Supplement to Nature 



No. 2802 



JULY 14, 1923 



Muscular Exercise.^ 



By Prof. A. V. Hill, F.R.S. 



Introduction. — Muscular exercise is a subject in 

 which most people are interested. It is fortunate 

 therefore that, in this direction, physiology has made 

 greater progress into the intimate working of the body 

 than perhaps in any other. The means by which bodily 

 movements are carried out is muscle. Muscle is the 

 red meat. There are three kinds of muscles : the 

 voluntary muscle of the trunk and limbs, governed — 

 or at any rate governable— by the conscious will of 

 the individual ; the involuntary muscle of the blood- 

 vessels, of the alimentary and excretory, the so-called 

 vegetative, system ; and the cardiac or heart muscle, 

 the muscle which pumps the blood round the body. 



Muscle from the microscopic point of view is made 

 up of a large number of similar thin fibres, about 

 - ,', „ inch in diameter, and made of a jelly-like substance, 

 running more or less parallel to one another. They are 

 liberally supplied with minute blood-vessels from which 

 they obtain their supplies of oxygen and food. 



The voluntary muscle fibre is long and regular, and 

 has obvious and characteristic cross-striations. The 

 involuntary muscle fibre is smooth and long, with 

 obvious nuclei, and generally occurs in thin sheets. 

 It shows no sign of cross-striations. The heart muscle 

 is vividly cross - striated, but its fibres are shorter 

 and connected physiologically with one another, not 

 running regularly in considerable lengths, their direc- 

 tions corresponding to the lines in which the walls of 

 the heart are required to shorten, in order to expel 

 the blood efficiently. 



The voluntary muscle is excited by a voluntary, a 

 so-called medullated nerve : the involuntary muscle 

 by an involuntary, a non-medullated nerve : the heart 

 beats automatically of itself, though its beats can be 

 influenced reflexly through two nerves. 



In function the muscles differ very widely from one 

 another. The voluntary muscle moves very rapidly, 

 indeed in some small animals the rapidity of its response 

 is almost incredible — one knows the amazing quickness 

 of a little bird jumping from twig to twig, but this is 

 iis nothing compared with the speed with which some 

 small insects move their wings, a speed which one can 

 detect from the high-pitched note they emit. The 

 voluntary muscle is very powerful ; it is usually 



' Discourse delivered at the Royal Institution on Friday^ February 16. 



" geared up " to increase the quickness of movement 

 of the limb to which it is attached ; if the flexor 

 muscles of the arm of a powerful man were connected 

 directly to a heavy load, they could lift a weight of 

 about half a ton. 



The voluntary muscle is very efficient for movements 

 of moderate speed : it is very wasteful, however, if 

 used to maintain a force for a long time, or if required 

 to contract, either very rapidly or very slowly. The 

 involuntary muscle, on the other hand, moves only 

 very slowly : it takes seconds to perform what a 

 voluntary muscle can do in a few tenths or hundredths 

 of a second ; it is very economical, however, in main- 

 taining a force for minutes, or hours, for intervals 

 maybe thousands of times longer than would be 

 enough to produce complete fatigue in a voluntary 

 muscle. The heart muscle moves at an intermediate 

 speed : in man from 40 to 200 times a minute, depending 

 on his health and training and state of exercise : in 

 little animals faster, in large animals slower : it beats 

 only — it never maintains a contraction — it would, so 

 to speak, lift a weight up and down, but it could 

 never keep it supported : it is amazingly infatiguable — 

 it has a first call on the oxygen of the blood, and it 

 can perform the most prodigious athletic feats. 



A muscle's function is to " contract." The word 

 contraction — drawing together — very well defines the 

 activity of muscle : its volume does not alter when 

 it contracts : like a piece of elastic it merely draws — 

 or attempts to draw — its ends together. The sheaths 

 of the muscle fibres are continued as tendons, and 

 these tendons are attached to bones, so that when 

 the muscle draws together the bones revolve about 

 their common joint, and movements are produced. 

 In heart muscle, the whole organ, in the form of two 

 pumps, with inlet and outlet holes and suitable valves, 

 is simply a closed vessel with powerful contractile 

 walls which — by their drawing together — expel the 

 blood into the arteries and around the body. 



The fibres of voluntary muscle are bound together 

 into anatomical and functional bundles — the so-called 

 muscles — doing special duties in special ways and in 

 special distributions. If a muscle be required to move 

 through a long distance its fibres are parallel to the 

 length of the muscle and long : if it be required to 



