Your "side muscles" are called your obliques, and they are responsible for twisting movements and stabilizing your abdominals. Toning the obliques will give you a leaner, more upright look and will narrow your waist. If you are only looking to tighten and tone, work your obliques three days per week. If you are looking to build muscle, work them twice per week, but use variations like weights and inclines to make each move more difficult.
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Do the bicycle maneuver. Lay on your back with your knees bent and your hands behind your head. Contract your abs to lift your head and shoulders from the floor, and touch your elbow to the opposite knee. Repeat on the other side, keeping your head and shoulders off the floor. Repeat until your form fails, being sure to keep your elbows wide.
Do a set of side planks. Lay on your side, supported on your elbow with your hips square and your legs stacked. Contract your abdominal muscles to raise your hips off the ground until your body forms a straight line and only your forearm and bottom foot are touching the ground. Repeat until your form fails, and then switch sides.
Grab an exercise ball to perform stability ball knee tucks. Lay face down on top of the ball, planting your hands on the floor to brace yourself. Walk forward with your hands until your knees are on top of the ball. Contract your abs and bring your knees toward your chest to roll the ball toward you until you are in a tucked position on top of the ball. Slowly roll back out to the starting position. Repeat until your form fails.
Exchange the exercise ball for a medicine ball to perform medicine ball trunk rotations. Sit on the floor with your legs in front of you. Bend your knees to a 45-degree angle, and keep your heels on the floor. Keeping your back straight, hold the medicine ball in front of you and rotate your body from the waist, right and left. To make it harder, touch the medicine ball to the floor on each side. To make it even harder, try balancing on your tailbone and using your abs to keep your legs in the air. Keep the medicine ball directly in front of your chest -- do not reach with your arms. Repeat, alternating sides, until your form fails.
Grab a broomstick to perform the broomstick twist. Sit on a sturdy chair or bench and lay a broomstick across your shoulders and behind your head. Reach your arms up behind the stick, extend your arms, and grasp the stick. Rotate your torso back and forth from the waist, keeping your abs tight. Start slowly, and gradually build up speed until your form fails. Be sure to keep your spine straight and your abs tight.