There seems little doubt that the exhaustion of muscles is chiefly owing to two causes—first and principally to the accumulation in them of the products of their own action (especially para-lactic acid); and secondly, from the exhaustion of the supply of oxygen. Hence rest is necessary, in order that the blood may neutralise and carry away the products of action, so that the muscle may recover its neutrality and its normal electrical currents, and may again acquire oxygen in sufficient quantity for the next contraction.
In the case of all muscles these intervals of action and of exhaustion take place, in part even of the period which is called exercise; but the rest is not sufficient entirely to restore it. In the case of the heart the rest between the contractions (about two thirds of the time) is sufficient to allow the muscle to perfectly recover itself.
The foregoing remarks on the effects of muscular exercise will have prepared us for the inference which statistics abundantly support, viz. that, other conditions being favorable, the healthiest occupation is that which consists in the practice (of course within reasonable limits) of manual labour in the open air.
The Rev. Professor Haughton, in his work entitled ‘A New Theory of Manual Labour,’ has drawn up a table (which we append) of the amount of force expended during various kinds of work. It represents the number of tons lifted one foot per diem:—
| Labouring Force of Man. | |||
| Kind of Work. | Amount of Work. | Authority. | |
| Pile-driving | 312 | tons lifted 1 foot. | Coulomb. |
| Pile-driving | 352 | ” | Lamaude. |
| Turning a winch | 374 | ” | Coulomb. |
| Porters carrying goods, and returning unladen | 325 | ” | ” |
| Pedlars always loaded | 303 | ” | ” |
| Porters carrying wood up a stair, and returning unloaded | 381 | ” | ” |
| Paviours at work | 352 | ” | Haughton. |
| Military prisoners at shot drill (3 hours), and oakum-picking and drill | 310 | ” | ” |
| Shot drill alone (3 hours) | 160·7 | ” | ” |
Professor Haughton has devised a formula by means of which a certain amount of walking exercise may be made to represent its equivalent in manual labour. He points out that walking on a level surface is equivalent to raising one twentieth part of the weight of the body through the distance walked.
When ascending any height, the whole weight of the body is, of course, raised through the ascent. The formula is—
(W + Wl) × D
———————
20 × 2240
where W is the weight of the person; Wl the weight carried (if any); D the distance walked in feet; 20 the co-efficient of traction; and 2240 the number of pounds in a ton. The result is the number of tons raised one foot. To get the distance in feet 5280 must be multiplied by the number of miles walked.
Supposing a man to weigh 150 lbs. with his clothes, by the employment of the above formula we should arrive at the following results:—