It's fun to draw your favourite superheroes, but it's difficult to give them the same visual excitement you see in the fight scenes that practically pop off the page. Characters simply look flat when they just stand there. Giving characters motion--particularly synchronising the motion of two characters--is hard because one change in the body usually affects several others. The actions and reactions of skeleton and muscle groups have to be very well coordinated and replicated on the page to be convincingly drawn.
Draw the primary lines of action on the page with your sketch pencil. Determine which body extremities are the most important to the action and trace the line of kinetic energy through the character's body. Pay careful attention to how one character's line of energy is affecting another character's, and whether this is being achieved in unarmed combat or with a weapon, which will have a line of energy of its own.
Sketch a stick figure over the line of energy, paying attention to the perspective view of each body part. Build the stick figures up with "pipes" on the arms, legs, neck and torso, which will make them resemble the mass of a real person.
Build up the musculature of the stick figures. Look at your own body, making the characters' motions in the mirror to examine how the muscles stretch or flex. Don't pay exclusive attention to the primary muscles involved; note how heavy lifting causes the neck to tense, or how a punch affects leg positioning.
Draw the details--such as scars, tensed faces or spurting blood--onto your characters. Then ink the vital lines of the drawing. Let the ink dry and then erase the pencil lines from the page.
Exaggerate the action by creating effect lines. Show impact by creating lines that "blow out" from the point of contact. Illustrate motion by drawing straight lines that trail behind a moving object.
Practice drawing stick figures very quickly--aim for 30 seconds a figure--and draw several in different positions to determine which will look best for your scene.