• The activity of both sets of muscles, the diaphragm and the abdominal muscles, varies reciprocally. Thus, during inspiration the tonus of the diaphragm increases while that of the abdominal muscles decreases, and vice-versa during expiration. Hence there exists between these two muscle groups a floating equilibrium constantly shifting in both directions.

    Dave Gorman: The activity of both sets of muscles, the diaphragm and the abdominal muscles, varies reciprocally. Thus, during inspiration the tonus of the diaphragm increases while that of the abdominal muscles decreases, and vice-versa during expiration. Hence there exists between these two muscle groups a floating equilibrium constantly shifting in both directions.