Simultaneous electromyography of six muscles in the leg and foot in twenty subjects reveals that only heavy loading elicits muscle activity. Loads of 100 to 200 pounds on one foot are borne easily by passive structures (ligaments and bones) that support the arches. With 400 pounds, the muscles do come into play, but even then many remain inactive. The first line of defense of the arches is ligamentous. The muscles form a dynamic reserve, called upon reflexly by excessive loads, including the take-off phase in walking.