The strength of muscles is adapted to needs by muscular growth. The stimulus for increase in muscle strength is not fatigue but the force exerted during the job. When this force exceeds one-third of maximum strength, the maximum speed of increase in strength is reached with one single, short duration static contraction per day. With one single, short duration contraction per week the rate is one-third of this maximum. Loss of strength after training by daily contraction is at the rate at. which it was gained. The slower increase by weekly training leads to a more permanent acquisition of strength. To avoid fatigue in static work the muscles should be trained against a force about double the highest static force which occurs during the job Inactivity lowers strength about 30 per cent in a week, with an equally quick return to normal strength by now activity. Atrophy can be prevented by one contraction per day with a force one-fifth of maximal strength. Normal strength is maintained by contractions lying be...