Cannonball squats are an advanced leg exercise that challenges the quads in a unique way. The exercise can be done with bodyweight alone or by adding weight or resistance. At the right point in a training program, adding weight can accelerate muscle growth for those who have already been doing bodyweight cannonball squats for some time.
The cannonball squat is similar to prisoner squats in that it involves a narrow stance and is an intermediate step for those who have mastered the basic prisoner squat movement. It puts more emphasis on the quadriceps in your front thighs, but it also targets the hips and glutes. Some studies suggest that cannonball squats are slightly more challenging than regular squats, but others disagree (1, 2, 3).
A key to this exercise is keeping your back straight. You do not want to squat too deep, as this could put pressure on the knees and other joints in your body. If you are not strong enough to do a full cannonball squat, try starting with a figure-four squat. Stand with your back against a wall, then lift your right leg up and cross it over your left leg so that the foot of your support leg rests on the back of the left calf. Lower until you are almost touching your heels, then push back up to standing.
You can also add weight to a cannonball squat by performing them with a barbell on your shoulders or doing them as a goblet squat with a dumbbell in front of you, which makes the exercise more quad-dominant. A hack squat machine also allows you to perform these in a feet together stance, which emphasizes the quads even more.