Final answer:
Noticeable contraction of the intercostal muscles provides evidence of using accessory muscles to breathe, indicating that the patient may require extra effort for inhalation and exhalation, beyond what is normally engaged with the primary breathing muscles.
Step-by-step explanation:
The evidence that a patient is using accessory muscles to breathe is most directly indicated by noticeable contraction of the intercostal muscles. During normal breathing, the primary muscles used are the diaphragm and the external intercostal muscles. However, when a patient is using accessory muscles, this implies that additional muscles are required either due to difficulty in breathing or the need for deeper breaths. Forced breathing, also known as hyperpnea, engages not only the diaphragm and intercostal muscles but also accessory muscles like the scalenes in the neck during inspiration, and obliques in the abdomen during expiration. These muscles assist in increasing lung volume during inhalation and forcing air out during exhalation by lifting the thoracic wall or compressing the rib cage.