is it normal to develop a callus on the side of the thumb at the knuckle? I wonder if I'm flexing my thumb back and applying pressure at the inside of the knuckle and I should be using the pad or tip instead. I can take a picture of how I grip the racket. probably time to check "the bible" again!
I think the bible says that the thumb shouldn't wrap around the racket but only touch with the pad. but you could also tape your thumb to avoid calluses.
I tried hooking my thumb as you do for making a fist, much better. The sore bit on the inside of the knuckle was from leaving my thumb pointed up like a backhand, not ideal I don't think.
Thanks It was more about whether if was a sign of poor technique. I feel more power and comfort since I adjusted.
Pointing your thumb upwards for non backhand shots means you are not using the correct grip - it is bad technique. It sounds like you have now changed and improved your grip!