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Shorter distal forelimbs benefit bipedal walking and running mechanics: implications for hominin forelimb evolution

Citation

Yegian, Andrew; Tucker, Yanish; Gillinov, Stephen; Lieberman, Daniel (2021), Shorter distal forelimbs benefit bipedal walking and running mechanics: implications for hominin forelimb evolution, Dryad, Dataset, https://doi.org/10.5061/dryad.c59zw3r6p

Abstract

This file provides the data from the manuscript "Shorter distal forelimbs benefit bipedal walking and running mechanics: implications for hominin forelimb evolution" sin the American Journal of Physical Anthropology.

Objectives: Brachial index is a skeletal ratio that describes the relative length of the distal forelimb. Over the course of hominin evolution, a shift towards smaller brachial indices occurred. First, Pleistocene australopiths yield values between extant chimpanzees and humans, with further evolution in Pliocene Homo to the modern human range. We hypothesized that shorter distal forelimbs benefit walking and running performance, notably elbow and shoulder joint torques, and predicted that the benefit would be greater in running compared to walking.

Materials and Methods: We tested our hypothesis in a modern human sample walking and running while carrying hand weights, which increase the inertia (mass and effective length) of the distal forelimb, simulating a larger brachial index.

Results: We found longer distal forelimbs and the added mass increased elbow muscle torque by 98% while walking and 70% in running, confirming our hypothesis that shorter distal forelimbs benefit walking and running performance. Shoulder muscle torque similarly increased in both gaits with the addition of hand weights due to elongation of the effective forelimb length. Normalized elbow torque, which accounted for the effect on shoulder torque caused by the experimental manipulation, increased by 16% while walking but 52% while running, indicating that shorter distal forelimbs provide a greater benefit for running by approximately three-fold.

Discussion: Selection for economical bipedal walking in Australopithecus and endurance running in Homo likely contributed to the shift towards relatively smaller distal forelimbs across hominin evolution, with modern human proportions attained in Pleistocene Homo erectus and retained in later species.

Methods

The data contained here represent intersubject means and standard errors. The underlying data were collected using motion capture (Qualysys AB, Goteborg, Sweden) while subjects walked and ran on an instrumented (Bertec Corp., Columbus OH, USA). The subjects walked and ran normally and holding extra mass in their hands.

Usage Notes

The data contained here represent intersubject means and standard errors, and are presented graphically in the main paper. Elbow angle and torque, as well as shoulder angle and torque, are given as time series. The x-axis is percent of the stride cycle, and therefore is time-normalized. Note that statistical comparisons between data from the normal and added mass conditions must account for repeated measures, as each subject participated in all conditions.