Quads

Barbell Squat

Difficulty: Advanced

Stand with your feet shoulder-width apart. Maintain the natural arch in your back, squeezing your shoulder blades and raising your chest.
Grip the bar across your shoulders and support it on your upper back. Unwrack the bar by straightening your legs, and take a step back.
Bend your knees as you lower the weight without altering the form of your back until your hips are below your knees.
Raise the bar back to starting position, lift with your legs and exhale at the top.

Difficulty: Advanced

Find a box or bench that is roughly knee height or a few inches lower. (The lower the box is the lower the mobility requirement).
You can either place your toes or the top of your foot on the box. Point your front foot forward or slightly turned outwards and break at the knees and hips simultaneously.
Aim to get your front leg to hit parallel depth (hamstring parallel with the ground) or lower.
The further forward your front foot is, the longer the Range of Motion is at your hips. The further back your front foot is, the longer the ROM at your knees.

Difficulty: Beginner

Step forward with one leg.
Lower your body until your rear knee nearly touches the ground.
Ensure you remain upright, and your front knee stay above the front foot.
Push off the floor with your front foot until you return to the starting position. Switch legs.

Difficulty: Intermediate

Sit on the machine with your back against the cushion and adjust the machine you are using so that your knees are at a 90 degree angle at the starting position.
Raise the weight by extending your knees outward, then lower your leg to the starting position. Both movements should be done in a slow, controlled motion.