Once you have begun to master effect gesturing using arcs, and understand weight and how it affects an object, it will be time to move on to one of the most challenging parts of animation: the walk. Bipedal creatures (which include us) use an odd form of propulsion. In a sort of controlled fall, we move our weight forward through friction on the ground of one foot as the other moves to catch our weight before we fall again.
Such is human walking, running, strutting, sneaking, and every other kind of locomotion. Oh, but that is not all! Through this process of controlled falling, the bipedal creature twists, turns, and distorts along nearly every axis possible in order to maintain balance. It is amazing that although we do it every day without thinking, walking is an immensely complex set of motions.
As we talked about earlier, we all have weight (at least on our earth). In order to balance this weight in all the twists and turns our body does, nature has provided a fairly central point around which most general motion takes place. This center of motion on humans resides about halfway between our belly button and crotch.
As we turn, we turn around this point. As we bend, we bend at that point. When we walk, our appendages all flail at different speeds; first one foot is charging ahead while the other stays still, and then they swap roles. However, our center of motion always stays moving.
Try it. Get up and walk about the room, and notice that from your head to your crotch, you move at a more or less constant rate, while the rest of your body is altering rates or, in the case of your hands, even moving backward. Because we have this center of weight, we can base most all motion around it. We never notice it, but we walk in arches.
Although our mind compensates for the up and down movement, our center of motion and thus our torso and our head gently rise and fall as we walk forward. This is due to the fact that when both feet are on the ground during mid-stride, both legs are bent and at a diagonal to our bodies. Then, as the walk cycle continues and all the weight is transferred to one leg, this one leg is nearly completely vertical and we are nearly fully erect.
With this knowledge, this is often a good place to start a walk cycle. With the forward movement gently moving up and down, you have a good start to believable movement. In fact, if the camera is placed from the waist up and we can see the up and down movement, we understand that the character is walking even if we never see his legs.