Power walking or speed walking is the act of walking with a speed at the upper end of the natural range for the walking gait, typically 7 km/h to 9 km/h (4.5 to 5.5 mph). To qualify as power walking as opposed to jogging or running, at least one foot must be in contact with the ground at all times (see walking for a formal definition).
Power walking has been increasingly recommended as an alternative to jogging for a low-to-moderate exercise regime, for instance 60–80% of maximum heart rate, HRmax. At the upper range walking and jogging are almost equally efficient, and the walking gait gives significantly less impact to the joints. When used in this way, an exaggerated arm swing is often used.
Power walking/speed walking is often confused with race walking, which has rules to define what counts as walking fast and is also a popular Olympic level event.
Reference: Wikipedia Power Walking