@timely
Along with what Steve just posted, there is another exercise that involves laying on your back and having your ankles up on a 5" diameter foam roller tube, relaxing your legs, letting them go completely limp and letting their own weight flatten and pull down on your knees. You can hold it for as long as you want or can stand. You can use a rolled up towel or blanket as the roller, as long as it lifts your ankles up high enough. Not sure how this would work for a severe bend, but if you are working on those last few degrees to 0 it really helps.