Review article on spine kinematics of quadrupeds and bipeds during walking.

Sheep, pigs and goats are the most commonly used large animals for preclinical spinal applications. However, there is an increasing number of voices challenging the suitability of quadrupeds, as the obvious postural differences give reason to presume significant differences in the spinal kinematics compared to humans. Rather, it is often questioned if primates did not represent the more suitable experimental animals due to their ability to bipedal walking. Both perspectives, however, have never been systematically addressed. Therefore, the present paper reviews the existing literature on in vivo spinal kinematics of quadrupeds, primates and humans during walking and critically discusses the comparability between these species. Surprisingly, no studies were found for sheep, pigs or goats. Instead, the literature search yielded 50 studies on quadrupeds, horses (n = 37) and dogs (n = 13), primates (n = 10) and humans (n = 61). In general, the kinematic data revealed a high level of heterogeneity and often demonstrated methodological deficits, e.g., insufficient number of measured animals. More kinematic variables were comparable between humans and quadrupeds than between humans and primates. Differences in spinal characteristics, however, could also be found amongst quadrupeds themselves (horse vs. dog). In conclusion, using of a particular animal species as a model for spine research requires its characterization. Cross-species extrapolations are ineligible. Furthermore, the review revealed significant differences between the bipedal walk of primates and humans. The gait alone thus does not constitute a valid argument for the superiority of primates over quadrupeds as experimental animal models for human spine research.