Stepping forward to lunge is a move everyone who plays squash knows. However, recent research indicates that stepping backward is actually better if you want to build lower body strength.
When you step forward and land your foot on the floor, the knee joint has to provide a hand brake to the movement, causing extra pressure. Your quad muscles have to contract strongly to slow you down which adds to this pressure because they are attached to your knee cap.
Just like a forward lunge, a backward lunge still targets the front leg. The muscles do the same amount of work but without the joint pressure because the impact (and hand brake action) is removed. The back foot taps down lightly and returns to the front keeping the load on the front leg.
Backward stepping lunges are safer for the lower back because you’re able to maintain an upright posture. With the first action of the backward lung being a backward step it’s much easier to keep your trunk vertical. Keeping your muscles well-conditioned with backward lunges can also decrease your risk of injury. It also helps with recovery of movement out of the front corners.
Before taking on lunge varieties (such as lateral and reverse lunges) learn to do the basic front lunge with proper form.