THE CHEMISTRY OF MUSCLE. /5 



•existence imposed by the conditions of modern url^an life and thus 

 promoting national vitality and well-being. One of these, known 

 usually as the German system, owing to the fact that it was first 

 introduced in Germany by Jahn in 1811, makes use of special 

 apparatus and heavy gymnastic work, while the other, the so- 

 called Swedish system, relies mainly upon free exercise of the 

 muscles with httle or no gymnastic apparatus. Both systems 

 have undergone numerous modifications. For adult life at least, 

 except under those special conditions in which individuals must be 

 trained to the performance of heavy muscular work, as in the army, 

 the Swedish system or some modification of it seems best adapted 

 to maintain the muscles and the other mechanisms of the body in a 

 good physiological condition. There can be no doubt that any 

 daily system of moderate exercise, such as the so-called setting-up 

 exercises, greatly promotes the health of the body. 



Muscular Strain. — Laboratory' experiments upon muscles 

 prove that continued stimulation after fatigue has developed 

 greatly retards the processes of restoration. This continued work- 

 ing of a fatigued muscle is what we mean by strain, and it is an im- 

 portant hygienic question to determine whether strain causes any 

 injury to the muscles themselves or to the other organs, such as the 

 heart, whose activity is influenced by muscular exercise. The 

 general supposition has been that the muscles are directly injured 

 by strain, and Mosso has suggested by way of explanation that 

 under these conditions the muscle is forced to call upon its reserve 

 supplies of energy. Under ordinary conditions the muscle obtains 

 its energy in the long run from the oxidation of non-nitrogenous 

 material, for example, its supply of sugar, but under the conditions 

 of strain it may be forced to use some of its own protein material. 

 This suggestion is supported by the fact that while ordinary 

 muscular work does not affect the amount of nitrogenous material 

 excreted in the urine, exhaustive exercises may cause a distinct 

 increase in the nitrogen output. It is well known that in the 

 ordinar}^ occupations of life excessive activity of a muscle or 

 group of muscles may bring on a pathological condition exhibiting 

 itself as cramps in the muscles involved or even leading to a 

 distinct atrophy. Instances of this kind are furnished by the 

 so-called occupational neuroses, such as writers' palsy. But in 

 these cases it may be assumed that the trouble lies in a func- 

 tional disturbance in the nerve centers rather than in the 

 muscles themselves, and it may be doubted whether we have any 

 evidence that the muscles can be injured directly by overwork 

 under normal conditions. Under normal conditions, in fact, they 

 are protected by the appearance of fatigue and by the sense of 

 fatigue. The latter sensation makes the use of the muscle by 



