I do this day in and day out with people, training people all over the country for both events and weight loss. Here's a solid ab series for you - it's four, one minute sets. There's no rest between minutes, but rest as much as you need throughout the four minute series. In the beginning this is going to be very hard, but try to do it three to four days per week.
Minute 1 - do as many full sit ups as you can Minute 2 - hold a tight plank position on your elbows Minute 3 - do as many unweighted v-twists as you can Minute 4 - do as many full sit ups as you can to finish.
Have fun.
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