If you have sore knees and need to replace lunges:

First, see a doctor and have them diagnose the problem. Second, see a therapist and have them treat the issue. Third, see a trainer and have them assess where you need to work on flexibility and how you can include some “knee-friendly” exercises in your workout program.

These exercises include:

Lying Hip Extensions (also known as Lying Hip Bridges)
1-leg Hip Extensions
Lying Hip Extensions with your feet on the ball
Stability Ball Leg Curls