Berberine and Metformin act the same in terms of the specific issue at hand. I believe there is only one human study of this issue for berberine, and numerous animals studies for berberine, while as you know it's plentiful for metformin.
Here's where I read the same interpretation of metformin on high intensity VOX capacity:
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Next comes the potential impact that metformin may have on exercise performance. In the study “Invalid Link Removed”, it was shown that metformin can significantly decrease VO2 max (–2.7%), peak heart rate (–2.0%), peak ventilation (–6.2%), peak resting energy expenditure (–3.0%), and actual exercise duration (–4.1%). While these reductions were certainly slight, and potentially non-significant for the average exercise enthusiasts, they do suggest that for a professional athlete (especially a professional endurance athlete), the use of metformin may spell the difference between a 1st place finish and not even making the podium! An even newer study, recently reported in the New York Times Invalid Link Removed, highlights that metformin has also, compared to a placebo, been shown to hamper endurance and limit mitochondrial proliferation in response to an exercise protocol! And as for muscle – the bad news is that Invalid Link Removed.
Metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults: A randomized, double‐blind, placebo‐controlled, multicenter trial
I can admit that anything that says "less muscle" will stick in my mind and then it's very hard to get it out.
Maybe prozac can help with that?