This was written by Charles Poliquin, taken from:
http://www.t-mag.com/html/body_88cp.html
Q: In your "Poliquin Principles" book, you briefly mention one-armed chin-ups. Are these just showoff exercises, or do they have some value to a bodybuilder? How about one-armed push-ups, same deal?
A: The one-armed chin-up isn't exactly a showoff exercise, as very few individuals can even dream of doing them (well, for those individuals, I suspect that it's a showoff exercise at times).
It's been estimated that only one out every 100,000 trainees has the genetic potential to achieve a single one-armed chin-up. The athletes most likely to be able to do one or more are mountain climbers or gymnasts.
Once, one of my client's bodyguards who, aside from being a sharpshooter, was also a very accomplished mountain climber. He told me that he really liked my concept of changing tempos and that he was getting much stronger. So I asked him what exactly it had done for him, and he explained that it had really improved his chin-ups. To illustrate his great strength gains, he handed his Glock to his partner and then proceeded to do a full-range, one-armed chin-up, taking 20 seconds for the concentric phase and 20 seconds for the eccentric phase. What was even more impressive was that he was doing it with only his middle finger wrapped around the bar.
There was another mountain climber who worked for our National Ski Team that performed 23 of them in front of me while using a pronated grip. He did them while holding onto the diving board of a drained swimming pool.
Both of these examples were quite slender and didn't sport excessively muscular arms. But obviously, they have superior motor-unit recruitment abilities. So, the direct applications of one-armed chins are rather limited because of genetic factors. Furthermore, this movement would be considerably harder for the average bodybuilder, as the rest of his body is generally a lot more massive than that of the average mountain climber or gymnast.
One-armed push-ups are more readily accessible to the average person as they require much lower levels of maximal strength. After all, if Sylvester Stallone can do them...
In my opinion, a more impressive form of the one-armed push-up is to have only the contra-lateral foot on the ground when doing them. If you're doing one-armed push-ups using the right hand, your left arm is extended in front of you, and your right foot is kept a few inches off the ground.
I first saw these being done by the late Kay Baxter at the Pro World Bodybuilding Championships in Toronto 14 years ago. What I like about this advanced form of the one-armed push-up is that it requires a much greater range of motion than the classic Rocky ones, and you also need to fire a much greater amount of motor units to stabilize yourself.