Or riding a KX250 in the tight south eastern woods
I get it within the first 10 miles, regardless of training, or warm up routine. Once I stop and stretch my forearm muscles and tendons by holding my arm out straight, palm out, fingers up, and pulling back on my fingers it goes away with in a minute and I don't get it again.
It is the action of blood filling the muscles to capacity resulting in pain and the temporary ability to properly control said muscles. Also known in medical terms as Hyper permeation,