Flexibility is the range of movement that you have in your joints. Some say that this definition should only apply if there is no exterior help to move the joint, but this is not correct. If your joints have a greater degree of movement when aided by an external force, then that degree of movement defines the flexibility.
Hence, if you strengthen you muscles, and these act as the external force, then the flexibility of your joints will allow a greater range of movement than were your muscles unable to exert that degree of force.
To put it simply, if you have a great degree of flexibility in your joints, then the stronger you are, the more you will be able to put that flexibility to use in the range of movement you have at your disposal. It goes without saying that the more flexibility you have in a martial art the better.
However, people do not have the same range of flexibility throughout their bodies. Even more specifically, flexibility in one range or direction of motion at a specific joint does not imply flexibility in another. Hence that fact that you have complete ability to complete perfect front splits does not infer that you have the same control over side splits, even though it is the same hip joint that is involved. The two are not connected. A suitable flexibility exercise program, therefore, should be designed to provide good flexibility in all joints in all directions of movement, relevant to the martial art studied.
Flexibility training should be carried out in conjunction with strength training in order that the range of flexibility achieved can be used to its full potential by the muscles that move the bones in the joints. There is no truth in the belief that you must trade flexibility and strength. Supreme strength simply means supreme use of the flexibility gained through training.
However there is more than this in a martial art. Tension and relaxation are of supreme importance. Tension is equated with power and strength while relaxation is equated with speed and flexibility. Russian Cossacks used to train with their sabres by standing waist deep in water and slicing into the water with their blades. As they became weary they would learn to relax on the downstroke and tense and use their power on at the moment of strike.
The results were incredible and awesome. A Cossack could slice a man from shoulder down to saddle with one strike with only a light sabre. All due to using relaxation and power at the correct times. This can be simulated by using a large rubber eraser. About three inches by an inch square is about right. Hold it in the fist and explosively compress it. Do this using your core power. Compress hard with maximum explosion, just as if you were punching. Then relax just as quickly. Repeat this during the day explosive compression and quick relaxation. Eventually you be able to carry out this rapid-fire tight-loose-tight sequence without thought.
This is a relaxation technique, but cannot replace the experience of the fight since fear of being hit cannot help tighten you up. However, if you do this for a few hours every day (easier than you think since the eraser can be carried in your pocket) you will find that you can relax your fist until the moment of the strike when you require maximum power.
Whether in boxing or karate, this will improve your performance and strength. Combine this with your flexibility training and you will be on the first rung of achieving greatness in karate or any other explosive martial art. Russian training methods help you to develop this total control over relaxation and explosive power in the use of your strength and flexibility.