The best and most common time to wear your chest strap is during any physical activity—running, walking, gym sessions, cycling, or sports. It shows how your heart reacts to exercise in real time. This helps you know if you're working too hard or not hard enough. If your goal is fat loss or stamina building, staying in the right heart rate zone matters a lot. A chest strap helps you stay in the “fat burn” or “aerobic” zone longer.