During inspiration, which of the following muscles contracts?

During inspiration, the primary muscle that contracts is the diaphragm. This dome-shaped muscle is located at the base of the thoracic cavity and plays a crucial role in the breathing process. When the diaphragm contracts, it flattens and moves downward, creating a negative pressure in the thoracic cavity. This negative pressure allows air to be drawn into the lungs, facilitating inhalation.

While the intercostal muscles also play a role in expansion of the chest wall during inhalation, the diaphragm is the main muscle that initiates the breathing process. The rectus abdominis and external oblique muscles are primarily involved in forced expiration or activities requiring trunk movement, rather than in the inspiration phase. Thus, while the diaphragm is the chief muscle for inspiration, the intercostals support this process, but the diaphragm's contraction is fundamental to initiating breath.