If you are in the martial arts community should know that grip and hand strength are a very important aspect of their fighting capabilities. Be it on the offence when you need to use your hands for punching, where your hands need to be tough to prevent injury to you and to inflect more damage to your opponent. Or on the defense when you need to be able break from the grip of your opponent and then apply your own crushing hold.
You should be doing the more common workouts for grip training such as the hand grippers, pinch grip and wrist curls but due to the nature of fighting your wrists will twist at different angles and giving them special attention will really help. For your wrist, hold a weight plate with your fingers on the flat side and your thumb on top. With your palm facing up place your wrist on your knee or better yet on the edge of a table. Then rotate your wrist so you move the plate in a circle, try going both directions. You will have to find a weight that suits you but don't make it easy.
For leverage, try something heavy like a sledge hammer. Hold your arm at your side and just rotate the hammer upwards then back down using only your wrist. You can adjust your grip up and down the handle to make it harder or easier. If you do not have a sledge hammer you can substitute for something else, like a long 2 x 4 or broom. For another variation hold your arm out in front of you and rotate the weight in circles using only your wrist.
For most people the wrist is the weaker link of the arm, if you can strengthen this you will be way ahead. Having a stronger wrist will really present it's benefit when you start doing twisting motions and you are still able to hang on.
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Grip Strength to discover more info on how to improve your grip strength. Tim Archbold's interests are Fitness Training and Health.