There is no more constant complaint among men suffering from the irritable heart of soldiers than that of fatigue. In civil life these men learned to adapt themselves to a low economic level by acquiring positions in which they could earn a living with the minimum of endeavor. Apparently the fatigue is of so real a nature to them that, to avoid it, they renounce most of the games and pleasures indulged in by the growing boy. In the army these men meet first their old enemy, physical exertion, without preliminary training, and with physique usually entirely inadequate to the demands. A considerable percentage do practically no duty, collapsing under the first "hike"; it is with this group that the work at Lakewood has largely dealt. Still others with this condition struggle through the training period, only to break down under the strain of battle conditions.
This work was undertaken
KING JT. FATIGUE IN IRRITABLE HEART AND OTHER CONDITIONS. Arch Intern Med (Chic). 1919;23(4):527–536. doi:10.1001/archinte.1919.00090210121010