In Why ab workouts are a waste of time personal trainer Steve Steinberg gets it right about how the only way to achieve abdominal definition is to reduce the fat around your belly. And because spot reduction is a myth, the way to do this is primarily improving your diet and increasing muscle mass (which burns calories throughout the day), and only secondarily with cardiovascular exercise. So if what you're looking for is definition, you never have to work your abs again.
What proper abdominal exercises do is build abdominal muscle, which actually increases your girth, and if your waist area is not slim enough, you won't see any definition regardless of how strong your abs are.
Everybody has a #sixpack. But ab workouts won't help reveal it! http://t.co/B74pZVWCQF #GetFit — CNN Health (@cnnhealth) August 29, 2014
While lunges, or split leg squats, if you want to reduce impact, should be a part of any fitness routine, they should not replace leg presses but compliment them, as they work your lower body differently than presses or standard squats. But they're impact on abdominal muscles is minimal.
If you're goal is to strengthen your core muscles, which generally consists of your abdominal, oblique, and lower back muscles, then consider the best ways to contract them.
AbsThe first thing to consider is where you pivot. These muscles primarily control how the torso contracts. They are engaged when you bend your hips, but the primary muscles used when bending at the hip are in your hips. Working these hip flexing muscles is important, but should not be mistaken, as they commonly are, with strengthening the three areas mentioned. As an example, hold your arms straight in front of you with you palms up will engage your biceps, but you primarily working you shoulders. If you want to work your biceps you should perform some type of curling motion. Similarly, if you want to work your abdominal muscles then performing a standard sit-up where you bend at the hip is not ideal. Ideally you want to contract your chest to your pelvis. By doing this you're primarily engaging your abdomen. In order to do this you need to stabilize opposite your contracting muscles, which is your lower back, not your buttocks as you would in a standard sit-up. For this reason a crunch, where you use your abdominal muscles to lift your shoulders and your hips off the floor, is more effective than a sit-up.
The next thing to consider is range of motion. Ideally you want to move from a stretched position, through neutral, to a contracted position. The problem with crunches, is that your most stretched position is neutral at best, and that's probably not even the case as you don't really begin to work your abdominal muscles until your shoulder blades and butt are higher than your lower back.
In order to begin from a truly stretched position you need to stabilize under your lower back - opposite your abdomen - without stabilizing your butt or upper back. You can do this by placing a foam roller under your lower back. You want to balance your lower back on the roller with your butt and upper back raised in the air. If this is too intense you can wrap the foam roller with a towel or mat to create a wider, softer base. A "stability" ball is too wide and flexible of a base and should only be used as a last resort. As you use your abdominal muscles to contract your pelvis to your chest you have to make sure stabilizing on your lower back and not rolling up, so that your butt is resting on your base.
This motion allows you to achieve the maximum range of motion while focusing on the desired muscles.
Lower BackThe same principles can be applied to working your obliques and lower back, although some modifications are necessary due to the differences in the way those muscles work.
Working your lower back is pretty straight forward, but is often neglected. The easiest method, which is the equivalent to the crunch, is to lie down on your front and contract your upper back to your butt, ideally lifting both off the floor. However, like the crunch, this does not achieve the optimal range of motion.
Lower back machines, either Nautilus style weighted machines or the ones your lean over forward, as well as straight leg dead lifts and good mornings all achieve the desired range of motion. The important thing to remember is that you want to drop your torso down close to your legs as if you're touching your toes, and lift your torso in a controlled manner until your torso is as close to vertical as you can contract. How close to vertical you can contract will depend on how your hips are stabilized. You can definitely lift your torso to a vertical position if you're standing. Your may be able to lift your torso into a vertical position if you're leaning over at a 45 degree angle. You're less likely to be able to lift your torso into a vertical position if you're stabilized on a 90 angle (horizontal) back machine. This is the full range in which you want to strengthen your lower back, and contracting beyond vertical is counter productive.
ObliquesThe most common oblique exercise I see is standing upright with a weight in one hand and leaning to the opposite side. But considering how quickly your body weight offsets the weight you're holding, it is apparent how little is being accomplished with this motion.
However, it is much less comfortable to balance on a foam roller with your ribs. In addition your obliques are more involved in contracting your torso to your hips than the muscles in your hips. Therefore the best way to work your obliques is to stabilize your hips while you lift your torso sideways.
The equivalent to a crunch would be to lie on your side on the floor, stabilize your lower body by hooking your feet or legs under something, and lift your torso up and towards your hips.
A more effective method is to perform the same contraction while stabilizing your hips with your torso lifted off the floor. There are a number of ways to accomplish this. If you have a lower back machine that you lean over, then you can stabilize yourself sideways. You can also lay down sideways on a decline bench, hooking your legs around the top. You can also lay down sideways on a lat pull down machine, hooking your feet around the pads the would normally hold your thighs down. In any of these positions, the idea is to contract your torso from a flexed position towards your hips using your obliques, not your hips.
I hope this helps. Let me know how it works for you and I'm happy to answer any questions.