The quadriceps are the muscles on the anterior side of the thigh. A quadriceps stretch can be done after exercise while your body is still warm. It can also be done in a dynamic warm-up to losen up. The quad muscles have a huge responsibility in knee movement. Often times when the knee is irritated and hurting it is because the quadricep muscles are tight and need to be stretched.
A quad stretch is done by taking one hand and grabbing the foot on the same side of the body. Pull up on the toe as you bring your heel towards your butt. You can hold the stretch for 60 to 90 seconds as a static stretch, post workout. Before a workout you want to stretch it dynamically by walking and pulling back each leg quickly in between strides.