Ive been practicing a couple of months now but ive realised my picking hand still needs a lot of work.

Especially when palm muting, my hand tires out very quickly. I can only play about 30 seconds with a piece that involves a lot of palm muting without going out of sync/rhythm.

Are there any exercises to build up my picking strength and endurance?
play a lot of palm muting?
Doing things that are hard and tough will always make you better and/or stronger.
Learn any Iced Earth/Demons and Wizards song and just palm mute the galloping sections
I do it all the time till it aches and i cant keep in rhythm, however I havent noticed or seen any improvements. I mean 30 seconds is shite. I should be able to play at least 3 minutes.
^ don't do it till it aches.... you should be playing until it gets tired, not painful just tired. play till it hurts and you might damage something
I get the same thing when i play MOP, however, for me it wears off as soon as I get to a not-so-palmuted part. After that I can play without problems.

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make sure you are not tense when playing as well. If you are making your arm, wrist, picking hand, fingers, or anything that is part of your picking mechanics tense, it will cause you to tire out faster than if you were loose in your playing. Those are things that you will develope with practice, but usually if you are getting tired as hell, it's due to stressing the muscles.

Just practice putting a lighter touch on the muting(it doesnt require a lot of pressure to get that good palm muted chunkage) and make sure to keep your picking hand loose and fluid, you should noticed it becomes almost effortless to chuck out those riffs all day. 9 times out of 10 when you get really worn out from only a short time playing, it's due to too much stress in your mechanics.
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Cheers Sinister that was really helpful a Dime-point to you!

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