Giraffes and hominins: reductionist model predictions of compressive loads at the spine base for erect exponents of the animal kingdom

Abstract

In humans, compressive stress on intervertebral discs is commonly deployed as a measurand for assessing the loads that act within the spine. Examining this physical quantity is crucially beneficial: the intradiscal pressure can be directly measured in vivo in humans, and is immediately related to compressive stress. Hence, measured intradiscal pressure data are very useful for validating such biomechanical animal models that have the spine incorporated, and can, thus, compute compressive stress values. Here, we use human intradiscal pressure data to verify the predictions of a reductionist spine model, which has in fact only one joint degree of freedom. We calculate the pulling force of one lumped anatomical structure that acts past this (intervertebral) joint at the base of the spine, lumbar in hominins, cervical in giraffes, to compensate the torque that is induced by the weight of all masses located cranially to the base. Given morphometric estimates of the human and australopith trunks, respectively, and the giraffe's neck, as well as the respective structures’ lever arms and disc areas, we predict, for all three species, the compressive stress on the intervertebral disc at the spine base, while systematically varying the angular orientation of the species’ spinal columns with respect to gravity. The comparison between these species demonstrates that hominin everyday compressive disc stresses are lower than those in big quadrupedal animals. Within each species, erecting the spine from being bent forward by, for example, thirty degrees to fully upright posture reduces the compressive disc stress roughly to a third. We conclude that erecting the spine immediately allows the carrying of extra loads of the order of body weight, and yet the compressive disc stress is lower than in a moderately forward-bent posture with no extra load.

Comparative finite element analysis of the first thoracic vertebra in artiodactyls

“…1 Models are homogenous (no spongiosa); *Force values before scaling. References: 2 Costeur et al (2019); 3 Cristofanelli et al (2005); 4 Dittmann et al (2014); 5 Günther & Mörl (2021); 6 Riek & Gerken (2010); 7 Simmons & Scheepers (1996); 8 Stadelmann et al (2020); 9 Vander Linden (2021); 10 Zsoldos et al (2010).…”

“…To define a compressive load for the loading scenario, we considered the weight of the head and neck segment. The estimated percentage for the head‐neck segment varies between 10 and 18% of body mass (data for G. camelopardalis, Lama glama and Equus equus ; Costeur et al, 2019; Cristofanelli et al, 2005; Dittmann et al, 2014; Günther & Mörl, 2021; Riek & Gerken, 2010; Simmons & Scheepers, 1996; Zsoldos et al, 2010). Since most studies on head‐neck weight percentage described G. camelopardalis and L. glama , we calculated the 10th/18th percentage of mean body mass for both specimens.…”

“…Since most studies on head‐neck weight percentage described G. camelopardalis and L. glama , we calculated the 10th/18th percentage of mean body mass for both specimens. Also, an angle lower or higher than 0° between the head‐neck segment and T1 in a lateral view in all species is expected to a certain point, therefore we used an “estimated neck angle” of about 30° (given for G. camelopardalis ) for the compressive load of our loading scenario (Supplementary online material: ) (Günther & Mörl, 2021; Müller et al, 2021a). Since the morphology in T1 of L. glama is more similar to other T1 of artiodactyls than the “cervicalized” T1 of G. camelopardalis , L. glama was used as reference to calculate the relative compressive loads in our other specimens.…”

“…Also, an angle lower or higher than 0°between the head-neck segment and T1 in a lateral view in all species is expected to a certain point, therefore we used an "estimated neck angle" of about 30°( given for G. camelopardalis) for the compressive load of our loading scenario (Supplementary online material: SI 9, 10) (Günther & Mörl, 2021;Müller et al, 2021a). Since the morphology in T1 of L.…”

“…Habitual head‐neck posture, combat behavior, and mass of the head‐neck unit directly influence the amount of compressive load exerted on T1 (Christen et al, 2012; Günther & Mörl, 2021; Jovanović & Jovanović, 2004; Vander Linden & Dumont, 2019; Vander Linden, 2021). Differences in posture, overall size and relative length of the neck will impact the stress regime within T1 (Arnold et al, 2017).…”