According to a recent study, no it is not. Take a read and see what you think.

The following references suggest not:

References
1) Mounier, Rémi, et al. “Antagonistic control of muscle cell size by AMPK and mTORC1.” Cell Cycle 10.16 (2011): 2640-2646.

2) Musi, Nicolas, et al. “Metformin increases AMP-activated protein kinase activity in skeletal muscle of subjects with type 2 diabetes.” Diabetes 51.7 (2002): 2074-2081.