Your butt muscles, collectively known as your glutes, are one of the largest muscle groups in your body. Strong glutes help with all types of movement, including walking, lifting, and of course running. Squats, lunges, and deadlifts are great moves to train your backside, but they're not the only butt exercises out there. Mixing up your routine will help you avoid boredom and work this important muscle group in different ways. We chatted with four top trainers about their go-to butt…