Did a little research on this awhile back,the only thing That I haven't been able to find is whether its ability to increase insulin sensitivity was mainly in muscle cells or fat as well . Patients lost fat in there mid section which is a good sign that it does favor muscle more than fat, but you have to remember these people already had type 2 diabetes so lowering total insulin levels and blood glucose would help anyway. Could be like R-ALA and too much would start to improve sensitivity in fat cells as well as muscle, will try to find some more research.
When it comes to insulin sensitivity, it really doesn't pay to worry about whether it's selective to muscle tissue or not. What is important is that you get insulin quiet as fast as possible. The less resistant cells are, no matter what type, the quicker we get blood sugar down, the quicker insulin stops being released. The less time we spend with insulin active, the more time we have to burn stored fatty acids.
If you're exercising with a decent amount of intensity more than twice a week, then worrying about whether your fat cells stored a few grams of sugar as triglycerides isn't productive. You'll use that up in your sleep, IF YOU ARE INSULIN SENSITIVE.
Think of insulin as a nagging girlfriend. At some point you just want her to shut up. The less she is bitching at you the better you feel, period. It doesn't make things better if you get her to bitch at you about something different. The point is, she is still nagging you. What you want to do is get her out of the room as quickly as possible so you can move on with more enjoyable things.
Besides, a muscle cell can only store so much glycogen. You can make it sensitive, but if there's no room there's no room.
If you still insist on trying to increase muscle sensitivity and decrease fat sensitivity, then go with IGF-1. This is about the only thing I've seen that has science backing it up as a selective glucose disposal agent. But be forewarned. IF you are insensitive to insulin because of a shitty diet, and if you consume enough carbs at one sitting, then all the igf-1 is going to do is make your blood sugar soar. Your muscles will only store so much. At that point you have now choked your liver and fat cells from storing anything and you are now sitting at 200+ blood glucose because you ate a tub of ice cream and drank a liter of pineapple juice. GH15 approved!!!
But if you read your glucose levels and see that it hangs out at 120+ a few hours after eating, then it really doesn't matter where you store the sugar, just get it the fuck out of your bloodstream, for your health AND your figure.