There seems to be some antagonism between metformin and exercise.
Researchers from Canada looked at the effects of metformin or placebo with exercise on detailed exercise physiological variables in patients with Type 2 Diabetes.
"In conclusion, our study reports several novel findings regarding the concomitant use of metformin and exercise, specifically: 1) increased HR during exercise with metformin, 2) higher plasma metformin concentrations with exercise, and 3) nonadditive effects of metformin and exercise on the glycemic response to feeding. In our opinion, the magnitudes of these effects were small but have the potential to reduce the effectiveness of this therapeutic combination in diabetes treatment. Additional research could help optimize the concurrent use of these important and widely prescribed treatment modalities for diabetes."
1. Boulé NG, Robert C, Bell GJ, Johnson ST, Bell RC, Lewanczuk RZ, Gabr RQ, Brocks DR. Metformin and exercise in type 2 diabetes: examining treatment modality interactions. Diabetes Care 2011 Jul;34(7):1469-1474. Available from: http://www.ncbi.nlm.nih.gov/pubmed/21602430