Hey .
I've played guitar for a few years now.. Mostly blues/rock and roll.
I've been trying to play a lot of metal using fast down strokes (i.e. thrash or even stuff like lamb of god, a lot of metal) and am finding it impossible to keep up .. my whole arm tenses up to the point where i cant play, no matter how much I try or persist (this isn’t a new thing) same thing happens with galloping (i.e. umm a lot of Iron Maiden lol) from my shoulder to my wrist goes tight as f*ck and I cant play for more than 1 minute ....
...so what i'm asking is .. am I doing anything wrong? like a technique?

I can tremolo pick like anyone else and keep going for a 7 minute long song. I have become very good at alternate picking through scales fast. Ive had to turn to alternate picking alot of the riffs im talking about (eg. MoP by Metallica), but it doesn’t sound right. but not fast down strokes or gallops its really bumming me out.
Cheers for any help.
how did you get good at tremolo? practice
how do you get good at downstrokes?
I think you get the point
if you can't do downstrokes well, you just need to practice more
I know I did (and still am)
Practice, and make sure you relax your arm. No other way to do it.
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wankings good practice... but na just start real slow getting the movement and technique and just build up in speed
i had a little bit of that. my solution was just keep slowy increasing the speed till i got it down and now i can downstroke just about anything.
Don't feel alone bro, I can't do it either. Like Raining Blood the three notes close together, it's pretty damn hard.