Accurate head-related transfer functions (HRTFs) are an essential aspect for the binaural presentation of virtual acoustic environments. HRTF measurement systems, however, differ greatly between institutions. One difference is whether the participants are standing or sitting during the measurement procedure. Previous publications indicate, that while the overall HRTF measurements in different systems yield comparable results, knee reflections while sitting down may affect the measured HRTFs, especially at lower frequencies. To further investigate how these reflections affect the HRTF, boundary element simulations of a sitting and standing artificial head with added lower extremities were conducted. The spectral effects and their spatial variance of the two different postures were analyzed and compared to previous measurements from different labs.