It does look more like a running sproing, and that's for a reason. A flying, powerful kick needs one out of two things that I can think of. Preferably both, depending on how far into the motion the character is.
1. It needs a sense of motion. Yours has this, so that's all good. But, the second thing confuses things.
2. It needs to show energy. Energy focused at whoever's being kicked, and energy that's been put into gaining speed in order to execute the kick. Yours has energy, as well. But, it looks like the energy comes from the wrong side of the body, so to speak. The leg you want to be kicking actually looks like it's being used to push the character in the other way. In fact, it looks just like he's about to do a walljump, the foot to the left starting to lean onto the wall...
I can think of two things to change the situation. First off, his kicking leg NEEDS to be compeltely straight. Otherwise he'd absorb the power of the kick, himself, and nothing would happen! Except, maybe, someone breaking a leg... Second, the other leg needs to show a sign that it's just been used to propel the character into the air, toward his target. You might want to make him lean back some to show how much power he's got stored for the attack. Just like Skull's reference, in fact...
And, I could say more thing about the stance, but I won't... It'd just be pointless rambling beyond this point. Just keep what I said in mind.
