Popular as crunches, sit ups and leg lifts are, they are NOT the best exercises for the lower abs.
The best exercises for the lower abs strengthen your abdominal wall without stooping the upper body forward, compressing the lower spinal segments, or weakening the pelvic floor. Although sit ups, diagonal crunches (elbow to opposite knee) and double leg lifts are popular in the gymnasium and sporting cultures they can cause a low back problem, or at the very least make an existing one worse. As such, they are not the best abs exercise.
Reverse curl ups is like doing sit ups-in-reverse, knees to chin. These are the 'number one' best exercise for the lower abs.
The 'number one' best exercise for the lower abs is reverse curl ups. This very same exercise helps recruit the pelvic floor muscles which act as a contractile sling across the base of the abdominal cavity. The pelvic floor muscles are often forcibly weakened by the bearing down effects of excessive sit up exercises. In later life, this can lead to incontinence.
With the action of bringing the knees to the chin with reverse curl ups there is an associated 'overflow action' that helps incorporate the anterior pelvic floor into the overall action. The idea, when drawing up the knees towards the chin, is that you also draw up the pelvic floor, scrinching the pelvic floor muscles around the urethra (the tube that brings the urine from the bladder to the outside of the body) to tighten through the contractile sling.
The reverse curl ups exercise is a small scale action, where initially it may be difficult to get any sense of movement and rounding of the lower back in attempting to lift the bottom off the floor. With continued effort over the weeks and months this will change, although at all times it's important to localise the action to the low back, instead of flexing and extending at the hips. Reverse curls ups are the Step 3 of the BackBlock routine. Ideally, in a lower abs strengthening session, you should perform 30 reverse curl ups in one go.
Oblique twists are possibly THE most effective best exercise for the lower abs & for improving the cosmetic appearance of the abdominal wall. This exercise specifically targets both the external and internal oblique muscles, which are an X-crossing sheet across lower abdominals.
The arms at shoulder level should not rest upon the floor, as this makes it all too easy to use counter-pressure through the arms to raise the very heavy legs. Ideally, the arms should hover approximately 10cm above the floor at all times, so that you have to use your oblique muscles and pelvic floor muscles alone, as strenuously as possible. Not having your legs crossed at the ankles also recruits a stronger action of the pelvic floor during oblique twists.
The oblique twists are supremely effective a reducing a slack and protruding lower belly. They principally activate both the internal and external oblique muscles and in so doing, also nip in the sides of the waist creating the enviable hour-glass figure.
The hardest part of the oblique twists action is changing direction and pulling the legs back the other way, and this is when it's tempting to push into the floor with the arms, that must be resisted. It is also important to keep the thighs together and not lift one leg at a time when swinging the legs up and over the body the other way. Ideally, you should perform 15 oblique twists to both sides in one lower abs strengthening session.
The final best exercise for lower abs is legs passing. This is a disarmingly complex exercise, made simple in the doing. The legs going up and down reciprocally requires both concentric (pulling in) and eccentric (paying out) control of opposite sides of the abdominal musculature. This dual action strengthens the abdominals but also in a sense 'tricks' over-active back muscles into switching off. At the same time, this third best exercise for lower abs helps restore a more relaxed swinging action of the hips during walking. Free swing hips often disappear when you've had a back problem for any length of time.
Legs passing is a highly sophisticated exercise that tricks the back muscles into switching off, while also strengthening the lower abs.
You should initiate the legs passing exercise by pressing the whole of the low back into the floor and keeping it there. This stabilises the spine and makes it safe to lift the legs.
Legs passing is often done badly, at the risk of making a back problem worse. The usual mistake is straightening out each leg as you take it to the floor, as in a sort of bicycling action of the legs. At all costs this should be avoided as it can can cause shearing strains across the lumbar spine which will feel like a strain and could make the back very painful - and set you back! Ideally, you perform 15 bilateral legs passing in each session.