There are many ways to strengthen the forearms with forearm exercises. Using a sledgehammer is a great way to increase forearm strength. Another piece of equipment similar to a sledgehammer is a lever bar. They both place a weight at the end of a rod that you grab.
A sledgehammer has a fixed weight at the end unless you modify it. To make the exercise more difficult, you work your way down the handle as you get stronger. If you get to the end of the handle you need to get a heavier hammer. As you get farther down the handle, the weight develops more and more momentum as you get it started moving. This places more stress on the joints and ligaments so it is a good way to strengthen them. But there is less work done by the muscles when momentum increases, resulting in relative rest during a part of the repetition and less stimulation to the muscles.
The lever bar is generally shorter with a handle at the end away from the weight. You add weight to the end of the bar instead of working your way down the handle. It doesn’t develop the increased momentum that a sledgehammer develops as you get toward the end of the handle. Alternating workouts with the sledgehammer with lever bar workouts can help the muscles, ligaments and joints to strengthen in a balanced manner.
There are some nice commercial lever bars available, but you also can easily make your own. The easiest to make is to load a weight onto one end of a dumbbell bar and leave the other end empty without a weight so you can grab the bar.
This home-made bar can be modified to have a thick handle by placing a piece of PVC slightly wider than your hand on the handle end of the dumbbell bar. The thicker handle will stimulate the grip muscles more than the plain dumbbell bar. One of the videos using it can be found here.
The exercises to do with the lever bar are the same as you would do with a sledgehammer for the forearm muscles. Radial deviations, ulnar deviations, pronations and supinations are the four main forearm exercises, and would take an entire article to explain. But videos of the exercises can be found at the links above.