Contraction of the intercostal muscles and the diaphragm causes the thoracic cavity to and air pressure in the lungs to

Contraction of the intercostals and diaphragm cause  the thoracic cavity to expand, and the air pressure in the lungs to decrease.

this decrease in pressure then pulls in air from outside, thus inspiration occurs. 

People who utilizes the progression principle will ensure their exercise program increases in frequency, intensity, time, and type. the changes need to happen gradually in order to ensure they make gains in their fitness program. a person who doesn't utilize the progression principle can still gain health benefits from exercising, but will not continue to improve in cardiovascular fitness.