Final answer:
The target heart rate (THR) range is calculated during exercise using the heart rate reserve method, accounting for individual differences in resting and maximum heart rate.
Step-by-step explanation:
The condition for calculating the target heart rate (THR) range using the heart rate reserve method is during exercise. When assessing the THR, it's important to consider an individual's resting heart rate and maximum heart rate, which can be influenced by factors such as fitness level, age, and any cardiovascular disorders present. During exercise, the heart rate increases which allows for the determination of the cardiac reserve. This then can be used to calculate the THR for optimal cardiovascular benefit. Maintaining exercise within the THR ensures the heart and lungs are efficiently conditioned without putting undue stress on the body.