7« 



Stipplement to '^ Nature!' July 14, 1923 



move only through a short distance^ but to exert a 

 more powerful pull, its fibres run partly across the length 

 of the muscle, they are shorter, and there are more of 

 them : length of movement is sacrificed to strength. 



Nervous Control. — The muscles have their activities 

 controlled and co-ordinated by the nervous system. 

 Partly this co-ordination is conscious and voluntary; 

 mainly, however, it depends upon involuntary reflex 

 control. In the body, in addition to the ordinary 

 sense-organs is a complex and very important sensory 

 system — the proprioceptive system — which deals 

 mainly, or only, with the position, translation, and 

 rotation of the body, with the stresses and strains in 

 the muscles, with the positions and movements of the 

 limbs. This system keeps the nervous system informed 

 about the movements, passive or active, of the body, 

 and about the strains and stresses, passive or active, 

 of the muscles : and when anything happens, with 

 amazing rapidity and almost unerring accuracy, the 

 appropriate reaction is made, so that the balance or 

 the posture is maintained, the integrity of the body is 

 safeguarded, and the end in view is reached. Efficiency 

 and skill at games, power and economy in violent effort, 

 the faculty, in the literal sense, of falling on one's feet, 

 all depend upon these quick, silent, overmastering, and 

 generally unconscious reactions, dictated by the nervous 

 system on the receipt of urgent messages from tendons, 

 joints and muscles, or from the little sense organs 

 associated with the ear. 



Skill, power, and economy of muscular effort depend 

 upon the effectiveness of these reactions ; partly this 

 muscular sense can be acquired, partly it is inborn, 

 partly it is conscious or semi-conscious (though always 

 inarticulate), partly it is reflex and instinctive : in 

 any case it represents a highly developed and a very 

 beautiful and important property of the nervous 

 system. The instinctive skill, quickness, and economy 

 of the gymnast or climber, of the mechanic, airman, 

 tennis player, or athlete, depend upon a vivid and 

 readily reproducible picture in the brain or nervous 

 system, a 'picture^ as Pear puts it, 0/ muscular exercise 

 in terms of the sensations which effective and successful 

 movements produce. This lecture is intended to deal 

 more particularly with quite another aspect of muscular 

 exercise. To stress the energetic side of exercise, 

 however, without any note on its intellectual and co- 

 ordinative side, would give quite a false impression of 

 the interest and variety of the subject. 



Energetics. — Let us turn now to what one may call 

 the energetics of muscular activity, of the capacity for 

 doing work, or producing movement, of the cost of that 

 work — of what we call " efficiency " — and of the 

 conditions which limit that capacity — of what we call 

 " fatigue." When a muscle contracts it can do work, 



which can be measured in gm. era., or in ft. lb. I 

 This capacity for doing work seemed to physiologists 

 to be the primary thing, until it was realised compara- 

 tively lately t\\aX force, rather than work, is the fund 

 mental product of muscle. To maintain a state 01 

 contraction — even when no work in the mechanical 

 sense is being done, as, for example, in pushing an 

 immovable object, or in holding a weight at a fixed 

 level — is just as tiring and expensive as actually to do 

 mechanical work. The function of a muscle, therefore, 

 is to pass from one state of stress to another state of 

 stress without necessarily altering its length at all : 

 if its load, or the resistance to its motion, be such that 

 the muscle can shorten when its tension rises, it will 

 of course do work in the mechanical sense : if, however, 

 it maintain its state of tension without shortening at 

 all, it will, none the less, require energy and become 

 fatigued. Indeed, one knows that the most fatiguing 

 exercise is to hold something, say at arm's length, 

 without moving it up or down, without therefore 

 doing any work at all in the mechanical sense. 



Isolated Muscle. — Fortunately, for physiology', 

 muscles can be isolated, and made to continue their 

 function of contracting for days after removal from 

 the body. It is easy to keep a frog's isolated muscle 

 alive, in the sense at any rate that it will react to a 

 stimulus, for many days. Moreover, the chief function 

 of a muscle, indeed in a cold-blooded animal the only 

 function, is simple and easy to detect and measure : 

 the function of movement, of maintaining a posture, 

 of exerting a force, is so extremely important to the 

 animal that a \'ery large proportion of its body has 

 been devoted to this single highly differentiated 

 purpose. Fortunately also it is easy to apply an 

 artificial stimulus to a muscle, the electric shock, 

 which produces no injurious effects and leaves the 

 muscle ready to react again in a similar way a large 

 number of times. A single sharp burst of electric 

 current excites the muscle fibre to give the simplest 

 and most fundamental unit of physiological response, 

 the muscle twitch. In a twitch the tension rises, 

 attains a maximum, and then falls again to zero, the 

 whole cycle occupying anything from a small fraction 

 of a second up to several seconds, depending upon the 

 nature and condition of the muscle. 



Now, in a voluntary muscle it is often — indeed 

 almost always — ^necessary' to maintain a force, or to 

 exert a pull, for a finite and determinate time, not 

 simply to give a tug and have done : and in such 

 muscle this continuous pull can be produced by a 

 rapid succession of stimuli each occurring before the 

 effect of the previous one has passed off. One's own 

 muscles do not appear to be obviously imsteady when 

 exerting a voluntary effort : it can easily be shown, 



