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The cat righting reflex is a cat's innate ability to orient itself as it falls in order to land on its feet. The righting reflex begins to appear at 3–4 weeks of age, and is perfected at 7 weeks. They are able to do this as they have an unusually flexible backbone and no functional clavicle (collarbone). The minimum height required for this to occur in most cats (safely) would be around 30 centimeters. Cats without a tail also have this ability, since a cat mostly moves its hind legs and relies on conservation of angular momentum to set up for landing, and the tail is in fact little used for this feat.
After determining down from up visually or with their vestibular apparatus (in the inner ear), cats manage to twist themselves to face downward without ever changing their net angular momentum. They are able to accomplish this with these key steps:
In addition to the righting reflex cats have a number of other features that will reduce damage from a fall. Their small size, light bone structure, and thick fur decrease their terminal velocity. Furthermore, once righted they may also spread out their body to increase drag and slow the fall to some extent. A falling cat's terminal velocity is 100 km/h (60 mph) whereas that of a falling man in a "free fall position" is 210 km/h (130 mph). At terminal velocity they also relax as they fall which protects them to some extent on impact. However, it has been argued that, after having reached terminal velocity, cats would orient their limbs horizontally such that their body hits the ground first.
With their righting reflex, cats often land uninjured. However, this is not always the case, since cats can still break bones or die from extreme falls. In a 1987 study, published in the Journal of the American Veterinary Medical Association, of 132 cats that were brought into the New York Animal Medical Center after having fallen from buildings, it was found that the injuries per cat increased depending on the height fallen up to seven stories but decreased above seven stories. The study authors speculated that after falling five stories the cats reached terminal velocity and thereafter relaxed and spread their bodies to increase drag. Scientists in Massachusetts have recently discovered that the cat's ability to spread its legs out to increase drag when reaching terminal velocity would explain the decreased injuries sustained above seven stories because they wouldn't reach terminal velocity before then. Professor David Stevenson said "we simulated the cat's weight and size and found the terminal velocity to be 60mph which would more than likely result in severe injury or death to the cat when falling from this speed, but once we took into account the cat's ability to right its self and spread its body out this reduced the terminal velocity to only 53mph. This 7mph difference is massive and would almost certainly ensure the cat's survival. There however is always the possibility that the cat may not manage to right itself so this is far from a conclusive experiment and we do not condone the throwing of cats from anything."