Look at your brake lever angle, when your in your "attack" position decending with your finger on the lever your forearm, hand and braking finger should all be inline.
Also are you single finger braking? If not you should be, using 2 fingers means your pulling lots of tendons simultaniously whiles also trying to grip the bar with your remaining fingers and move your arms about, just too much going on, also are yo a bit keen on the brakes? (be honest with yourself here) if your grabbing late fistfuls of brake and tippy toing through corners you'll be doing alot more hand/forearm work than you really need to and make sure your levers are a sensible distance from the bar…
Poorly set up brake levers and the wrong technique are the number one cause for "Arm pump" it has very little to do with having super muscly arms, technique comes with practise so find a nice set of corners in your local woods and practise, practise, practise….
Good luck…