Pullups are great for developing back muscles and that is the main reason for doing them. Sometimes however we are not able to do series of pullups needed to develop back muscles because our forearms get trired too quickly and fingers cannot hold onto the pullup bar fo long enough.
Hanging on a pull-up bar
The best exercise to develop your forearms is to hang on the bar with your arms straight. Try to do this exercise at least a few times a day for 20-30 seconds and your forearms will get stronger in no time.
If your forearms are strong but you want to make them even stronger, we suggest training with a towel. It's the same exercise as the one above. The only difference is that you wrap a towel around the bar. The bar gets thicker and the towel spins on it. To hold on to it, your forearms have to work much harder.