The best indicator for measuring the intensity of a workout is:

Heart rate is the most effective indicator for measuring the intensity of a workout because it provides a direct measurement of cardiovascular exertion during exercise. When you engage in physical activity, your heart rate increases to supply more oxygen to the muscles, which indicates how hard your body is working. Monitoring heart rate allows individuals to assess whether they are exercising within their target intensity zone, which can be determined based on their age, fitness level, and health status.

While vigorous breathing can indicate a higher intensity, it is more subjective and can vary significantly from person to person. Muscular soreness may suggest that a workout was challenging or new, but it is not a reliable measure of workout intensity, as soreness does not always correlate with the effort put forth during the session. Total time involved provides contextual information about the duration of exercise but does not inherently convey how intense the workout was; longer workouts can be at varying intensities. Therefore, heart rate remains the most quantifiable and objective way to assess how hard one is training during a workout.