Rb I 7800 Å (left panel) and ZrO 6474 Å (right panel) spectral regions in massive Galactic AGB stars. The pseudo-dynamical models (red lines) that best fit the observations (black dots) are shown in four sample stars. For comparison, the hydrostatic models are also displayed (blue lines). The location of the Rb I line and the ZrO bandhead are indicated by black vertical lines.