Lack of Resistance: Abs are muscles just like your pecs and lats, so why not train them in a similar manner? Doing sets of endless reps of crunches is not going to build impressive abs any more than body weight push-ups will develop an Arnold-like chest. Do not be afraid to perform weighted abdominal exercises that limit you to just 12-15 solid reps if you really want some “deep boxes” in your midsection!
Anchoring Feet: One mistake I constantly see even advanced lifters make is to anchor the feet under pads when doing crunches or various crunch machines. This not only shifts much of the emphasis off the abs and onto the hip flexors, but can also result in lower back pain over time.
Reps Too Short/Fast: If there is any muscle group that should be exercised with slow, controlled reps it’s the abs. Pay careful attention to achieving a full range of motion from stretch to contraction, with a tight squeeze at the peak. Fast reps will do nothing to overload the abs and will more likely just result in injury.
No Intensity: So many trainees are working under the false notion that all it takes to create a truly wicked 6-pack are a few easy sets of crunches and leg lifts at the end of a workout. But like I mentioned earlier, the abs are muscles just like any other, and if you really want to build bricks into your belly you have to train them with similar intensity to other areas of your physique. Don’t be afraid to put a serious hurting on your abs!
Wrong Movements: Just like you hit the quads or triceps with various movements that work the muscle through various planes of motion, the same needs to be done for your abdominals when serious about your 6-pack! When designing an abs workout for myself, or clients, I feel it is important to begin with a movement that focuses most tension on the lower midsection. These include all type of leg lifts with bent or straight legs. Next up should be an “upper” ab movement, preferably using resistance, where the trunk is “curled” toward the knees, such as with various types of crunches. Finally a twisting type exercise is then necessary to develop the external obliques and intercostals.