In all likelihood it doesn't. Firstly, no toxic effect was noted on the mice, which indicates that grape seed extract does not impede normal cell life and division but mostly/only cancerous cells. For sure Grape seed extract has no negative effect on the general cell repair activities of IGF-1. As for cell division, that is still unclear.
Secondly, muscle cells are different than most cells. They do not normally divide and they have many nucleii, whereas other cells have one nucleus each and divide regularly.
In all likelihood the further research in that direction will confirm that grape seed extract inhibits cancerous cell growth but not normal cells, otherwise some degree of toxicity to this stuff would be found out.
Now wether or not grape seed extract will prevent myocytes from dividing, we do not know for sure. Studying how exactly myocytes divide would yield better understanding of this.
