Most mechanical reliability models for optical fibre assume the same crack growth parameters (n and B) for both proof testing and in-service life. However, this assumption leads to inconsistencies in our understanding of how the fibre behaves in the laboratory and in the field. In 1983 Ritter at al. suggested that proof-testing and in-service crack growth parameters differ according to their respective environments. Recent high-speed strength tests of abraded optical fibres indicate that crack-growth parameters change as a function of time under the same environmental conditions. It was suggested that at high processing speeds, the presence of crack growth in Region It of the K-V relationship should be entertained. In this work, the impact of Region II crack growth during proof testing was explored and the effect on mechanical reliability predictions evaluated.