You haven't felt the burn in until you've mastered cossack squats. Here's exactly how to do perform the move correctly, according to a trainer. Squats never fail to provide the best and worst burn.

Bend one knee and push your butt back to squat down to one side. Keep your other leg straight, and as your lower down ...

You don't need lunges to build stronger legs with knee pain; just try this personal trainer's alternative, called the suitcase deadlift.