Conversationally, people say cats have four legs. That means they have four knees and zero elbows, right? But they use their front legs like arms, so maybe they have two knees and two elbows? The answer isn't straightforward. First you've got to learn some cat anatomy.
Cats walk around by moving their front and hind limbs, so it's only natural we'd assume they have four legs. Take a closer look, though, and you'll see the front pair collapse forward, much like your arms, while their back legs collapse backward, much like your legs. Given this revelation -- plus their propensity to paw, claw and grasp at things as if their front paws were hands -- it makes sense to refer to their front legs as arms and their back legs as legs. There's a problem, though: You're looking at the wrong part of their legs.
Elbows & Knees
There are two primary limb joints: hinge joints and condylar joints. Without getting too technical, human elbows and ankles are hinge joints; human knees are condylar joints. It's similar in cats, but the locations are different. Human knees and elbows are both mid-limb; for cats they're much higher, closer to the shoulders and hips. Here's the short story: Cats have knees on their back legs and have elbows on their front legs, so they have two knees and two elbows. If you're arguing from a functional standpoint, however, you can argue they have knee- and elbowlike joints on each leg, which makes for four knees and four elbows.
Back Legs & Front Legs
It seems like your cat's back legs bend the opposite direction as yours, but that's not the case. What look like your cat's lower hind legs are actually the top part of his feet. And the same goes for his front paws -- essentially your cat perpetually walks tiptoe. A quick glance at a cat skeleton in an anatomy book confirms this rather handily. It also shows cats have patellas, or kneecaps, on their back legs but not on their front legs. This makes for two knees and two elbows per cat.
Symmetry & Function
With a broader understanding of cat anatomy you've confirmed your cat has two knees and two elbows. Makes sense, right? Hold on. Oricom Technologies -- a Colorado-based robotics developer --- makes the argument that your cat's legs display a complex geometry in which each leg on each side of the cat's body mimics its counterparts. As such, the wrist on your cat's front legs bend similarly to the knees on his back legs. Likewise, the wristlike joints on his back legs bend akin to the elbows on his front legs. Cats use each leg as if it has both a knee and an elbow, so you can argue they have four knees and four elbows. It's not a traditional take on anatomy, but it's functionally true.
- Charles D. Newton, of University of Pennsylvania School of Veterinary Medicine: Normal Joint Range of Motion in the Dog and Cat
- Kenyon College: Cat Anatomy Tutorial -- Muscle Function
- Washington State University Extension, 4-H Youth Development Program: Cat Anatomy and Physiology
- DifferenceBetween.Net: Difference Between Elbow and Knee
- Oricom Technologies: Basic Anatomy of Mammals
- Penn State Behrend: Skeletal System
- Orange Coast College: Cat Skeletons -- Forelimb & Pectoral Girdle
- Penn State University: Upper Extremity Muscles -- Forearm (Lateral)
- Penn State University: Upper Extremity Muscles -- Forearm (Medial)
- Moggies: The Body of the Cat
- Mascotia: Cat's Skeleton, Muscles and Movements