FRONT PLANK LEG LIFT The classic core exercise. Bringing in the leg lift will add a strengthening element to the hip extensors and the lower back (and therefore the muscles stabilising the back of the spine as well as the front of the spine), as well as strengthening the muscles controlling rotation, thus making it an all 'round back strength exercise. I often recommend this exercise to patients who can benefit from having more stability around the lowest part of the spine, an area that has to deal with joining the mobile spine and the more stable bony pelvic ring.
Doing the movement slowly and deliberately will give you the best results: this is how the brain has time to understand how the body moves and learn how to control fine movements effectively. As a rough guideline, start with 2 sets of 10 repetitions (5 on each side), to gradually build up to 3 sets of 16 repetitions.