The Sliding Foot
by
James Rooney, D.V.M.
This essay on basic mechanics is concerned with the action of the fore and hind feet from impact to midsupport. The results of this theoretical study will be compared to the limited amount of experimental data available. The basic hypothesis is that the angular movements of the fetlock and coffin joints and the sliding of the hoof on the surface are closely interrelated equilibrium functions which can be simply expressed algebraically.
This is a modified version, corrected, of an earlier version of this work on this site.
For those of you who find working through this sort of material onerous, here is a summary with the hope that you may pursue the full story. It is proposed, by me, that the angular movements of the fetlock and coffin joints are normally in phase with each other during normal locomotion. The amount of slide can be determined by use of a simple algebraic relationship between the angular displacement of the coffin and fetlock joints, The amount of sliding is somewhat greater for the hind than the fore foot and is related to the amount of angular displacement of the two joints. The amount of sliding of the forefoot at the gallop is about 1.5-2.0 cm and for the hind foot about 2.0 cm. The reason for the difference between the fore and hind is explained.
In these photographs of Muybridge it is apparent that the digits of the fore and hind legs have different kinematics as sketched in Figure 1 and by the photograph of a pacer from Muybridge, Figure 2.
 Figure 1 |
This is somewhat difficult to follow but: 1 is the impact of the leg, either fore or hind. 2 is the position of the hind leg just after impact as shown by the solid lines. 2’ is the position of the foreleg as shown by the dotted line. Both the coffin and fetlock joints have rotated together in the foreleg while the coffin has rotated before the fetlock in the hindleg. 3 repeats for the foreleg and indicates that the hind leg is now in essentially the same position. 4 is the position of both fore and hind toward the end of the support period.
 Figure 2 |
Since we need to do a bit of bastard algebra, the rotation of the fetlock will be z1 (or z1) while the rotation of the coffin will be Z2 (or z2). Z3 will be defined below.
After the fore foot impacts with the surface, the fetlock (z1) and the coffin (z2) joints move virtually simultaneously, so that movement of the leg is as shown during the first half of support (figure1). After the hind foot impacts, the coffin joint (z2) moves before the fetlock (z1), rather than simultaneously as in the foreleg.
It is known that both the hind and forefeet slide on the surface at the faster gaits. This “slide” is a translation of the hoof on the surface considered to be caused by the rotation, z 3, Figure 3
 Figure 3 |
The mathematical expression of the hypothesis for the angular displacements
and slide of the fore leg is:
Z1 -(z2+S) = 0
[1]
where S, slide, is
S = z 3.
By convention, counterclockwise is positive and clockwise is negative,
so that z1 is positive while z2 and z3 are negative.
The equation is the same for the hind leg: z1-(z2+S) = 0.
Fore leg: Using numbers taken from the photographs of Muybridge: if the angular displacement z2 = 500, a reasonable value at fast gaits, then z1 should also equal 500. It is obvious, however, that z1 is greater than z2, and there must be an equivalent amount of slide for the hypothesis of equilibrium to hold true. At the faster gaits z1 is about 700, so that 700-500=200 and 200 must be present as an equivalent slide, Figure 3.
The amount of slide is calculated as follows: the angle, 200, in radians, is multiplied by the vertical dimension, 5cm., an average distance from the compromise center of rotation of the coffin joint to the ground surface. (5 cm is a compromise number and can vary somewhat from foot to foot and horse to horse.)
0.349 x 5=1.74 cm of slide
This value compares to that found by Pardoe et al (2001): the slide of the forefoot at the trot on concrete was about 2 cm. Back et al (1995), however, calculated a slide of 5.2 cm for the forelimb at the trot on a treadmill.
Hind leg: During the first part of support of the hind leg, z2 moves to, or nearly to, 500 before z1 moves. Z1 of the hind leg is a larger angular displacement than z1 of the foreleg. By measurements on the photographs of Muybridge (for example, Fig. 2) the hind leg z1 is about 40 greater, or 240, than of the fore leg so that:
0.418 x 5=2.1cm
Back et al (1995) found 10.8 cm slide for the hind limb at the trot on a treadmill. The results are not in good agreement, but the qualitative difference between the sliding of the fore and hind foot are in agreement, at least, with the sliding of the hind foot about twice that of the fore foot.
The present calculations were compared with the angular displacement data of Roepstorff et al (1999). They averaged data for six horses at a trot on a treadmill at ~3-4m/s. and compared the results with standard iron horseshoes and barefoot. The slide calculations were done, by me, as described above and showed that barefoot or shod the amount of sliding of the fore and hind feet agreed closely with the values obtained in the present study.
The angular movements and sliding of the foot in this study are based only on the kinematics of the fore and hind legs. The report of Yxklinten, et al (1998) on barefoot, steel shod, and plastic polymer shod horses, and the work of Roepstorff and Pardoe indicate that the fundamental action can be discerned whether the foot is shod in several different ways or is unshod.
Bibliography:
Back, W, Schamhardt, HC, Hartman, W and Barneveld, A. (1995) Kinematic differences between the distal portions of the forelimbs and hind limbs of horses at the trot. American Journal Veterinary Research 56: 1522-1528.
Muybridge, E .(1957).Animals in Motion. Dover. New York
Pardoe, CH, McGuigan, MP, Rogers, KM, Rowe, LL and Wilson, AM. (2001) The effect of shoe material on the kinetics and kinematics of foot slip at impact on concrete. Equine Veterinary Journal, Suppl. 33: 70-73.
Roepstorff, L, Johnson, C and Drevemo, S. (1999) The effect of shoeing on kinetics and kinematics during the stance phase. Equine Veterinary Journal 30: 279-285.
Rooney, JR and Robertson, JL (1996).Equine Pathology. Iowa State Univ.Press.
Rooney, JR (1998).The Lame Horse. Meerdink. Neenah.
Rooney, JR (2005) http://www.horseshoes.com/farrierssites/sites/rooney/index.htm
Yxklinten , U, Johnson, C, Roepstorff, L and Drevemo, S. (1998). Olov Original and the biomechanics of horses – a comparative study between horses shod with traditional horseshoes, rubber horseshoes and barefoot. Commercial report, Swedish Agricultural University, Uppsala.
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