To show expressive movement when drawing movement lines in cartoons or comics, adjust the lines' intensity, volume and length according to the scale of the movement in question. Speed lines, as they're called in comics, can be used to highlight a character's movement or to draw the viewer's eye toward the character. Japanese manga frequently uses radial speed lines, also called focus lines, to emphasize a character.
First, determine the scope of the movement. As an example, assume you're drawing a character at full super-speed motion. The direction of the lines depends on the direction of movement; running straight requires lines to be drawn straight back from the character. Try to make the lines the same color as the character to simulate an afterimage-like effect. The lines should appear to blur out the actual surface of motion like arms or legs.
Less intense motion requires shorter, fainter lines. Look at virtually any Flash comic for reference inspiration for speed lines.
Western comics and Japanese manga handle speed lines differently; in Western comics, the background stays in focus while speed lines are drawn around the character. In manga, the background is reduced to a blur of lines while the character remains in focus, as though the observer is moving with the character. If the character is in the center of the panel and lines radiate from him, it indicates the movement toward the foreground or background.