Supine cycling opposite elbow to knee ("Crunch, cross-body" Core/abdominal strengthening, cycling legs)

Lie on your back and lift your legs up so your hips and knees are at 90 degrees.
Place your hands behind you head and tuck your chin to your chest, lifting your head up into a crunch position.
Keeping your lower back pushed into the mat, pedal your legs like you're riding a bike.
Bring your opposite knee to opposite elbow as you pedal your legs.