Whether it's repetitive motion or heavy lifting, using your hand can cause your forearms to tighten up causing you pain.
Stretching the flexors and extensors in the forearm is an easy way to prevent injuries and pain.
Typing and movements like a tennis backhand stroke can overwork the extensors. Stretch these muscles by making a relaxed fist with your arm extend in front of you. Rotate your fist downward. You can also assist the rotation downward with the other hand.
Holding on to things will tend to tighten the flexors in the forearm. Stretch these muscles by extending your arm in front of you with an open hand and the fingers pointing up. Take your other hand and place the palm against the fingers. Pull the four fingers back towards the elbow stretching the flexors.
You can also do self massage for these muscles by grabbing the forearm and using the pads of the finger tips. Hook the fingers down into the tender points of the muscle and add a back and forth motion.