Recently I've decided to concentrate on my picking technique and I have noticed that when I play fast downpicking parts my arm tenses up and I don't pick from the wrist I seem to turn my whole forearm but when I alternate pick I pick from the wrist, has anybody got any advice on how to downpick from my wrist without tension?
There is really no secret to it, you just have to analyze your own playing and take action to make it as relaxed as possible.

Looking at stuff such as:

How hard do you hold your pick? Do you have to hold is so hard in order not to drop it and play properly?

Are you anchoring? Are your fingers floating around or tucked in?

Are you making small motions when you down pick? Is your hand relaxed or are you tensing up?

Look at this stuff, and then try doing everything you can to get the most relaxed and efficient technique you can. Even if you have to slow down to 30 bpm to do it.

If you have a teacher make him look at your technique and give you advice aswell. If you don't i suggest you get into learning to analyze your own playing and finding solutions. That's one of the things you have to do if you are going to be self taught in my opinion, being able to critique yourself.
Fusion and jazz musician, a fan of most music.

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Thanks for the response. I don't hold the pick very hard, I don't know if it is classed as anchoring but I rest my hand on the bridge, my fingers are tucked in but I need to work on smaller motions, my arm is only tense when I play fast parts. Do you have any advice for making smaller motions?
Then slow down. Practice slow and once you feel comfortable and your arm is not tense any more, you can increase the speed a little. Little by little, it takes some time, but once you reach a speed level with no tensed arm, you'll just fly.