I got into a terrible motorcycle accident—you can read about it on my website [leroycolbert.com]— and that put an end to any ideas I had of ever stepping on a bodybuilding stage again, because I could no longer train my legs. But I could still work my upper body, and I loved training. So it wasn't until after the accident that I got my arms to 21".
Basically, there are no 'secrets' to building big arms, but there are specific ways you can train them that will make a difference. The first and foremost is to prioritize the muscle group you want to grow. The body only has so much energy for each workout, so you want to work any lagging muscle groups at the beginning. It makes such a difference when you train a bodypart first in a workout as opposed to second or third.
Beyond that, I would say that you've got to train with a rapid cadence. You're not throwing the weight around, but you want to move it quickly and under control. Look at it like this: If you have two guys doing curls and one does 10 reps in 30 seconds while the other guy does 15 reps in that same time, the second guy just did more work and his biceps are going to grow bigger.
For biceps all I did was alternating dumbbell curls and concentration curls. Sometimes Marvin [Eder] and I would do burnouts at the end of the workout. We'd stand facing each other and I'd pick up a 110-pound barbell and do 10 reps, then hand it to him. Then he'd do 10 and when he was done he'd hand it back to me and I'd do maybe eight and hand it back to him. We'd do this until we couldn't even lift our own arms anymore, much less the barbell.
For triceps there were bent-arm pullovers—which no one thinks of as a triceps movement—lying French curls, pushdowns, and sometimes we'd do bench dips. Those would give you a really good pump.