The Passive Forces
During quiet or eupneic breathing, expiration is usually passive. With contraction of the inspiratory muscles to bring about inspiration, elastic components of the lung are stretched. The potential energy stored in the elastic features of the lung is released with relaxation of inspiratory mucles. Elastic recoil of the lung is normally sufficient to bring about expiration without contraction of expiratory muscles. However, with high ventilatory rates, the muscles of expiration comprising the abdominal group and the internal intercostals are important.