Muay Thai knee strikes are one of the most powerful attacks in any martial art. It possesses an explosive knockout power - especially flying knee strike - and it is a great tool for wearing down an opponent in a clinch. A trained fighter can deliver a blasting knee strike that can break the rib cage.
Front knee strike:
This is one the most common knee techniques: thrust your knee forward to target ribs/liver/abdominal section of your opponent. What's important is that you must thrust your hips forward to deliver this strike and not only lift your knee upward. It is useful in a clinch - aside of side knees - where it targets your opponent's lower body sections to wear him down. Then you can hit him in the head from a dominant - like a double collar tie - position when the opportunity arises. This is a powerful technique which works well in the ring and in a street fight too.
Side knee strike is also known as a knee slap: very useful in a clinch fighting cause you don't need much space to apply it properly. You can also use it outside of a clinch against a charging opponent. This technique doesn't use forward thrust. In the clinch, the power comes from a rotating movement of your knees. Outside of the clinch, it comes from rotating hips and the knee - from the outside towards the opponent - at the same time. It targets the ribs and both sides of the abdomen. Knee strike from a clinch is not as powerful as an outside knee strike but it does a lot of damage when used constantly.
One of the most powerful and spectacular Muay Thai attacks is flying knee strike also known as a knee bomb. To do it jump up on one leg and strike with the same leg's knee. Don't jump with one leg and strike with another - that's not how you do it. The desired area of the strike is the head. If the opponent is too far from you then you might want to spring forward more than an upward and hit him in the chest. It is also one of the best ways to close the distance and get into the clinch. This attack possesses an explosive knockout power.
Knees are also great to use in combinations:
with elbows: step in and start with left knee and then follow up with right elbow
Or mixed up with kicks and punches:
Knees are tough by nature so you don't have to condition them as much as shins. Another thing is that you target rather soft areas of your opponent's body, like face, abdominals, and thighs. The most common injury you may experience are bruises on a softer inside part of a knee. You should condition them on a heavy bag until they get noticeably tougher.
Also take a look at:
Muay Thai techniques
Muay Thai knees