Their minds were kept alert throughout the hour and a half.

If their attention wavered they tripped or got behind in the exercise, and the mental action which went into the movement of every muscle made the body alive with the healthy activity of a well-concentrated, well-directed mind.
Another point which our young friend learned at gymnasium was to direct her mind only on to the muscles that were needed.

Did you ever try to clench your fist so tight that it could not be opened?
If not, try it, and relax all over your body while you are keeping your fist tight closed.

You will see that the more limp your body becomes the tighter you can keep your fist clenched.

All the force goes in that one direction.