The objective of the present study is to improve knowledge of the dynamic human neck behavior in vivo, an essential aspect in rear end impact protection optimization. Neck characterization is performed in the frequency domain by a vibration loading of the volunteer thorax. Inter-individual differences and gender influence are also investigated. The mean modal characteristics of the human head-neck system extracted from the 30 experiments and the related standard deviation are defined by two natural frequencies located at f1=1.68±0.69 Hz and f2=7.91±0.74 Hz. For all subjects the first vibration mode was a flexion-extension motion and the deformed mode shape related to the second natural frequency was a head translation also called retraction motion. When male and female were distinguished, only small differences appeared, i.e. for the male, mean value and standard deviation of the two natural frequencies of f1=1.83±0.96 Hz and f2=8±0.8 Hz against f1=1.53±0.28 Hz and f2=7.83±0.68 Hz for the female group. Muscle effect was also analyzed and it was possible to demonstrate that the contacted condition shows a major increasing of the first natural frequency value, especially for males. The new data recorded on human being in vivo constitute new validation parameters for mathematical neck models and dummy necks.