Most galloping quadrupeds have one airborne phase per stride, when the legs are bunched beneath the body. Sighthounds and cheetahs add a second, at full extension, when fore and hind limbs reach away from each other and no foot touches the ground. The extra flight phase lengthens the stride without requiring faster limb cycling.
The anatomy that permits this is a long and unusually flexible lumbar spine, which flexes and extends like a spring, together with a deep chest housing a large heart and lungs. Greyhounds sustain top speeds of roughly 65 to 70 kilometres per hour over a few hundred metres. The cheetah, using the same gait pattern, is faster still.
The breed is often described as an endurance runner because it is lean. It is not. Greyhound muscle is heavily weighted towards fast-twitch fibres suited to short anaerobic bursts, and racing distances are typically well under a kilometre. Off the track most greyhounds are notably sedentary, which is why the phrase forty-mile-an-hour couch potato has stuck to them.