The Diaphragm - a membrane of muscle and tendon, flexes to reduce ambient pressure in the thorax, and cause the lungs to compensate by drawing in air. Exhalation works in reverse.
A muscle that contracts shortens whereas a muscle that relaxes lengthens.
Relaxes
It Contracts
It relaxes
The bicep muscle. The tricep contracts to straighten the arm as the bicep relaxes.
The heart
The muscle that separates the thorax and abdomen and aids in breathing is called the diaphragm. It is a dome-shaped muscle that contracts and relaxes to regulate the volume of the thoracic cavity, allowing for inhalation and exhalation.
contracts - for antagonistic muscle pairs e.g. muscles in arm (when one relaxes, other contracts, and vice versa)
When the bicep contracts, the tricep relaxes. These pair of muscles are known as antagonists which means as one contracts, the other relaxes and vice versa.
It is called your diaphragm.
Diaphragm
The Diaphram