This is more of an isolation exercise for detail than a bulk builder, but it should always be a part of your leg workout for its effectiveness. Extensions will help you get that teardrop look that every bodybuilder longs for and you can beat your legs up good by locking out your knees and pausing at the top of the rep for a second or two.
Use either the 45-degree or old school 90-degree variety to get a real deep stretch and this is an exercise where you can put up some heavy weight for overall strength and power. Take a shoulder-width position with your feet on the platform and lower it until you are making a 90-degree angle at your knees with your upper and lower legs. Push it back up and stop right before locking your knees out before beginning the next rep.
While many people choose to use dumbbells for this movement, a barbell is actually the better way to go. It brings your stabilizer muscles and balancing into the equation and will make you work harder to complete each rep. But walking lunges may in fact aggravate your existing injury or out you in risk of one just as squatting will. So in that case, grab a set of dumbbells and play it safe. You still will get the benefit from lunging, which will give your quads a great two-way workout – stretching on the down leg and flexing on the up leg.
You can really up the reps on each set of dumbbell step-ups by alternating legs in two ways, meaning that you step up with the right and step down with the left and then do the reverse. So you will be working both legs, but with a higher impact/lower impact formula. Start off fairly close to the bench, as this will work your quads better. If you have to take a longer step to get there, then your glutes are engaged and begin to take over.
Besides your quads, you also have to work on the other lower body muscles – hamstrings and calves. Here are two exercises for them: