Final answer:
The 'high' classification of MET intensity relates to demanding exercises that require rapid ATP production through anaerobic and aerobic metabolism, such as running or competitive sports, and typically involve significant increases in heart rate and breathing.
Step-by-step explanation:
The “high” classification of MET (Metabolic Equivalent of Task) intensity refers to activities that require a large amount of energy in a short period of time. High-intensity activities demand that muscles produce more ATP (adenosine triphosphate), which is used as energy, leading to the combined use of both anaerobic and aerobic metabolism to meet the body's needs rapidly. Typically, any activity that significantly accelerates the heart rate and increases breathing can be considered high intensity, such as running, intense cycling, or competitive sports. It is crucial for individuals engaging in high-intensity activities to have both a good level of physical fitness and to ensure adequate rest and nutrition to support this level of exertion.
According to the 2018 Physical Activity Guidelines, moderate-intensity exercises can be sustained more comfortably over a longer period of time, about 1-hour per day, whereas high-intensity activities are shorter in duration but greater in effort, like an endurance program that includes 1-3 hours per day of moderate to high-intensity exercise.