This. My kicks (and punches) got a lot more powerful when I added heavy bag practice, which was a rare part of my early training. Makiwara, sparring, and practicing with the air - even striking into striking pads held by partners - didn't do nearly as much for power development.Absolutely agree. And kicking the air won't teach that effectively. And unless you've got an unlimited number of partners willing to take your best shots, sparring won't truly teach that either.
As I said in my first post here, you've got to hit things hard to learn that effectively. There's really no substitute for a good heavy bag to teach and develop stopping power. Well, maybe cows hanging in a meat locker like Rocky used to hit might be better. And you'll help your butcher tenderize the meat
I'll also add that flexibility matters. I'm not highly flexible at the hips, so high kicks lose power. I can pummel a ribcage, but kicks to the head are much softer.
