how to time and space your animation(loop/walk cycle especially),line by line, muscle, shapes, whatever its form or properties,so to avoid (unless wanted)
a. limping, unbalanced
b. jittering, where the lines shakes or despite moving correctly in motion and
c. floaty, feeling weightless and unnatural

the worse one is unbalance especially since i honestly feel the body walking/running is a complex matter so any sort of scientific or calculated input is most welcome to make the explanation somewhat simple and quick...