Most treadmills monitor intensity with hand sensors that measure your heart rate, but that's not the most accurate approach. A better way to gauge your effort is to use what's called the rate of perceived exertion, which involves ranking your sense of how hard you're working on a 1-to-10 scale, with 1 being low and 10 being high. For instance, a rate of 5 to 7 is consistent with a moderate-intensity level where you work hard, but can maintain a conversation and do not overexert yourself. http://www.tecmofitness.com/cardio-equipment/treadmill/