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Tricep presses.. Like a pressup, except you place your hands down near your waist, and keep your elbows pressed into your sides. That last bit being very important, its far to easy to let them stick out. One method is to get a training partner to stand above you and physically press your elbows into your sides

You shouldn't be punching with your arms alone

Work your shoulders for endurance

You should be punching with your legs, hips, waist, and shoulders

Wanna build punching muscles? how about Go outside and put on some techno music the really long songs and punch on the beat until the whole playlist is over different songs with differenet tempos and beats to build the ability to match and change your fighting rythem like lee always went on and on about.

Another great one to work on punches is punch at a tree as hard as you can and not hit it. Really builds gauging of distance and power.

Last good Punch muscle building excersize is climb in a pool with some kind of pad you can submerge and punch the bag as hard and often as you can for 10 mins. Make sure that you are about chin deep. If your in karate or some forms of gung-fu its good to submerge your mouth too so you can practice propper breathing techniques.

+1 for the endurance. Important.
-1 for the Lee. SHits got nothing to do with fighting rhythm.
-1 for the Tree drill - what do you learn from not hitting anything? It's not good to punch air full power.
+1 for underwater training. Not too sure about hitting a bag, but shadowboxing underwater will be enough.

In one of Buce Lee's many TV interviews he talked about finding a rythm in the fight and to match it or change it up and when you can match any rythm you can anticipate whats about to happen.

And the Not punching anything is that you build muscles that also stop the strikes wich are nice because the straight punch muscles dont help strengthen the muscles for other strikes such as backhand knife edge ridge hand or hammerstrike.

Hitting the bag underwater takes alot of effort to impart force its really frustrating, but when you do it often you will see your striking speed go alot faster.

Striking power comes from strike speed. F=MA Force=Mass times Acceleration hard to increase the mass of the hand by working out but its easy to increase the acceleration.

Try it out see if you like it after a month if you dont see an improvement then dont do it ^^

uhm ontop of agreeing with soul, punches and or attacks that have little to no distance such as famed actor/fighter Bruce Lee's one inch punch leaves little room for speed. it works in two ways like a Katana to a claymore. A katana uses the speed of a slice to slash through a target where as a Claymore depends on raw power to creat a driving force that cleaves a target. in terms of effectiveness it depends on what you'd go for. speed plus accuracy can do more damage but raw power and a lucky hit can mess somone up. Best example, the movie "Protector" with Tony Jaa, Bleach's Urahara Kiske's explanation, " A revolver can shoot six times, a Missile only once."

Here's an excercise more for building stronger punches. The only downside I see is if you do it wrong, you may actually lose punching speed.

Anyways, heres what you'll need: A makiwara/heavybag against wall, hell even a focus mit duct taped to the wall, basically you need a striking target that will not budge.

What you do:
Take your target (for simplicity's sake, i'll say, a makiwara) and position it to shoulder level. It is very important that it doesn't budge when struck full force with a punch. Now, what you do is punch it full force. When your hand impacts the target, maintain the intensity of the force in your blow for no more then 5 seconds. (Any more, & you risk slower punching speed.) when the 5 seconds is up, imediately follow up with the second punch, and repeat the drill as necessary.

Basically, you get used to punching the immovable target. So when you switch to a human target, the human body is nothing in comparison.

Just besure to maintain proper form, & start with a low rep to get your body accostomed to the exercise.

Not sure this info will help, since you mentioned you needed tricep work for better jabs, but hey, maybe someone will find this useful. ninja