Karpov, thanks for your advice.
Yeah, i had reduced the resolution to increase the speed, so there's no frames between bent lower leg and extended one. The original kick animation has this "swinging-at-enemy with-lower-leg", what looks rather slow and theatral, than a explosive release of power in someone's teeth.
The feet can kept on the ground by location constraints, but what about the moving parts? Also, how do you parent the bone of a weapon to the hand and do you delete it in a keyframe when it should be holstered?
For now, the change of speed is my main issue, which i solved my copypasting the poses from frame to frame (deleting the original with Alt+I) but i did take a look at the IPO curves, which and which parts of could be scaled. This feature promises more comfortable speed control for parts of the animation, and movement changes would be less of a mess.
It looks rather complicated, i'll post a result when i'm home.
Edit: here, a improved kick. The gif is somewhat sluggish, the original is twice as fast.

Also, a .3ds:
http://www.mediafire.com/?aahocs7myb8p4u9