Final answer:
The 'shortness of breath' zone, crossing from aerobic exercise to anaerobic exercise, is known as the anaerobic or red zone. This transition point occurs when your body's oxygen consumption can't keep up with the exercise intensity, causing a sensation of shortness of breath.
Step-by-step explanation:
The zone which is often referred to as the "shortness of breath" zone indicative of crossing from aerobic exercise into anaerobic exercise is the anaerobic zone. This is also known as the red zone.
When you transition from aerobic to anaerobic exercise, your body derives energy through a metabolic process that does not require oxygen. As the intensity of the exercise increases, your body's oxygen consumption can't keep up with the demand, thus creating a 'shortness of breath' sensation. This is a signal that you are working out at a very high intensity, and your body is shifting into the anaerobic zone. However, sustaining this level of exercise for longer periods is not possible for most people.
Learn more about Anaerobic Zone