Currently, personalization of computational human heart models is often performed by combining a carefully designed patient-specific heart geometry with a generic myofiber orientation. This is due to a lack of accurate in vivo fiber orientation measurement techniques. The myofiber orientation is a key determinant of the forces causing the contraction of the heart as it is used to construct the local material coordinate system. In this study, we use a mechanical whole-heart model to evaluate the impact of ventricular fiber angles and sheet angles on clinically relevant mechanical biomarkers of the left ventricle (LV): wall thickening WT , mitral valve displacement MVD, net LV twist angle θ and ejection fraction EF. We show that the mechanical function is strongly dependent on the fiber direction. In the evaluated fiber angle configurations, WT ranged from 33.38 % to 52.48 %, MVD from 5.85 mm to 18.19 mm and EF from 46.83 % to 59.95 %. For some fiber angles, θ was negative indicating a change of the apex rotation direction. Additionally, θ was the only biomarker undergoing significant changes for different sheet angles. For WT, MVD and EF the impact of sheet angle combinations was negligible (< 4.6 %), unless changes in the active stress tensor were introduced. Even with the altered active stress tensor, the change between different sheet angle combinations is small compared to the change between different stress tensor formulations. Considering the changes introduced by varying the fiber angles, developing ways of personalizing the fiber orientation or systematically considering its uncertainty appears important.