" As the weight gets heavy, there will be a pronounced tendency to allow your chest to drop down to meet the bar, completing the rep from the top down instead of from the bottom up. When this chest drop becomes excessive, the weight is too heavy. And “excessive” is a rather subjective concept here. Someone might decide that no chest drop is allowable, in which case heavy weights cannot be used in the exercise. Or someone might decide that as long as the chest can be touched with the bar, the rep counts. This degree of variability is one of the things that distinguish an ancillary exercise from a primary exercise: if a large degree of variability is inherent in the performance of an exercise, it cannot be judged effectively or quantified objectively. For this reason, the barbell row makes a very good ancillary exercise but a very poor contest lift. "

― Mark Rippetoe , Starting Strength: Basic Barbell Training