the first time I did the plank, I stop as soon as my body starts to shake but then I start timing it and it does shake after 20 seconds....that tells you that I'm so out of shape at that time (235 lbs) but now, I could do my normal s sets of 90 secs with 30 secs in between...I know I could do more but don't want to make the same mistakes I did when I end up hurting my back.

This looks even harder. You know when to stop when your body starts shaking?

@bob....here's a better picture of planking....LOL :-D

*that's me in my old cubicle with my couch...I miss that cubicle :-(