Final answer:
The participant's heart rate in the Rockport one-mile walk test should be measured immediately at the end of the walk to accurately estimate aerobic fitness levels.
Step-by-step explanation:
When conducting the Rockport one-mile walk test, the participant's heart rate should be measured immediately at the end of the walk. To ensure accurate results, it is usually suggested to measure the heart rate during the last minute of the walk or right after completion while the body is still in a state of exertion. The heart rate measurement helps in estimating the participant's aerobic fitness level.