You can go into most gyms and watch people that train their abdominals and a common trend will usually emerge. Abdominals are usually trained with a large number of fast repetitions. I personally believe when it comes to getting great abdominals the old saying couldn't be more true "Quality over quantity." I don't feel that you need to perform sets of 100's of reps to produce quality abdominal muscle. In fact I believe that when you perform high volume sets like that you more often than not tend to actually shift focus away from your abdominals.
This occurs because when performing high repetitions of an abdominal exercise increased speed becomes very common and the goal of the exercise is then shifted from efficient abdominal work, to doing whatever it takes to finish the set. This includes using momentum, allowing other muscles to take over the movement, and shortening the range of motion. All three of these techniques equal decreased efficiency and effectiveness of the exercise.
If repetitions are performed in slow and controlled manner, your abdominals will be forced to do the majority of the work without any assistance. Also when training abdominals I feel that in order to get the most effective workout as possible you need to slow down both the concentric(muscle contraction) and eccentric(muscle lengthening) portions of the exercise.
When you watch people doing fast paced high volume sets of abdominal work they virtually eliminate the eccentric portion of the exercise. Why would you just let gravity take over during the eccentric portion of the exercise to return you to the starting position and eliminate all of its advantage? That's essentially like doing only half of the exercise and therefore only receiving partial benefit. During the eccentric portion of the exercise not only can you get a good stretch in the abdominal, but by controlling the speed you will also increase the time that it is under tension. If you perform your repetitions in this manner you will find that not only can you not do as many repetitions, but you no longer need to in order to have a great workout.
There are lots of ways to train abdominals, but for me slower is definitely better. If you feel like you're not getting the best results from your abdominal training sessions, I suggest you slow things down a bit and focus more on the quality of your repetitions and less on the quantity.
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