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First published online February 13, 2009
Journal of Experimental Biology 212, 713-721 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.019885

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Walking, running and the evolution of short toes in humans

Campbell Rolian^1,*,, Daniel E. Lieberman¹, Joseph Hamill², John W. Scott³ and William Werbel¹

¹ Department of Anthropology, Harvard University, Cambridge, MA 02138, USA
² Department of Kinesiology, University of Massachusetts, Amherst, MA 01003, USA
³ School of Medicine, Vanderbilt University, Nashville, TN 37232, USA

View larger version (11K): [in this window] [in a new window]   Fig. 1. Schematic model of musculoskeletal function in the second toe during propulsion. For simplicity, only the action of the flexor digitorum longus is depicted. (A) During propulsion, the component of the ground reaction force (GRF) acts on the distal toe pad at a distance R from the MTP joint center of rotation (black circle) to create a dorsiflexion moment at this joint. This moment is balanced by a plantarflexion moment produced by the digital flexors, which exert a force (Fflexor) acting at a distance r (lever arm) from the joint center of rotation. (B) Longer segments are hypothesized to increase the external moment arm (R'), increasing the dorsiflexion moment and digital flexor forces required to stabilize the joints (F'flexor). Other abbreviations: fdl, flexor digitorum longus tendon; MP, middle phalanx; MT, metatarsal; PP, proximal phalanx.

View larger version (27K): [in this window] [in a new window]   Fig. 2. Method for calculating external moments acting on the MTP joints in the force plate (A) and pressure plate trials (B). Black dots represent kinematic markers on the first (MTP1) and fifth (MTP5) metatarsophalangeal joints and distal phalanges of the first (DP1) and third toes (DP3). The image in B shows the peak pressure, in kPa, recorded from each sensor cell over the entire stance event. This composite image is used to determine the outlines of the hallux and lateral toe pressure zones. Note that, in this individual, there was little pressure detected under the fourth toe, and none under the fifth toe. COP, center of pressure; COP H, hallux center of pressure; COP Lat, center of pressure for the lateral toes as a group.

View larger version (6K): [in this window] [in a new window]   Fig. 3. Effect of increasing toe length on peak digital flexor force in the lateral toes, estimated using multiple regression equations derived from the study sample (see text and Table 7). Dots and vertical bars represent the predicted peak forces and their 95% confidence intervals, respectively, for four hypothetical individuals having the same body mass, contact times and toe-out angle, but with toe lengths ranging between the sample extremes (Short, Average and Long), and scaled to A. afarensis lateral toe length (afarensis-like). The afarensis-like individual was obtained by increasing the average hallux length by 40%, although it should be noted that the hallux of A. afarensis probably was not that long. The dotted line represents the average digital flexor force in the hallux (Table 2).

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