Final answer:
Walking at a brisk pace of 4.5 mph is classified as a moderate physical activity according to the 2018 Physical Activity Guidelines.
Step-by-step explanation:
Walking at a brisk pace of 4.5 mph would receive a moderate physical activity classification. According to the 2018 Physical Activity Guidelines, moderate-intensity activities are those in which you can talk, but not sing.
This level of exertion includes activities like walking at 5 km/h, which is approximately equivalent to 3.1 mph. Therefore, walking at a slightly faster pace of 4.5 mph is still within the moderate-intensity range.
Engaging in moderate-intensity aerobic activity is part of the guidelines which suggest adults should aim for at least 150 to 300 minutes of such activity per week for substantial health benefits.