Cats and dogs have a digitigrade stance. That is that they perpetually walk on their toes, as opposed to their toe nails like a deer or flat-footed like a bear. This puts their heel off the ground. People often mistake the heel for a backwards facing knee but a close examination of the anatomy shows that they have a knee that points much the same way as ours. The knee is easy to miss when looking at the animal because the knee is about at the same level as the belly. The knees of some digitigrade animals such as dogs or cheetahs are easier to see because they are distinctly below the level of the belly. On the front limb, the wrist is low on the leg and slightly angled forward. Most of the limb that you see below the belly is a long forearm (radius and ulna).
In addition to the six muscles you have seen in the bear and the deer, there are two more ones that make prominent bulges on the bodies of cats. Cats use their forearms dynamically and have more developed back and shoulder (latissimus dorsi and deltoid) muscles than you see in other quadrupeds.
Print out the mammal anatomy worksheet and follow the In the step by step guide below, adding one muscle at a time. Then envision how these muscles show through skin and fur. This will help you memorize the shapes and locations of the muscles better than just reading this post. Click on the first image to start a sideshow with step by step details.