What you are feeling now is why many people continue to juice. The pumps aren't as good, and the weight on the scale continues to drop, no matter what you seem to do. The good thing is, you got up to 195 naturally... which means, you can do it alone. Muscle memory is an incredible thing. As I'm 5'7, I can relate that because of our frame, we look bigger. You must have been one big mofo. Due to the fact that you stopped bodybuilding in general, and probably ate for shit... of course you are going to shrink. However, this isn't the same reason a juicer crashes when he comes off his cycle. Regardless of the food he eats or the amount he trains, he can't be as big. To answer your questions...
1. When do you even begin to consider juicing?
When your feel your natural limit has been hit... and you have plateaud. This means, you should not touch a dart until you are back up to 195, naturally again. And, at that point... why you'd want to is beyond me.
2. How quickly does juice "aid" in the gains?
Juice adds quickly to gains. I've gained 10 pounds in the past 14 days I've been back on.
3. Do you think simply relying on muscle memory will get me back up to 190lbs much quickly than the juice?
Not quicker than the juice, but once you start eating your proper diet again, and smashing the weights... I think you will be surprise with your muscle memory. If you juice to get back up to 195, then what happens when you crash? Obviously, you're going to lose some of that weight. At 5'8, 195 and 10% bodyfat, you sound like you have a body that doesn't require juice.
Get back in the gym naturally and see how it goes bro.
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