The Made In Chelsea graduate seems to spend half her life in the gym and as with any good workout, as she gets fitter, the exercises get harder.
If you frequent the gym with a personal trainer or a class, it's common to feel some serious despair that it just isn't getting any easier.
This is not because you're failing at fitness, it is in fact because you will naturally push yourself harder - the challenges get longer, the number of reps get bigger.
And in Millie's case, the plank is in motion.
With mats under her trainers she pulls herself forward in the plan position for as long as possible. She calls it 'the plank from hell'.
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Millie Mackintosh's workouts
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