Just taking a guess at this based on my motocross days but this sounds like a classic case of "arm pump" which some people go as far as having surgery to fix (fasciotomy). Typical when you have a death grip on the bar.
Check out some of the remedies some of those guys are doing. When I was starting to get it the only solution is to relax my grip.
Thanks for the replies. I agree with Dylan that technique is important, but it only gets me when conditions are really gusty and messy. I'm going to try out some windsurfing gloves from a mate (current gloves are 2mm so pretty thin). I've spent a few months on and off working out with the hand grips, so think its time to go more hardcore in this area and introduce a greater range of activites (something like this: https://blog.paleohacks.com/forearm-exercises/). If anyone has tried other exercises successfully let me know. Cheers.
If you are serious about working your grip there is literally ONE exercise I recommend, not those dumb scrunchie things, not that paleo stuff, you want to do the "rope and bar"
Get a rope, a bar, put a rope on the bar and do this:
Back when I used to do forearm exercises I would put on a 25 pound weight, start with whatever weight you want and exercise this twisting exercise until you can't do it anymore. Forearms are like calves you have to work them incredibly hard to get any results, it's not like a bicep or quad where you can hit it with some weights and see results, you need to blast them until you can't move them at all to get them stronger. kiting is still the best exercise for your forearms and 2mm gloves is pretty thin, so maybe you just need to kite more? Anyway if you want the single best exercise for the forearms do the rope, bar and weight twisty thing.
Just taking a guess at this based on my motocross days but this sounds like a classic case of "arm pump" which some people go as far as having surgery to fix (fasciotomy). Typical when you have a death grip on the bar.
Check out some of the remedies some of those guys are doing. When I was starting to get it the only solution is to relax my grip.
I come from a closed circuit motorcycle road racing background and yes I agree, the only solution is to relax your grip. Making arm muscles stronger only means you can hang on tighter when in fact you should be doing the opposite.