Elbow-to-knee

Start by lying flat on your back with your knees bent and feet flat on the ground.

Place your hands behind your head with your elbows pointing outwards.

Engage your abs and lift your upper body off the ground, bringing your right elbow towards your left knee.

At the same time, bring your left knee towards your right elbow, creating a twisting motion.

Pause for a moment at the top, then slowly lower your upper body and extend your legs back to the starting position.

Repeat the movement, this time bringing your left elbow towards your right knee and your right knee towards your left elbow.

Continue alternating sides for the desired number of repetitions.

Recruiting larger muscle groups to "help" move the weight instead of isolating Abs — swinging momentum or shifting the torso takes tension off the muscle this exercise is meant to train.

Flaring the elbows too wide or letting the hips sag/pike, which shifts load off Abs and stresses the shoulder joint.

Rushing through reps using momentum rather than controlling both the lifting and lowering phase, which is especially important since there's no external load to force good tempo.

Skipping a warm-up set at lighter load/intensity — even "beginner" exercises benefit from a rehearsal set to groove the movement pattern before working sets.