Final answer:
The external intercostal muscles are the synergists to the diaphragm during inspiration, helping to expand the thoracic cavity by raising the rib cage.
Step-by-step explanation:
The external intercostal muscles are the synergist muscles to the diaphragm during inspiration. These muscles assist the diaphragm by aiding in the expansion of the thoracic cavity. As the external intercostals contract, they raise the rib cage, leading to an increase in the volume of the thoracic cavity and thus facilitate inspiration. Unlike the external intercostals, the internal intercostal muscles aid in expiration by drawing the ribs together, which decreases the volume of the thoracic cavity. The innermost intercostal muscles also act as synergists for the action of the internal intercostals during expiration.