Don't know if this helps, but.... I rock climb, and forearm pump is something you get every time. As soon as you get it you lose all the strength in your fingers. I've wouldn't describe it as a numbness though. In 7 years of climbing I've never not had it. It takes a lot longer to get now! Just keep excercising it and it'll take longer to get. Have a look at some climbing sites for ideas on excercises, or better still - climb!