What is the mechanics of inspiration during rest and exercise?

During inspiration the diaphragm contracts flattening, and the external intercostal muscles contract pushing out the ribs and sternum. This increases the volume and decreases the pressure within the thoracic cavity, the atmospheric pressure is greater than in the lungs so air rushes in down the pressure gradient. To allow for a faster breathing rate and a greater volume of air to enter the lungs additional muscles are recruited during exercise. The most important of these is the sternocleidomastoid in the neck which pushes the sternum out, and the pectoralis major and minor, which pushes the ribs further out increasing the thoracic cavity volume.