A small, flat butt might not seem like an issue, but for many people it is a concern. A flat butt not only does little to fill out a pair of jeans, but is often accompanied by small, weak muscles. To bulk up your butt, begin a structured strength training program with butt-specific exercises. The American College of Sports Medicine recommends using 70 to 85 percent of your one-repetition max for four to six sets of eight to 12 repetitions to bulk up your butt.
Perform an elevated stationary lunge. Hold a dumbbell in each hand, with your palms facing in. Place your right foot on the top of a step or other elevated surface. Position your right knee so it is aligned directly above your right ankle. Scoot your left leg back until both your knees are at 90-degree angles. Pull your belly button into your spine, stabilize your abdominal muscles, and straighten your torso. Keeping your upper body upright, slowly lower your hips straight toward the floor. Do not allow your right knee to move forward as you lower down. Once your back knee nears the floor, pause, and then push up to return to the starting position. Complete the desired number of repetitions, then switch leg positions and repeat.
Perform an elevated bridge. Lie face-up on the floor, with your knees bent and your feet resting flat on a step or elevated surface. Hold a plate weight across your hips, securing it in place with both hands. Pull your belly button into your spine and contract your abdominal muscles. Maintaining this position, push through your heels to lift your hips off the floor until they become aligned with your shoulders and knees. Pause at the top of the movement, squeezing your butt muscles, then slowly lower back to the floor.
Perform a weighted squat. Place a barbell on a squat rack and load it with an appropriate weight. Position yourself underneath the bar so it is resting across your upper back and shoulders. Grasp it with an overhand grip to secure it in place. Pull your belly button into your spine, stabilize your core, and stiffen your torso. Maintain this position and straighten your legs to remove the bar from the rack. Walk a few steps away from the rack so the bar can clear it when you squat. Adjust your stance so your legs are shoulder-width apart. Bend forward slightly from your hips while bending your knees to lower into a squat. Continue down until your thighs are parallel with the floor. Push through your heels to straighten your legs and lift your torso to return to the starting position.
Perform a BOSU lateral squat. Position a BOSU ball on the floor with the rounded side up, so you have plenty of free space around it. Stand with your right side toward the ball, and hold a dumbbell in each hand at shoulder level, with your elbows bent. Place your right foot in the center of the ball and your left foot firmly on the floor. Straighten your back and stabilize your abdominal muscles. Squat down as if you were sitting in an imaginary chair. From the bottom of the squat, jump laterally so your right foot lands on the floor on the opposite side of the ball and your left foot lands in the middle of the ball. Repeat the squat and jump back to the other side.
Perform a weighted lunge. Stand with your feet hip-width apart and hold a barbell across your upper back and shoulders, securing it with an overhand grip. Pull your belly button into your spine and contract your core muscles. Take a big step forward, keeping your torso as straight as possible. Lunge forward until your front thigh is parallel to the floor. Push off the heel of your front foot to return to the starting position. Repeat the lunge with your other leg.