calisthenics / isolation
Get Started With Oblique Crunch
Sit with your knees bent to 45 degrees, heels touching the floor, and lean back at a roughly 45-degree angle with your back straight.
Bring both hands behind head with slightly bent elbows, elevate and rotate your torso to the right, trying to touch your right elbow with left knee.
Return to the starting position and repeat with alternate sides.