Cardio-respiratory endurance allows the body to perform physical activities for longer periods of time with less effort through continuous or interval training exercises that elevate the heart rate for at least 20 minutes 3 times a week. Muscular endurance can be improved through circuit training exercises that gradually increase the time or number of repetitions performed at each station. Regular training results in increased lung capacity, more oxygen delivery to working muscles, an enlarged heart that pumps more blood, and an aerobic threshold allowing sustained activity.