ab crunch with cable and resistance band

although a little bit of instability is good during the exercise from time to time to make it a little bit more challenging, but it should not be too much that you sacrifice the range of motion itself. when you want to target the six-pack muscle or the rectus abdominis, you need to flex the spine for contraction and extend it for relaxation which can happen from the upper part of the torso(thoracic part by curing the chest) or the lower part of the torso(lumbar part by hip extension/flexion). these options are limited when you are on your knees because they are going to throw you out of balance, so you cannot be focused and stable to perform them correctly.