Final answer:
Respiratory fatigue in healthy individuals commonly occurs with high exercise intensities, involving accessory ventilatory muscles during forced breathing or hyperpnea. It is less common under normal circumstances due to the oxidative endurance of the diaphragm, but high intensity or extended submaximal exercise can lead to fatigue.
Step-by-step explanation:
In healthy individuals, respiratory fatigue may occur with high exercise intensities (>80% to 85% peak oxygen consumption). This condition typically arises when there is a high demand for oxygen and the muscles responsible for breathing are overworked. During forced breathing or hyperpnea, which happens during vigorous exercise or activities like singing, both inspiration and expiration are driven by muscle contractions. The primary breathing muscles include the diaphragm and intercostal muscles, but accessory ventilatory muscles in the neck, such as the scalenes, and the abdominal muscles, including the obliques, also play significant roles. These accessory muscles help increase lung volume and force more air out of the lungs during forced expiration.
While respiratory fatigue is less common in healthy individuals under normal circumstances due to the oxidative nature of the diaphragm muscle, it can be experienced during periods of high-intensity exercise or extended submaximal workloads. This is when accessory muscles become involved to sustain the increased ventilatory demand, potentially leading to fatigue if the intensity is maintained for an extended period. Additionally, exercise-induced hyperpnea begins at the start of exercise, not when an oxygen deficit is present, indicating that other factors, including neural mechanisms and motor neuron activation, play a role in this respiratory response.