Element abundance ratios hold important clues to understanding the evolution of stellar populations, through the varying timescales of different nucleosynthetic contributors. Newly measured and compiled [Mg/Fe] ratios in the MILES stellar library are used to confront models of star spectra. Such models have been used in recent years to provide estimates of differential changes in spectral line strengths, due to enhancements in [α/Fe]. In this paper we test one widely used set of theoretical element response functions. Using magnesium as a proxy for all alpha elements we test the reliability of these theoretical response functions against empirical observations, and thus the reliability of current methods of measuring element abundance ratios in the stellar populations.