What tool is commonly used to monitor intensity during exercise?

A heart rate monitor is the tool commonly used to monitor exercise intensity because it provides real-time feedback on the heart rate, which is a direct indicator of how hard the body is working during physical activity. By comparing the heart rate to individual maximum heart rate zones, a personal trainer can effectively assess whether a client is working at a moderate, vigorous, or maximal intensity. This information is crucial for ensuring that clients are exercising at an appropriate intensity to meet their fitness goals, whether it's weight loss, improved cardiovascular fitness, or training for athletic performance.

While body fat analyzers and calorie counters provide valuable information regarding body composition and caloric expenditure, respectively, they do not measure exercise intensity in real-time. Fitness trackers can be equipped with heart rate monitoring features, making them potentially useful for gauging intensity as well, but they also often include additional metrics that may overshadow the primary focus of intensity measurement. Therefore, a heart rate monitor stands out as the most direct and efficient tool for this specific purpose.