When comparing both vertical and horizontal pulling exercises, upper trapezius muscle activity is greater upright rowing movements, while middle trapezius muscle activity seems to be higher during horizontal rowing movements, such as the bent - over row. (strengthandconditioningresearch.com)
In practical terms, both upper and middle trapezius display high muscle activity at knee flexion angles of 31 — 60 degrees. (strengthandconditioningresearch.com)
It appears that the middle and upper trapezius display superior muscle activity during the bent over row, while middle trapezius muscle activity seems not to be affected by stability at the hand or seated row technique. (strengthandconditioningresearch.com)