How do I strengthen my obliques?

Holding weights, simultaneously lift the right weight as close as you can to your right armpit (bending elbow toward ceiling) and lower the left weight toward your left ankle. Then, lower the right weight and lift the left one. Throughout, keep your chest lifted and eyes straight ahead. Curve your spine from side to side to increase movement. Work up to 20 reps each side.

I would recommend Hip Ups and Side Planks.  Do not do "side bends" with weight.  I often see people holding a dumbell and bending to each side.  This is a good way to injure your spine.  A better exercise is to get into a Side Plank:

In the Side Plank Position, let your hips on the floor side come down a few inches then pop back up an inch or two above the starting point of the plank.  Do this 10-15 times on each side and you will understand why this is such a great exercise!

Cable rotation is a great exercise to strengthen the oblique muscles.  Set the cable machine at chest height.  Stand next to it holding the handle with your arms straight in front of you.  Keeping your core engaged, rotate your body away from the machine allowing your back foot to pivot.  Return to the starting position. 
A great way to strengthen your obliques is to perform the side iso-ab.  Perform this exercise by following the technique described below. Lie on your side with your feet and legs placed on top of each other. Place your forearm on the ground with your elbow located directly under your shoulder.  Draw-in your abs and contract your glutes.  Next, lift your hips and legs off of the ground until your body forms a straight line from head to toe. Make sure to keep your weight placed on your forearm and feet the whole time.  Hold the top position for a few seconds then slowly lower your body to the ground before repeating for the desired number of repetitions.  If this exercise is too challenging, consider trying to perform it from the knees instead of the feet, or perform the standing cable torso iso-rotation as an alternative.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.