If you want a Strong Core:  Skip the Crunches and do these 5 Exercises. 
Bird dog:

Bird dog. (Illustrations by The Washington Post )
On your hands and knees (in table top pose), stay centered with shoulders over hands and hips over the knees. Raise your arm forward until the hand is at the same height as the shoulder. Be careful not to shift hips laterally or extend the back out of a neutral position. “Reach” the opposite leg behind (vs. lifting it). Keep the core engaged and ensure the “reaching” of the leg is coming from the glutes. Hold for a few seconds and then switch sides.
Dead bug:

Dead bug (Illustrations by The Washington Post)
Lie flat on the back with arms along your sides. Lift the right knee toward the chest while lifting the left arm toward the floor behind you. Bring the knee and arm back to starting position. Switch sides. Maintain a neutral posture in the lower back by pulling the navel toward the spine.
 Side-lying plank:

Side-lying plank. (Illustrations by The Washington Post)
Lie on your right side with knee and elbow bent, and then push into the right forearm and right knee to lift the hip off the ground. This is the least taxing variation of side-lying plank. Keep the right hip forward and up, maintaining a straight line from the knee up through the hip and shoulder on the right side. Hold for a few seconds and rest. Repeat 10 times and then switch sides.

Bridge. (Illustrations by The Washington Post)
Lying on the back with the knees bent and the feet about hip-width apart, lift the hips gently off the floor. Engage through the glutes when lifting. This pose has you moving opposite to the flexed position you are in while sitting at a desk.
Narrow squat:

Narrow squat. (Illustrations by The Washington Post)
Start by standing with your legs hip-width apart. While maintaining a straight back, sit back as far as you can (as if you were sitting down in a chair) into the squat. Keep the shin relatively upright and avoid poking the knees out in front of the feet, as that can be stressful on the knees.