A good part of it might be that my upper-arms are essentially straight down, so it's just biceps. Holding your arms out / forward will tire you out, and it's pretty easy to do that in a lot of desks / setups, especially if they're too high. That, and I tend to move around and/or stop and think a lot. In part because I should, in part because otherwise I get stiff. That tendency to not sit still for long has helped in a lot of other ways too, though it makes me hate long drives :|