"Using the FITT principle is one way to remember the guidelines for physical activity and create a prescription for improvement in your health-related physical fitness. Frequency is how often a person performs a health-related physical activity. Intensity is how hard a person exercises during a physical activity period. Intensity can be measured in different ways, depending on the related health-related component. Time, also known as duration, refers to the amount of time or repetitions when performing a physical activity. Type is the kind of exercise or physical activity a person is performing. For cardiorespiratory fitness, frequency is 3–5 times per week, increasing as you get into better shape; intensity is 50%–60% of heart rate reserve, increasing as you get into better shape; time is 20–60 minutes of continuous activity, increasing as you get into better shape; and type includes running, cycling, swimming, and activities that use large muscles. For flexibility, stretching should be a regular part of the warm-up and cool-down; stretch all major muscles and joints and hold for 15–30 seconds; perform 1–3 repetitions; continue for 10–20 minutes, increasing the time as you get into better shape; and perform static stretches and controlled dynamic stretches. For muscular endurance, perform activity daily for some muscle groups and 3–4 times per week, increasing frequency as you get into better shape; perform at less than 50% of your predicted 1-repetition maximum, starting with body weight and then adding resistance; perform 15 or more repetitions for 1–3 sets, increasing intensity as you get into better shape; continue for 30–60 minutes, increasing the time as you get into better shape; and use resistance training such as body weight, tubing, medicine balls, or free weights. For muscular strength, perform activity 3 times per week, using different muscle groups each time you work out and increasing frequency as you get into better shape; use 60%–80% of 1-repetition maximum; perform 8–12 repetitions for 1–3 sets, increasing intensity as you get into better shape; continue for 15–60 minutes, increasing the time as you get into better shape; and use resistance training such as body weight, tubing, medicine balls, or free weights."