At the 27th Stapp Conference an analysis was presented of human head-neck motion in lateral flexion (1)*. Based on this analysis performance requirements for mechanical necks in this type of impact were formulated.This study extends the analysis to head-neck response in frontal flexion. Results will be presented of dynamical tests with human subjects conducted by the Naval Biodynamics Laboratory (NBDL) in New Orleans. Two of these subjects were also included in the analysis of lateral flexion. It will be shown that the mechanical (mathematical) system with two ball and socket joints describing the head-neck response in lateral flexion is also suitable for forward flexion. Geometrical parameters are identical for both impact directions in contrast to the dynamical properties which show significant differences.By this mechanical analog the observed head-neck motions are completely defined. Results will be compared with earlier performance requirements proposed by Mertz et al. (2,3).Evaluations of the Part 572 and Hybrid III head-neck systems show that both necks are stiffer than any of the volunteers in the impact ranges tested.