Above is the JACK KNIFE exercise. Touching fingers-to-toes is the most difficult and demands the fullest range of motion, but also produces the best results. Limiting the range of motion is how beginners can scale back the exercise to fit their abilities.
The above are SEATED KNEE TUCKS. Watch the variations in hand position, each changing the difficulty level.
Above are SITUPS. This is the old standby abdominal exercise. Do your best to keep your feel on the floor, and do not pull on your head or neck. Make the movement easier by crossing your arms across your chest. Ensure a full range of motion for best results.