Originally Posted by
dscheidt
pain or numbness in the palm and thumb is usually the median nerve, which runs through the depression in the center of the palm at the wrist. (The "carpal tunnel", of carpal tunnel syndrome fame.) There's a lot going through a pretty small space (there are, as I recall, nine tendons and the median nerve.), and if anything is inflamed, the others can be adversely effected. The median nerve provides innervation to the palm side of the thumb, index, middle, and part of the ring finger, as well as the fingernails on these fingers.
Stress on the carpal tunnel can be caused by hand/wrist angle, putting weight on the wrong part of the hand, and a bunch of other things (like too soft grips that deform to put pressure on it). They're mostly fit related.
MTBerJim says it happens in mountain biking too. I have the same problem both in road biking and mountain biking. On a mountain bike, I can lose my whole hand on a fast bumpy downhill. I can mitigate the problem on both bikes by using padded gloves and better padding on the bars. I'm not talking about squishy grips but I don't use hard rubber or the more common grips found on mountain bike either. My worst instance of the problem occurred on a week long mountain bike tour on a Miyata Ridge Runner equipped with the original hard rubber, no vibration damping grips. It took 6 weeks for me to get the feeling back in my hands.
If it happens on multiple bikes, I doubt that the fit issues would be the same on each bike, especially with the longer cockpit of the mountain bike.