The Quadriceps are 4 (obviously) muscles that work together to extend your knee. They are a strong muscle group that often gets overly tight and pulls hard on the lonely tendon trying to hold them together at the bottom. Stretching, foam rolling and barbell smashing them will help loosen them up if they are getting tight. I would suggest that you put the majority of your focus on front squats, back squats and lunges when training your quads. ~ Berceste