There are at least two separate regions within the hamstring muscles (upper and lower) that appear to respond differently to the same resistance training exercises. (strengthandconditioningresearch.com)
The great question facing researchers was why these conditions were seen in two widely separated regions, on opposite sides of the equator, at the same time. (techtimes.com)
Those small differences could expand into large separate regions over time. (newscientist.com)