I can do underhand pullups quite comfortably (working the biceps), but I find the overhand ones more difficult, almost certainly because my back muscles aren't as strong as those at the front.
On Sunday I did a mix of under and overhand pullups (70 & 25 respectively), and tricep dips.
On Monday I did six sets of ten overhand pullups in the course of an hour (I was doing karate practice). I felt very strong and did them better than those the day before. But over the course of the day and even now, my inner forearms hurt when I open my hand fully, or flex my hand in (palm faces the body). The ache is below the wrist, in the middle. Images on Google show this could be the Flexor Carpi muscles, I guess strained.
My question is why would these muscles ache and not my upper back (which I assume is what gets worked in overhand pullups)?
UPDATE: I no longer have this issue. I switched to wide-arm pull-ups as advocated by Arnold Schwarzenegger and Nick Aldis, doing 50 reps over 3-5 sets on a regular basis and it is going well. I believe my forearm muscles were not used to the exercise and I was overtraining. Rest and working on flexibility have both really helped.