Narrowly defined, fitness refers to the characteristics that enable the body to perform physical activity. These characteristics include flexibility of the joints, strength and endurance of the muscles, including the heart muscle, and a healthy body composition. A broader definition of fitness is the ability to meet routine physical demands with enough energy reserve to rise to a sudden challenge. This definition shows how fitness relates to everyday life. Ordinary tasks such as carrying heavy suitcases, opening a stuck window, or climbing four flights of stairs, which might strain an unfit person, are easy for a fit person. Still another definition is the body’s ability to withstand stress, meaning both physical and psychological stress. These definitions do not contradict each other; all three describe the same wonderful condition of the body.