The computer simulation of NBDL data for human subject dynamic response in −Gx and +Gy impact sled acceleration vectors yielded mechanical constants pertinent to the design of a Biofidelic Manikin (BFM). Improved neck model biofidelity was achieved by decreasing the degree of coupling between initial curvilinear head translation and head angulation in −Gx simulations. A long neck linkage, i.e., a neck link with pivot points relocated from anatomic T1 (and/or the occipital condyles), satisfactorily decouples head translation and head angulation and improves the prediction of head-neck motion. Simulation results in −Gx and +Gy are shown for a neck model with pivot points at the occipital condyles and the lower neck pivot point located inferior and posterior to anatomic T1. In −Gx, the magnitude and duration for condyles extension, in particular, is improved while satisfactory acceleration response is maintained. Exercising the neck model at −6Gx and −15Gx showed that it was necessary to use nonlinear stiffnesses for the lower neck joint in flexion.Two modes of condyles extension motion were seen in the volunteer subjects in the −Gx acceleration vector. The first mode is extension with rebound (some subjects demonstrating flexion). The second mode shows peak extension followed by further increase in extension motion. The two modes of extension motion appear to depend on a pre-test condition - probably muscle tensing. Passive neck models investigated in this study display only mode one response, i.e., extension with rebound.