Temporal Changes in Arikara Femoral Torsion Daniel J. Wescott and Deborah L. Cunningham, Texas State University 0 5 10 15 20 25 30 35 40 45 % asymmetry EC PCC DC Femoral torsion, a measure of the twist in the shaft from proximal to distal (Figure 1), is a biomechanically relevant feature that can aid in reconstructing activities in past populations. At birth femoral torsion is approximately 31°, but it decreases to 15° - 18° by adulthood and is generally symmetical 1 . Average femoral torsion has been reported to differ among populations and between males and females within populations, with females generally exhibiting higher torsion angles or greater anteversion 2-4 . Several studies have observed high femoral neck anteversion in Native Americans, especially among females 5-6 . Basgall 6 observed significant femoral torsional asymmetry among Northern Plains females that she linked to sitting posture. Habitual sitting postures, such as “W-sitting” (Figure 2), where the thigh is medially rotated are known to result in high torsion angles 7-8 . Previous research by the authors 9 observed a significant temporal trend among Arikara females in femoral strength asymmetry associated with increased crop production for trade (Figure 3). Here, we investigate sexual dimorphism and temporal trends from the mid- 16 th century to the early 19 th century in femur diaphyseal torsion among the Arikara. Figure 1. Asymmetry in femoral torsion reflected in subtrochanteric orientation Figure 2. W-sitting posture Figure 3. Femoral subtrochanteric asymmetry among Arikara We examined asymmetry in femoral torsion and head diameter (FHD) among three Arikara variants ranging in date from 1600 to 1845 (Table 1). The Arikara resided in villages along the Missouri River of South Dakota. They are traditionally considered horticulturalists, but they also hunted, fished, and gathered wild plants 10-11 . The Arikara were actively involved in trade. As trade networks became more important, the Arikara females increased their production of crops to have surplus for trade 9 . Computed tomography scans were taken at subtrochanteric with the femur oriented in standard sagittal and coronal planes 12 . Torsion was assessed by measuring theta, the angle between the mediolateral axis and maximum bending rigidity, at subtrochanteric (Figure 4). While this definition differs from traditional measures of torsion, it provides an accurate measure of the relative twist in the shaft. ANOVA with post hoc multiple comparison tests was used to determine sexual dimorphism and temporal differences. Females and males were examined separately for temporal trends. Asymmetry was also regressed on mean date of site occupation. Figure 4. Subtrochanteric theta Sexual Dimorphism Significant sexual dimorphism was observed for theta, theta directional asymmetry, theta absolute asymmetry, and FHD. No significant sexual dimorphism was observed in FHD asymmetry. Temporal Trends Males: No significant temporal trends were observed in theta , asymmetry of theta, or FHD. Females: Significant differences were observed in theta asymmetry between Historic Arikara and EC and PCC (Figure 5). There was also greater variation in asymmetry among the Historic Arikara. No significant differences were observed between EC and PCC. There was no temporal trend in FHD asymmetry. There is a slight positive correlation between absolute theta asymmetry and mean site date (Figure 6). HA PCC EC ABSOLUTE ASYMMETRY IN THETA- FEMALES ABSOLUTE ASYMMETRY IN THETA - MALES HA PCC EC DIRECTIONAL ASYMMETRY IN THETA - MALES DIRECTIONAL ASYMMETRY IN THETA - FEMALES EC PCC HA EC PCC HA 8.6 11.7 31.6 8.6 4.9 27.2 6.8 6.6 7.9 1.4 1.9 2.8 Sexual dimorphism in femoral torsion could be related to sex differences in hip biomechanics, but Hunt and Williams 5 found no significant sexual dimorphism in torsional asymmetry among Terry Whites and Blacks or the Puye from New Mexico. They did observe sexual dimorphism within Native American populations from Illinois, Hawikuh from New Mexico, and Arikara, and suggested that asymmetry in femur torsion is likely the result of culturally influenced biomechanical stressors. Temporal trend could be related to an increased sedentary lifestyle 13 , but the trend is only seen in females, making this an unlikely explanation. A previous study 9 showed increased strength in the left femur of females that we associated with intensified activities involving field production, planting, and harvesting necessary to produce surplus crops for trade. While this study cannot demonstrate the cause of asymmetry in femoral torsion, it is likely linked to a common sitting posture observed in historic photographs from the Great Plains (Figures 7- 9). Females habitually side-sit with one thigh medially rotated causing asymmetrical stress on the proximal femora. Males tend to sit in a cross-legged position (Figure 7) and therefore do not habitually internally rotate their thighs asymmetrically like females. Introduction Figure 5. Box plots showing variation in subtrochanteric theta by Coalescent variant for females and males FEMALES MALES Materials and Methods Results Discussion and Summary Figure 6. Temporal trend in subtrochanteric theta asymmetry by mean site date for females and males Figure 7. Historic photograph illustrating sexual dimorphism in sitting posture (Richard Throssel Collection 2394-5-3) Figure 8. Historic photograph illustrating female child side sitting Figure 9. Historic photograph illustrating sitting position of a Crow woman preparing hides (Richard Throssel Collection 2394-1-10) Historic Arikara females may have spent a greater amount of time in a side-sitting position during activities such as hide-processing, quillwork, and sewing to produce surplus for trade. In addition, Historic Arikara females may have become involved in these activities at an earlier age than in previous temporal variants. Variant Dates Female Male Extended Coalescent (EC) 1500-1650 27 13 Postcontact Coalescent (PCC) 1650 – 1780 48 52 Disorganized Coalescent or Historic (HA) 1780-1845 78 101 Table 1. Sample Size Asymmetry R-L Asymmetry R-L Asymmetry |R-L| Asymmetry |R-L| R 2 = 0.2372 R 2 = 0.1143