First, you have to decide whether you want to focus on improving speed, power, or a technical aspect of your kicks during a single workout session. Or maybe you just want to condition your shins to make them tougher.
Once you have decided where to start to do a proper warm-up and stretching before you start kicking a bag. Some people prefer to do some shadow boxing before kicking a bag or Thai pads - it certainly doesn't hurt to do so.
If you want to kick faster take a look at this drill. What's important here is to keep the same pace. You ill notice how the pace improves from session to session.
There are two important factors when you want to increase your kicking power: the first one is physical and it is the overall fitness and strength. There are no shortcuts here, just hard regular training. And the second one is technical: the right stance, the torque, etc. This is why sometimes a smaller person can kick harder than a big dude with no proper technical skills.
Practice various types of kicks, not just a round kick. A push kick [a teep] is a very effective weapon and can be used both: defensively and offensively. With this video, you can also improve your push kick and push kick defense
And last but not least: shin conditioning. Having tough and well-conditioned shins is crucial for a Muay Thai practitioner. In Muay Thai all kicks, except push kick, are delivered with a shin and not with foot. This what makes them so effective and so painful for the opponent.
Shins are also much tougher since the kick connects with just one large bone and not with multiple small foot bones. That said, shins must be properly conditioned to make them harder.
Not every kick will land on a soft spot. If you have ever kicked your opponent with full power in his pelvis you will know what I mean. Many times an opponent will block it with his own shin.
And if the fighter's shins are tougher than yours you will have enough just after a few kicks. So condition your shins by kicking a heavy bag and then slowly progress to a harder banana bag.
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