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?