1Anthropology, University of Florida, 2Evolutionary Anthropology, Duke University
March 27, 2015 , Archview Ballroom
We investigated how long bone cross-sectional dimensions (CSDs), i.e., area moments (I) and section moduli (Z), reflect the change in mechanical loading associated with the onset of walking in an archaeological sample. Reduced major axis regression residuals informed about over- or underestimation of CSDs by diaphyseal length. ANCOVAs tested for homogeneity of slopes across age groups. Ratios provided measures of relative rigidity and growth velocities. Bootstrap estimated regression slopes informed about scaling of CSDs to length across age groups (0 to 60 months [mos]). CSDs signaled distinct periods of postural changes. Such morphological data can be used to investigate further the factors affecting subadult long bone growth.
CSDs of upper limb (UL) bones increased faster than lower limb (LL) bones in the first year, presumably related to crawling. Relatively high CSD growth rates at 6-9mos and 18-24mos potentially reflect body weight shift to the LL and a response to more habitual bipedalism, respectively. High rates in the latter age interval were maintained during the 24-36mos period despite slowed length growth potentially due to weaning stress. LL length growth always outpaced CSD growth, potentially to allow for adult limb proportions. In older children, CSD growth increased in the LL bones, presumably to counteract the increasing bending strain. Similar patterns in the non-weight-bearing fibula perhaps signal the increased use of plantarflexors and evertors as gait matures. AP and ML CSDs did not differ in growth, supporting past studies showing that bone is not necessarily reinforced in the axis of maximum strain.