Final answer:
The diaphragm contracts during inhalation, including during vigorous physical activity when forced breathing is active, but not during exhalation. So, the correct answer is option b.
Step-by-step explanation:
The diaphragm contracts during inhalation, whether it be during quiet breathing or vigorous physical exercise. During inhalation, the diaphragm and external intercostal muscles contract, expanding the rib cage and lung volume. This action creates a lower pressure within the lungs compared to atmospheric pressure, drawing air into the lungs. In contrast, exhalation is generally a passive process, where the diaphragm relaxes, allowing the lungs to recoil and air to be forced out due to increased pressure within the lungs.
However, in the case of vigorous physical exercise, forced breathing, also known as hyperpnea, involves both inspiration and expiration due to muscle contractions. In this scenario, additional accessory muscles come into play for both forced inhalation and forced exhalation, but the diaphragm contraction is specifically associated with inhalation, not exhalation.
Therefore, the correct answer to the question 'The diaphragm contracts during:' is b. Inhalation, during vigorous physical exercise.