To date, the diagnosis of whiplash injuries\u00a0 has been very difficult and largely based on subjective, clinical\u00a0 assessment. The work by Winters and Peles Multiple Muscle\u00a0 Systems\u2013Biomechanics and Movement Organization. Springer, New York\u00a0 (1990) suggests that the use of finite helical axes (FHAs) in the neck\u00a0 may provide an objective assessment tool for neck mobility. Thus, the\u00a0 position of FHA describing head-trunk motion may allow discrimination\u00a0 between normal and pathological cases such as decreased mobility in\u00a0 particular cervical joints. For noisy, unsmoothed data, the FHAs must be\u00a0 taken over rather than large angular intervals if the FHAs are to be\u00a0 reconstructed with sufficient accuracy; in the Winters and Peles study,\u00a0 these intervals were approximately 10 degrees.<\/p>\n
In order to study the\u00a0 movements\u2019 microstructure, the present investigation uses instantaneous\u00a0 helical axes (IHAs) estimated from low-pass smoothed video data. Here,\u00a0 the small-step noise sensitivity of the FHA no longer applies, and\u00a0 proper low-pass filtering allows estimation of the IHA even small\u00a0 rotation velocity omega of the moving neck. For marker clusters mounted\u00a0 on the head and trunk, technical system validation showed that the IHAs\u00a0 direction dispersions were on the order of one degree, while their\u00a0 position dispersions were on the order of 1 mm, for low-pass cut-off\u00a0 frequencies of a few Hz (the dispersions were calculated from\u00a0 omega-weighted errors, in order to account for the adverse effects of\u00a0 vanishing omega).<\/p>\n
Various simple, planar models relating the\u00a0 instantaneous, 2-D centre of rotation with the geometry and kinematics\u00a0 of a multi-joint neck model are derived, in order to gauge the utility\u00a0 of the FHA and IHA approaches.<\/p>\n
Some preliminary results on asymptomatic\u00a0 and pathological subjects are provided, in terms of the \u2018ruled surface\u2019\u00a0 formed by sampled IHAs and of their piercing points through the\u00a0 mid-sagittal plane during a prescribed flexion-extension movement of the\u00a0 neck.<\/p>\n