estrellita,
We are told that if you already exercise reduce by half and build back up slowly. If you don't already exercise wait about 3 - 4 weeks for your body to adapt to the ketosis and the reduced calories; then begin slowly.
With the reduced calories we don't really have enough for heavy exercise. The general concern with very low calorie diets is that that you go into starvation mode, hang on to fat, and reduce muscle mass. However, with Medifast, we are well balanced with proteins and carbs to maintain muscle mass. I've lost 50 pounds and using various calculators, lost no muscle, only FAT!
I intend to kick up the cardio work and start adding some strengthening exercises to build tone -- would like to have done more by now, but......
Exercise is definately part of the long term plan with Medifast, but begin slowly.
Good luck!