To make a good animation of a dynamic movement, you need to actually redraw a lot of the anatomy rather than do small modifications. It's more work
but that's the secret.
Study a similar animation, like Double Dragon's bat-swinging here :
https://www.youtube.com/watch?feature=player_detailpage&v=jo6R6LqpNNY#t=250This case has only 4 striking frames, but you'll notice they are quite carefully chosen (In animation they are often called key frames, as they show something at a "key moment" of the action). The animation goes "full circle", and the bat only stops when it's completely behind the character.
Also, note that the body (shoulders) goes in the opposite direction as the bat, to show that the character compensates for the weight. Weight is important.
If you use it as a reference, be careful that Double Dragon anatomy has issues of "short thin legs - big head", you should probably not rely on these body proportions. (It's even worse in a later Techmos game, "The Combatribes". I think it was assumed as a kind of "SD" style, it afforded enough room to put more details on arms and faces )
edit: Found another kind of reference : See the playable characters animations of Golden Axe :
http://www.spriters-resource.com/genesis_32x_scd/goldenaxe/With 4 frames too, they clearly show the same principles.
