There are lots of tools out there that are both free and paid for that will let you ease the burden of character animation in After Effects. From DUIK, to BAO Bones, to Rubber Hose – character animation scripts are a stones throw away. All will allow you to rig up a character and offer a simple way to animate arms and legs.
Although most of them will offer something unique, they will all basically let you animate arms and legs using IK or FK, or both. Those systems can be really helpful when creating complex animations. You can get away with not using anything, too.
Ae has a pretty robust (albeit cumbersome) shape layer system. You can use shape layers and their special attributes to animate a character’s arms, legs, and fingers even.
Showing how this is done is Motion Graphics Artist Evan Abrams. Evan walks through how he animated a simple character loop of a fellow that is pretty excited about lifting heavy weights.
For short loops, animating this way is a simple way to get things done, without going to 3rd party tools.