Exertion refers to the physical or mental effort put into an activity. It can involve various forms of work, such as exercising, lifting heavy objects, or concentrating on a challenging task. The level of exertion can vary based on the intensity and duration of the activity, affecting how tired or fatigued a person feels afterward. In physical fitness, exertion is often measured to ensure that individuals are working within their limits. Activities like running, swimming, or weightlifting require different levels of exertion, which can be monitored using tools like heart rate monitors or perceived exertion scales. Understanding exertion helps in achieving fitness goals safely.