Lateral Step Up
This exercise can be accomplished on most any size step from a simple 2X4" board up. Stand so that one leg is on the step and the other foot is hanging free. Slowly lower yourself until the foot that's dangling just brushes to the ground, then come back to the starting position. It's important to keep your leg correctly aligned so it may be good to do this in front of a mirror for awhile to get the feel for proper alignment. Most people have a tendency for the knee to fall a little to the inside, and you'll need to correct this by tightening your buttock muscles and slightly rolling your foot to the outside. Select a step height that feels comfortable to you and doesn't cause knee pain. If you feel discomfort, check your knee alignment and try again. Slow smooth movements are best - you should feel this working in your quadriceps muscle on the front of your thigh