In specifications of high performance human centrifuges, different definitions of the term "G-load onset rate" occur. This paper compares four of these definitions and discusses the motivation and its advantages and disadvantages from the technical point of view. The four definitions are applied to a high performance human centrifuge to generate Gz load versus time profiles with constant rates of G onset. In order to illustrate and measure the differences between the different methods, some evaluation parameters like "overall transition time" (time between two different g levels), and "lower transition time" (time to reach the constant onset rate from the lower g-base level) are developed. Further, the influence of a tolerance threshold for the Gx- and Gy-load components in a Gz load profile on the evaluation parameters are discussed. The admission of a relative small tolerance threshold (< or = 0.3 g) leads to a drastic reduction (40%-50%) of the evaluation parameters.