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Research Article

Hip extensor mechanics and the evolution of walking and climbing capabilities in humans, apes, and fossil hominins

Elaine E. Kozma, Nicole M. Webb, William E. H. Harcourt-Smith, David A. Raichlen, View ORCID ProfileKristiaan D'Août, View ORCID ProfileMary H. Brown, Emma M. Finestone, Stephen R. Ross, Peter Aerts, and Herman Pontzer
PNAS April 17, 2018 115 (16) 4134-4139; first published April 2, 2018; https://doi.org/10.1073/pnas.1715120115
Elaine E. Kozma
aGraduate Center, City University of New York, New York, NY 10016;
bNew York Consortium in Evolutionary Primatology, New York, NY 10024;
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  • For correspondence: ekozma@gradcenter.cuny.edu hpontzer@hunter.cuny.edu
Nicole M. Webb
aGraduate Center, City University of New York, New York, NY 10016;
bNew York Consortium in Evolutionary Primatology, New York, NY 10024;
cDepartment of Anthropology, Lehman College, New York, NY 10468;
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William E. H. Harcourt-Smith
aGraduate Center, City University of New York, New York, NY 10016;
bNew York Consortium in Evolutionary Primatology, New York, NY 10024;
cDepartment of Anthropology, Lehman College, New York, NY 10468;
dDivision of Paleontology, American Museum of Natural History, New York, NY 10024;
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David A. Raichlen
eSchool of Anthropology, University of Arizona, Tucson, AZ 85721;
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Kristiaan D'Août
fInstitute of Ageing and Chronic Disease, University of Liverpool, Liverpool L7 8TX, United Kingdom;
gDepartment of Biology, University of Antwerp, 2610 Antwerp, Belgium;
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  • ORCID record for Kristiaan D'Août
Mary H. Brown
hLester E. Fisher Center for the Study and Conservation of Apes, Lincoln Park Zoo, Chicago, IL 60614;
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  • ORCID record for Mary H. Brown
Emma M. Finestone
aGraduate Center, City University of New York, New York, NY 10016;
bNew York Consortium in Evolutionary Primatology, New York, NY 10024;
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Stephen R. Ross
hLester E. Fisher Center for the Study and Conservation of Apes, Lincoln Park Zoo, Chicago, IL 60614;
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Peter Aerts
gDepartment of Biology, University of Antwerp, 2610 Antwerp, Belgium;
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Herman Pontzer
aGraduate Center, City University of New York, New York, NY 10016;
bNew York Consortium in Evolutionary Primatology, New York, NY 10024;
iDepartment of Anthropology, Hunter College, New York, NY 10065;
jDepartment of Evolutionary Anthropology, Duke University, Durham, NC 27708
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  • For correspondence: ekozma@gradcenter.cuny.edu hpontzer@hunter.cuny.edu
  1. Edited by Carol V. Ward, University of Missouri-Columbia, Columbia, MO, and accepted by Editorial Board Member C. O. Lovejoy March 1, 2018 (received for review September 10, 2017)

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    Fig. 1.

    Ischial morphology and hip mechanics. (A) The hamstrings muscle group exerts a force Fm, which results in a hip extension moment Fm × r, where r is the orthogonal distance from the Fm vector to the center of rotation for the hip. The resulting force at the knee, Fk, is equal to Fm × (r/B), where B is the orthogonal distance from the Fk vector to the center of rotation for the hip. The hamstrings group inserts on the proximal tibia and fibula, very near the knee, making femur length a useful proxy measure for B. The ratio r/B thus gives the DMA for the hamstrings group (i.e., the force Fk exerted at the knee for a given Fm). DMA is a function of hip flexion angle, Φ. Greater DMA allows the hip extensors to generate greater Fk, but also requires more shortening of the hamstrings group (i.e., muscle strain) per degree of hip extension. This graphic presents a chimpanzee, with its highly flexed hip. (B) Humans’ shorter and reoriented ischium results in lower peak DMA but a greater functional range of hip extension, enabling the hamstrings to hyperextend the hip beyond 200°, as shown here. (C) Ischial length (which defines maximum r) relative to femoral length (which defines B) in fossil and extant taxa. Solid line is the nonhuman primate linear regression (R2 = 0.89; P < 0.001), with dashed lines showing 95% prediction interval. The linear relationship test between femur and ischial length in humans yields a P value of 0.12. Arrows for fossil taxa represent ±10% range. Red dots, humans; black dots, nonhuman apes; blue dots, catarrhines; green dots, platyrrhines; green arrow, E. nyanzae; brown arrow, Ar. ramidus; orange, Au. afarensis; yellow, Au. africanus. (See SI Appendix, Tables S1 and S2.)

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    Fig. 2.

    Hip extensor DMA and locomotor mechanics. Red, Homo; gray, Pan; blue; Gorilla; purple, Pongo; green, hylobatidae. (A) Skeletally derived DMA in extant hominoids. Shaded regions represent 95% prediction interval for DMA in each taxon. (B) Bars represent mean range of flexion and extension with SDs while climbing (19) (dark colors), and while walking quadrupedally or bipedally on level ground (light colors). For Homo, only bipedal walking is shown. For level walking in Pan, both P. troglodytes (Upper) and P. paniscus (Lower) are shown. For hylobatidae, only vertical climbing is shown. (C) Hip moments (dimensionless, scaled to body mass and tibia length) in humans, chimpanzees, and bonobos. Red line, Homo level bipedal walking; gray line, P. troglodytes level quadrupedal walking; dashed line, P. paniscus level quadrupedal walking; dot-dashed line, P. paniscus 45° incline climbing; dotted line, P. paniscus vertical climbing. (D) Hamstrings electromyography activity in humans and chimpanzees and gibbons (25–27). (I) Human level bipedal walking; (II) P. troglodytes level quadrupedal walking; (III) P. paniscus 45° incline climbing; (IV) P. paniscus vertical climbing; (V) Hylobates vertical climbing. (E) Passive in vivo hip extension ranges for nonhuman primates (pooled sexes) (29) and for humans (30). (Data are in SI Appendix, Table S3.)

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    Fig. 3.

    DMA and pelvic orientation in fossil taxa. (A) Homo, pink; Pan, gray; Ar. ramidus, brown; Au. afarensis, orange; Au. africanus, yellow. Skeletally derived envelopes for hamstrings DMA. Shaded regions for Pan and Homo represent 95% prediction intervals. Shaded regions for fossils represent the full range of mechanically feasible pelvic pitch angles plus a range of reconstructed sacral breadths and femur lengths (see Methods). (B) The range of pelvic tilt angles for bipedal orientation is based on the requirement that some portion of the medial gluteals (red regions on the ilia) must be aligned vertically to oppose gravity and to stabilize the trunk during single-leg stance. In the most dorsal-superior orientation (Left), the anterior border of the medial gluteals is aligned vertically over the acetabulum; in the most ventral-inferior orientation (Right), the posterior border of the medial gluteals is aligned vertically over the acetabulum. (See SI Appendix, Table S5.)

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Hip extensor mechanics and the evolution of walking and climbing capabilities in humans, apes, and fossil hominins
Elaine E. Kozma, Nicole M. Webb, William E. H. Harcourt-Smith, David A. Raichlen, Kristiaan D'Août, Mary H. Brown, Emma M. Finestone, Stephen R. Ross, Peter Aerts, Herman Pontzer
Proceedings of the National Academy of Sciences Apr 2018, 115 (16) 4134-4139; DOI: 10.1073/pnas.1715120115

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Hip extensor mechanics and the evolution of walking and climbing capabilities in humans, apes, and fossil hominins
Elaine E. Kozma, Nicole M. Webb, William E. H. Harcourt-Smith, David A. Raichlen, Kristiaan D'Août, Mary H. Brown, Emma M. Finestone, Stephen R. Ross, Peter Aerts, Herman Pontzer
Proceedings of the National Academy of Sciences Apr 2018, 115 (16) 4134-4139; DOI: 10.1073/pnas.1715120115
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