Sparsely branched polyolefins often exhibit a thermorheological complexity, which was reported to be maskable by a modulus shift. However, the only physical background for a modulus shift is a density change, and this influence factor is only small in the relatively narrow temperature regime accessible by polyolefins. This paper deals with the question, how this modulus shift can be caused by experimental artifacts and real effects. The physical background of these two contributions to a vertical activation energy, as well as a meaningfulness of the application of a modulus shift, is found not to be given for polyolefins, when measuring only in a temperature range between 130 and 230 degrees C.