Squats, deadlifts, pushups on an incline.
Don't waste your time with situps.
and more planks, and side planks and one leg up plank one arm up plank, elevated planks, planks on swiss ball, mountain climbers on swiss ball farmer walks are good too.
There are about 2 dozen plank variations (google them) you can do. Other exercises to work low abs, leg raises, reverse crunches and leg raises on captains chair, bicyles. Obliques- russian twists, side planks, side hip presses.
The core includes the low back too. Do some weighted back extensions, supermans, alternating arm/leg extensions.
I never "feel" squats in my "core" but because you are stabilizing your entire body to get down and up again you are using your "core" as accessory muscles I guess.
I am currently doing New Rules of lifting for Abs and all the workouts in there are based on squats, deadlifts, and planks. Many of the exercises are "off loaded" meaning you are doing say a squat but in one hand is a 25 lbs dumbbell while the other is holding 35 lbs. There are lots of just one handed and one legged lifts so your focus is on keeping your body straight
Squats require a strong core just as they require strong legs (at least at higher weights). You need strong stabilizers to squat properly and at high weights. Squats are great for developing and strengthening those stabilizer muscles.
Planks are also great. You may never feel a burn on your core from them (I certainly don't) but they work it quite well. They will also work your lower back stabilizers quite good.
I used to be able to do planks for over 3.5 mins, but it took a bit of time to get there. Side planks are also great, and definitely more challenging.
There's a really good DVD that I use called "10 minute solution, pregnancy pilates," which includes a killer abs workout for after the baby is born. I started using it when I was pregnant, but frankly I still use it now because I really like the Standing Pilates sequence and the abs sequence. The abs workout has lots of modifications geared towards those of us with no core strength whatsoever, and it enabled me to strenghten my abs gradually.
If you are planking correctly you have to use your abs to hold yourself up in the proper plank position.
I'd be curious what muscle you thought it did work?
Look up 'Top 10 Most Effective Ab Exercises' on about.com. I have done bicycle crunches for several months now. You only do as many as you can do and over time slowly increase the reps. Check out about.com. There's plenty of ways and another way may suit you better. There's plenty of ways without any equipment too.
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