Final answer:
The active phase of breathing involving the contraction of the diaphragm and intercostal muscles is known as inspiration or inhalation, which increases thoracic volume and draws air into the lungs.
Step-by-step explanation:
The active phase of breathing where the diaphragm and intercostal muscles contract is called inspiration or inhalation. This process involves contraction of the diaphragm, causing it to move downward and the chest cavity to expand, and contraction of the external intercostal muscles, lifting the rib cage upward and outward. As a result, the volume inside the thoracic cavity increases, producing a negative pressure differential that draws air into the lungs. Conversely, expiration or exhalation is typically a passive phase where these muscles relax, decreasing the thoracic volume and pushing air out of the lungs. During forced breathing, such as during exercise or singing, other accessory muscles are also involved to facilitate the movement of air.