The hook can be a devastating kick to the body or head, thrown from closer in than a wheel, and can be thrown a different angles.
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Absolutely do NOT practice the wheel kick on a heavy bag for the reasons you describe. A hook would be safer on the heavy bag, but I can see it being hard on the kicking hip.
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IMHO, a heavy bag should only be used for linear kicks/strikes.
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Hmm from these I suspect that we have some misunderstanding. I see the early part of the thread was about what the kicks are called. Perhaps I wasn't clear.
When I train these kicks (unlike in kihon) I do not care about the kihon forms. I let power and balance as i get feedback from the sack guide me. If I can give the sack a good power, maintain balance and not feel any back joints, then it's good. Then I try to work on the speed.
What I call wheelkick, has the same range as the spinning hook kick. That you bend the leg before impact to get more power and protect your knee I do in both versions. That isn't the difference in what I mention.
There is only one difference between the spinning kicks for me, the spinning hook kick starts by chambering, and then you extend the kick say at 45 degrees and hook immediately before target. It spins faster. The wheel kicks STARTS with a roughly extended leg (but still relaxed) and it rotates all they way from the back to the target, acquiring more momentum (but it's slower), but in the end, just like the hoook kick I "hook" to make sure I do not have straight leg on impact.
To make sure your knee is find, the legt is slightly bent in impact. the degree of bending can be adjusted to the target distance.
What I find important is to keep the knees extension in line with impact, so there is no sideway force on the knee. Same as when doing power roundhouse kicks. Normally diagnal, but if kicking the leg it can even by slighlty downward, if kicking the lower rib, it cna be sligtly up.
Here is a decent explanation in line with what I mean
When I trained this lately, I often train only roundhouse on the bag, and i train combos such as 2 roundhouse, leg, spleen, leg, spleen then leg, then leg, reverse wheel to leg or liver to opposite side. What I trained to make this with balance, power and speed. And I seem to never get much power into the spinning hook, but the spinning wheel is good (but a but slower), there is why i train the combos with the faint.