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First published online November 28, 2008
Journal of Experimental Biology 211, 3889-3907 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.020578
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Stabilization and mobility of the head, neck and trunk in horses during overground locomotion: comparisons with humans and other primates

Donald C. Dunbar1,*, Jane M. Macpherson2, Roger W. Simmons3 and Athina Zarcades3

1 Department of Anatomy and Neurobiology, and Caribbean Primate Research Center, University of Puerto Rico School of Medicine, PO Box 365067, San Juan, PR 00936, USA
2 Neurological Sciences Institute, Oregon Health and Science University, 505 NW 185th Avenue, Beaverton, OR 97006, USA
3 School of Exercise and Nutritional Sciences, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, USA


Figure 1
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  		Fig. 1. Graphic depictions of Hypotheses 1–4. Although the hypotheses refer to different gaits, a standard walking figure is used for all illustrations in order to emphasize the differences in head, neck and trunk pitch predicted by each hypothesis. Hypothesis 1 (H1); the head is free to rotate more than 20 deg. in the pitch and yaw planes on a stabilized trunk during walks. Hypothesis 2 (H2); the trunk will pitch through more than 20 deg. on a stabilized head during canters. Hypothesis 3 (H3); the head and trunk will remain rotationally stabilized (≤20 deg.) during trots. Hypothesis 4 (H4); the neck will not be rotationally stabilized in the pitch plane because it must make large (>20 deg.) compensatory movements to isolate the head from the rotational influences of the trunk, thereby enabling independent head stabilization relative to space. Arrows indicate the direction and relative magnitude of pitch rotation.

 

Figure 2
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  		Fig. 2. Measured axes and angles in the pitch plane. (A) head axis (black) was a line passing through the ear and the lower lip, neck axis (red) was a line passing through the withers (3rd thoracic vertebra) and the ear, and trunk axis (blue) was a line passing through the dock (tail base) and the withers. (B) Head angle ({theta}) and trunk angle (β) relative to space, and head angle relative to the trunk ({alpha}). (C) Neck angle relative to space ({delta}), head angle relative to the neck (µ) and neck angle relative to the trunk ({epsilon}). Horizontal broken lines indicate the earth-horizontal space reference.

 

Figure 3
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  		Fig. 3. Video tracings of the walk (A), trot (B) and canter (C). Horizontal lines indicate limb contact with the ground.

 

Figure 4
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  		Fig. 4. Graphs of vertical displacements of anatomical landmarks for single, representative cycles of the walk (A), trot (B) and canter (C). Thick horizontal lines depict support phases for the left hind limb (LH), right hind limb (RH), left forelimb (LF) and right forelimb (RF).

 

Figure 5
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  		Fig. 5. Vertical translations of the head (A), neck (B) and trunk (C) during walks (black circles), trots (red squares) and canters (blue triangles). Translation of each segment is depicted by plotting the vertical displacement of its rostral landmark (y-axis) against that of its caudal landmark (x-axis). Broken lines have a slope of 1 indicating vertical segmental translation with no rotation.

 

Figure 6
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  		Fig. 6. Segmental pitch displacements for single, representative cycles of the walk (A,B), trot (C,D), and canter (E,F). Left column (A,C,E) depicts head (H–S), neck (N–S) and trunk (T–S) pitch relative to space. Right column (B,D,F) depicts pitch of the head relative to the neck (H–N), the neck relative to the trunk (N–T) and the head relative to the trunk (H–T). Negative values denote head angles below earth horizontal (0 deg.). Thick horizontal lines depict support phases for the left hind limb (LH), right hind limb (RH), left forelimb (LF) and right forelimb (RF).

 

Figure 7
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  		Fig. 7. Pitch rotations during walks, trots, and canters of the head vs the neck (A), the neck vs the trunk (B) and the head vs the trunk (D) relative to space, and of the head relative to the neck vs the neck relative to the trunk (C). Pitch rotations of the head vs the trunk during overground walks and slow gallops in two monkey species – bonnet macaques (Macaca radiata) and hanuman langurs (Semnopithecus entellus) – in the wild (E) are presented for comparison with graph D. Horizontal and vertical broken lines indicate 20 deg. threshold for stabilization. Monkey data (E) are from Dunbar and colleagues (Dunbar et al. 2004Go) and the graph is modified from Dunbar (Dunbar, 2004Go).

 

Figure 8
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  		Fig. 8. Equine head pitch rotation (solid line) and head vertical translation (broken line) during representative walk, trot, and canter cycles. Thick horizontal lines depict support phases for the left hind limb (LH), right hind limb (RH), left forelimb (LF) and right forelimb (RF). The highly in-phase relationship depicted in the walk example is characteristic of this gait. By contrast, the highly out-of-phase relationships depicted in the trot and gallop examples are common but do not occur consistently (see Fig. 9).

 

Figure 9
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  		Fig. 9. Head pitch (solid line) and head vertical (broken line) displacements during all walk (A), trot (B), and canter (C) cycles for all three horses. Data are smoothed (6 Hz cut-off frequency for walks, 8 Hz for trots and canters) and the mean values have been subtracted for each cycle to yield change in pitch cycle and vertical position. The angular and linear displacements of the head were consistently in-phase for walks but had shifting phase relationships for trots and canters.

 

Figure 10
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  		Fig. 10. Schematic drawing comparing vector components of equine and primate (human and monkey) vestibular acceleration in response to clockwise pitch rotation. Rectangle in the 3 o'clock position represents equine vestibular apparatus whereas rectangle in the 12 o'clock position represents primate vestibular apparatus. Circle with small arrowheads depicts direction of rotation about the pitch axis. Resultant vectors (Rh, Rp) and their horizontal (Hh, Hp) and vertical (Vh, Vp) components are depicted for horses and primates, respectively.

 

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© The Company of Biologists Ltd 2008