Possible Clues:

• Bodybuilder's muscle
• Muscle targeted by squats, informally
• Cheek muscle, for short
• Posterior muscle, for short
• Muscle strengthened by a StairMaster, informally
• Thigh-rotating muscle, informally
• Any of the buttocks muscles, for short
• Butt muscle
• Muscle strengthened by squats
• Rear muscle