I'm a big Vince Gironda guy. He was a genius but an asshole.
Anyway, the sissy squats is what I've added because at almost 45, I'm keeping as much stress off joints and ligaments but maximizing through isometrics, negatives, superset, giant sets etc...
Anyway, the sissy squat is just a filthy exercise for your quads. You will experience growth if done correctly and consistently. This is my favorite quad exercise.