In general, physical fitness refers to one's ability to perform physical tasks, especially as they relate to a sport or occupation; however, because physical fitness encompasses a wide variety of abilities, it means different things to different people. The main categories of physical fitness are aerobic endurance, muscular endurance, strength, speed, power and flexibility.Each main category of physical fitness covers a broad variety of activities. Aerobic endurance refers to the capability of the heart and lungs to perform at a high level for a long period of time, and activities that make use of aerobic endurance include running, swimming and biking. Muscular endurance and strength are on the same spectrum, yet they refer to distinct abilities. Muscular endurance is the muscles' ability to work for a long time against resistance, and it is most commonly used during high-repetition sets of weight training exercises like cable crunches and pull-ups. Meanwhile, strength refers to the maximum force muscles can exert against resistance at once. One-repetition maximums for the squat, bench press and deadlift are good examples of strength. Speed is related to strength, and it refers to one's aptitude for moving short distances quickly. Examples of speed include the 40-yard dash and the 100-meter freestyle. Power is what Olympic weightlifters use during the snatch and the clean and jerk. It combines strength and speed, and it refers to the body's ability to exert explosive force over a short span of time. Flexibility is frequently overlooked, but it is important because it facilitates the other aspects of physical fitness; it refers to the muscles' range of motion at each joint.