Strengthening arm muscles involves just a few basic exercises. And even though they're relatively simple, if you have not exercised in a while you should take it slowly. The muscle strength will build over time, giving you the extra power you are looking for in your golf swing, but you must be patient and not force the training further than your body can go.
Difficulty: Moderately Easy
Place your arm on a flat, comfortable surface with your palm up. Place the dumbbell in your hand and grip it. Keeping your elbow on the table, lift your forearm up off the surface and bring the weight toward you. Once you have pulled your arm in, slowly lower it back down. This works the muscles of the inside of the forearm as well as the upper arm. Repeat 10 times; then switch to the other arm.
Grip the dumbbell and place your arm on the table with the palm down. Slowly lift the arm up toward you without moving the elbow. Once up slowly lower the dumbbell back down. This works the muscles on the outside of the arm. Repeat 10 times; then switch to the other arm.
Hold the bar of the dumbbell so it is vertical, and lift it off the table keeping the elbow down. Pull the weight toward you and then slowly lower it. This works the sides of the forearm. You can also do this standing up with the arms straight. Slowly lift the weights up until the arms are horizontal to the floor and then slowly lower them. This will work more of the muscles in the upper arm and shoulders. Do 10 repetitions on each arm.
Tips & Warnings
It is best to start out with a light-weight dumbbell and slowly work your way up to heavier weights. You can test your strength with a 5-pound weight. If you have never used weights before, this will probably be a good starting point. If you don't show any signs of soreness a day after use, and the weights are easy for you to lift, then you might want to consider moving up to a heavier weight such as 10 pounds. If you feel pain when you lift the dumbbell, move to a lower weight.