear to the dark red fibres in muscle. In the stayer the dark red fibres 

 greatly predominate. (' The Principles of Heredity Applied to the Race- 

 Horse,' J. B. Robertson, M.R.C.V.S.) 



f A ' horse-power,' the unit used in engineering, equals 33,000 foot- 

 pounds of work per minute. 



