typically a ballistic phase where the runner is airborne with both feet in the air (for bipedals). Another difference concerns the movement of the centre of mass of the body. In walking the body "vaults" over the leg on the ground, raising the centre of mass to its highest point as the leg passes the vertical, and dropping it to the lowest as the legs are spread apart. Essentially kinetic energy of forward motion is constantly being traded for a rise in potential energy. This is reversed in running where the centre of mass is at its lowest as the leg is vertical. This is because the impact of landing from the ballistic phase is absorbed by bending the leg and consequently storing energy in muscles and tendons. In running there is a conversion between kinetic, potential, and elastic energy. There is an absolute limit on an individual's speed of walking (without special techniques such as those employed in speed walking) due to the upwards acceleration of the centre of mass during a stride – if it is greater than the acceleration due to gravity the person will become airborne as they vault over the leg on the ground. Typically, however, animals switch to a run at a lower speed than this due to energy efficiencies. Based on the 2D inverted pendulum model of walking, there are at least five physical constraints that place fundamental limits on walking like an inverted pendulum. These constraints are: take-off constraint, sliding constraint, fall-back constraint, steady-state constraint, high step-frequency constraint. Leisure activity Many people enjoy walking as a recreation in the mainly urban modern world, and it is one of the best forms of exercise. For some, walking is a way to enjoy nature and the outdoors; and for others the physical, sporting and endurance aspect is more important. There are a variety of different kinds of walking, including bushwalking, racewalking, beach walking, hillwalking, volksmarching, Nordic walking, trekking, dog walking and hiking. Some people prefer to walk indoors on a treadmill, or in a gym, and fitness walkers and others may use a pedometer to count their steps. Hiking is the usual word used in Canada, the United States and South Africa for long vigorous walks; similar walks are called tramps in New Zealand, or hill walking or just walking in Australia, the UK and the Irish Republic. In the UK, rambling is also