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In fitness classes I have often seen that crunches were done in a specific pattern, for example:

10 normal crunches
10 crunches in 2 steps (i.e. up hold 1 count, up hold 1 count, down, down) 10 mini pulsing crunches on the top
hold it 10 seconds

Or when doing bicycle crunches for example:

Do 15 very slow bicycle crunches and then do 15 very very fast ones.

What's the reason for the described variations concerning body-sculpting effects and body performance?

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Honestly: It's more to keep you entertained then designed to impact body-sculpting effects. Instead of doing 30 regular crunches and then 30 bicycle crunches, your instructors alternate variations of them so you don't get bored very quickly. Some people will denounce this and spout something about "muscle confusion", but that's a myth. –  user4963 Feb 4 '13 at 12:46
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Marco Leblanc is right. I just might add that in general crunches arent the best idea. You wont get 6pack abs by doing them, cause thats related to your BF%, and there are better exercises that strengthen your core without straining the back so much. A simple plank works your core nicely and does not ruin your back as much –  K.L. Feb 4 '13 at 13:44
    
I'll agree with the other comments. Here is some additional info on ab exercises. –  BackInShapeBuddy Jul 30 '13 at 9:12
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1 Answer 1

When you get trained to teach classes by a professional (company) they talk about keeping your audience focused and participating.

So there are two main reason for switching of exercises:

  1. After 10-15 reps a lot of people would zone out and lose focus. I have been told 15 is the magic number but opinions vary. I personally think 12 is a great number because it is one syllable and after that it is all 2 - you have to say eleven really quick so it is one syllable. You are counting in your head 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, THIR, TEEN, FOUR, TEEN... If you are doing something high intensity you either slow down, skip numbers, or lose count.

  2. A lot of people in class have trouble doing more than 10 of something. By doing the same thing over and over you are rubbing it in and leaving them out of an exercise for a while. So if a guy can only do 4 crunches at a time he is doing 40% of the crunches - that's OK. Better than doing a set of 30 and he does 11%.

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I know that none of these answers address performance because that has nothing to do with switching up the exercises. Wasn't trying to skate the question just giving an honest answer. The effects of only doing 10 crunches and variation are probably positive motivation to the untrained and probably negative because of lack of intensity on the muscle group to the trained. –  DMoore Jul 30 '13 at 22:16
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