Quads in swimming

I've been doing a lot of kick sets and trying hard to make sure that I use my glutes/hips to kick. One interesting method I read online is to imagine you are holding a coin between the buttocks while kicking. My question is this. I'm feeling some fatigue in the quadriceps while doing this, even though I'm sure I'm initiating the kick from the hips. I'm wondering if this is normal to some extent or a sign that I am not relaxing my lower leg enough on the down kick (i.e. I'm keeping it too extended). Should I be trying harder for that whip action where the lower leg lags behind the thighs on the way down (so it is completely relaxed) and locks on the way up? Thanks.