Legs are the core of main activities, you neglect them and you will injury yourself. This exercise combines dynamic stabilization with a simple yet effect leg workout. The lunge is an easy exercise that beginners and advanced athletes use to gain strength, power, and anaerobic speed.

By adding a sledge hammer, or equivalent tool, to the lunge you effectively have to activate more muscles and coordination within your body to complete the movement. This equals better muscle control, proactive injury prevention, and greater strength gains.

While there is an injury that this exercises targets, that does not mean it should not be used to help regain strength. When we state an injury we are specifically talking about an exercise that can speed up recovery, which almost always means a exercise/movement that targets the stability of a joint to perform a workout. Regarding the this exercise you would be more likely to perform:

Once you can complete these workouts with ease, no pain, and proper form, then you would start to perform this exercise.  

Also when it comes to "pains" felt when performing this exercise, asses your form and range of motion. If you have some one that can watch you, ask them to, or you can use a camera to film yourself.  You want to see:

All of these and more can cause injury and decrease the total amount of weight you can truly lift. There is nothing wrong with stepping away from heavy weight, to fix an underling problem. Do not let your ego define your success, all the greats have lowered weight or incorporated different exercises to fix problems. Which then allowed them to continue their journey to lift heavier weight. 

Remember, the "life goal" is to be able to lift just as heavy now as when you are 85, 95, or 110 years old