Impact excitation and frequency response measurements look promising as a nondestructive firmness measurement for apple. In this work the influence of the apple properties including firmness, local shape, damping and Poisson's ratio on the impact excitation is studied on the basis of a force model and using the excitation frequency and the internal stress as criteria. The change of the elastic modulus between 2 to 12 MPa, weight between 100 to 250 g, and coefficient of restitution below 0.8 does not influence the reliability of the excitation using the optimal excitation parameters. When the radius of the local shape in the contact surface of an apple is far less than 31 mm, the impact may not excite the spherical mode and even cause damage to the apple. Suggestions are given for the location of the impact excitation on the apple surface. Poisson's ratio is found to have little influence on the impact excitation.