Because the cell theory is technically correct. So why would the cell theory be disapproved if it is accurate?
Answer:
During quiet breathing, the main muscular effort is made in the inhalation process and the expiration process is mainly passive as it depends on the inhalation.
The expiration occurs due to the elastic recoil that causes a decrease in volume. In this process, the contraction of the diaphragm helps in the inhalation which is least affected by the intercostal muscle. During quiet breathing, the rate of inhalation and expiration time is relatively proportionate to one another.