Along with being aesthetically appealing, a well-defined, strong butt helps you to move with greater speed, agility and power. Building up your butt requires training all three muscle heads of the glutes: the gluteus minimus, gluteus medius and gluteus maximus, the largest muscle in your body. Maximizing muscle growth in the glutes can be done using strength-training and cardio machines.
Many of the strength-training machines targeted for your lower body will work the glutes, though certain exercises can be more productive than others. The Smith machine allows you to perform squats and lunges -- two glute-activating exercises -- with a balanced bar that makes it easier to remain stabilized during movement. The leg press targets your butt while also strengthening your quadriceps and hamstrings, and the lying leg-curl machine can help you to isolate your glutes and hamstrings for a solid improvement on your rear view.
The other half of the butt-building equation, cardiovascular exercise, helps you to burn fat off your rear while also contributing to the overall strength of your glutes. Both the elliptical trainer and the treadmill rely on your glutes to activate the forward hip motion. Increase the incline of either machine and the glutes become even more challenged. The stepmill fires up your glutes every time you push through your heel to climb to the next step on the never-ending staircase.