Introduction

1.0 The hoof capsule is a three dimensional multi-directional reinforced composite structure

(Reilly 1996) and its shape and morphology are influenced by the opposing forces of ground

reaction and descending body weight acting on it during static stance and locomotion. The

external shape of the hoof capsule reflects the distribution and magnitude of the stresses and

strains that occur in the tissues and structures of the hoof during weight bearing and

locomotion (McClinchy et al 2003) (Thomason et al 2004).

Conformation will influence the orientation of the distal limb segments and as such is

fundamentally related to the bio-mechanics of the hoof and its ability to distribute and

dissipate and absorb forces. Overload of or trauma to the hoof will cause the horse to adapt its

posture. This adaption will alter joint angles at the pastern, fetlock, elbow and shoulder

(Ridgeway 2003).

1.1 Rooney’s theoretical centre of pressure (CoP)

According to Rooney (Down loaded 2007) the vertical force of the ground reaction force

(GRF) is exerted all over the bearing surfaces of the hoof in contact with the ground. In

mechanics one considers that “spread-out” force to be concentrated at a single point called

the centre of pressure (CoP). That is done in order to simplify the calculations. It does not

mean that “all” the force is concentrated at that point; it means that one can account for the

mechanics of the foot if one considers that the dispersed forces are all concentrated at that

one point. Rooney gives good mathematical and diagrammatic explanations of the linear

forces acting on the hoof that allow them to be plotted accurately on radiographs (Fig 1).

Rooney also gives a simple definition of CoP as follows: that if a triangular support were to

be placed under the horse’s foot at the centre of pressure, then at mid stance the foot would

not tip forwards or backwards but balance.

Rooney explains how if the forces in the foot are not in equilibrium then the effects will be

substantial. One example is the newborn foal with flaccid flexor tendons. In this scenario the

extensor moments of GRF and the common digital extensor tendon are greater than the

opposing pull of the deep digital flexor tendon (DDFT) and so the toe of the foot extends

raising it off the ground. With the application of a plantar extension to the foal’s foot, the CoP

moves towards the extension and behind the centre of rotation distal interphalangeal joint

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(CoR Dip) where it then becomes a flexing moment opposing the extensor apparatus and

bringing the toe of the foot back down on to the ground.

Rooney’s work was one of the first papers to consider biomechanics (1969). His work on the

centre of pressure of the equine foot is based on the application of mathematics and Newton’s

laws of physics. However, it is highly theoretical as no actual force plate data was used or

available at that time.

Figure 1. Rooney’s Basic mechanics of the foot showing linear forces and moments

(Modified After Rooney 2007 download)

As can be seen from Figure 1, at mid stance the linear forces of the distal limb and foot are in

equilibrium,

GRF (Ground reaction force) – Body weight = 0

Forward influence of body weight – surface friction = 0

Body weight + Forward influence = - R (Downward resultant vector)

CoP (centre of pressure of GRF) + surface friction = R (upward resultant vector)

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Resultant vectors R – R = 0

In addition to forces and vectors of forces acting on the foot, moments must be considered.

The definition of a moment is the perpendicular distance from the force (CoP) to the pivot

point (CoR DiP). These moments have also been considered by Wilson et al (1998) The

moments are:

CE moment - extending moment from CoR DiP to the extensor process of P3 created by

tensile forces of the common digital extensor tendon and conjoining branches of the

suspensory ligament.

CoP moment – extending moment from CoR Dip to the vertical component of CoP.

DDFT moment – flexing moment from CoR DiP to the centre of the flexor surface of the

distal Sesamoid bone created by tensile forces of the DDFT.

The moments acting on the foot in static stance are also in equilibrium as well as the forces

acting on the foot and leg. The moment of the common digital extensor tendon and branches

of the suspensory ligament (CE) combine with the moment of the CoP (centre of pressure) to

balance out the moment of the DDFT.

DDFT moment – (CE moment + CoP moment) = 0

1.2 Wilsons mathematical CoP

Forces and moments have also been considered by Wilson et al (1998), in agreement with

Rooney (2007download) Wilson states that when considering foot ground interaction it is

useful to imagine that all the force transferred by the foot is applied at a single theoretical

point on the ground surface. Wilson however, used force plate studies to investigate the effect

of imposed imbalance on the CoP by the application of toe and heel wedges to create palmar

dorsal imbalance and medial and lateral wedging of the foot to create medio lateral

imbalance.

Wilson et al (1998) concluded that the application of a standard steel horse shoe had a

minimal effect on the point of force application of stance. The application of heel wedges

delayed the unloading of the heels, while toe wedges advanced the unloading of the heels.

The position of the CoP at mid stance was unaffected by the heel wedges suggesting that they

do not unload the heels as is so often claimed. This finding explains the author’s own

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personal experience of palmar elevation of the foot where even with frog support to correct a

broken back hoof pastern axis (HPA) the heels of the hoof became crushed. This effect is

counterproductive to the rehabilitation of the long toe low heeled horses hoof capsule as the

treatment exacerbates the condition. If there is involvement of strain or rupture of the DDFT

then elevation of the heels has been shown to reduce the tensile forces within the DDFT

(Riemersma et al 1996; Willemen et al 1999). This is due to the heel elevation inducing an

increase in the extension of the metacarpophalangeal joint and an increase in the flexion of

the proximal and distal interphalangeal joints (Bushe et al. 1987; Crevier-Denoix et al 2001;

Rooney, 1984). Load is then transferred to the superficial digital flexor tendon and

suspensory ligament (Lawson et al 2004).

Application of mediolateral wedges by Wilson et al (1998) resulted in the CoP moving

towards the wedged side, this effect was more pronounced on the medial side. This Wilson et

al (1998) cited was due to the possibility that with lateral wedges the horse can adopt a more

base wide compensatory stance which leads to increased medial loading. This response would

be difficult for the horse with medial wedges as the contra lateral limbs would interfere with

each other during locomotion.

Wilson et al (2001) uses a different method for calculating the theoretical centre of pressure

or point of zero moment (PZM). The distance from the forward most ground bearing point of

the toe to a vertical line dropped from the centre of rotation(CoR) of the distal interphalangeal

joint (DiP) was used to calculate the moment arm of the PZM (CoP) see Figure 2.

As this study does not have any force plate data for the purposes of calculation, the plotting

of Wilson’s CoP is based purely on his diagram (Fig 1 page161 in his 2001 study). The PZM

bifurcated the ground bearing surface between the centre of rotation and the toe, and

intersected the dorsal wall with the perpendicular moment arm. In the event of the bifurcated

measurement not intersecting at the dorsal wall with the moment arm the bifurcated

measurement will take priority. The author’s interpretation of Wilsons diagram is seen in

(Figure2)

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Figure 2. Wilsons PZM (CoP)(Modified after)

Wilson states that since the CoP lies dorsal to the centre of rotation of the DIP joint (Schryver

et al.1978; Wilson et al. 1998), the GRF acts to extend the DIP joint. This is balanced by the

flexing moment of the DDFT. Fig. 2

Unlike the COR the COP (centre of pressure) is not a fixed anatomical point and will move or

change during the landing, loading and breakover phases of the temporal stride pattern due to

the changing position of the horses centre of mass see figure 3 .The COP originates at the

point of first impact, in the majority of horses this is the lateral heel, even though to the

human eye they appear to land level, this is because the landing phase of the horses stride

happens so fast that the human eye cannot detect the lateral landing pattern of the foot.

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Therefore the sound well conformed horse trotting in a straight line should appear to land

level (M.C.V. Van Heel et al 2004) see figure3.1

Figure 3 Lateral view showing trajectory of CoP during stance phase of temporal stride

pattern. The Percentage timings are after Wilson et al (2001).

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Landing Phase of Stride 0% - 20% Limb extended forward underneath head away from centre of mass (CoM) Lateral heel first point of impact. CoP moves rapidly forward towards mid location in foot

0% - 20% Impact (origin of CoP)

Mid Stance 50%

Fetlock descends under peak load and limb becomes vertical as CoM advances towards it. The CoP remains in a mid location from 20% onwards to 80% of stance phase.

Initiation of Breakover 80%

Heels begin to unload as tensile forces in DDFT overwhelm the extending moment of the CoP. CoM passes over foot as limb is retracted under body. Heels lift of at 80% of stance.

Toe Off 100%

From 80% onwards the CoP travels rapidly forward as heels lift and rotate around the toe. The CoM is now well advanced of the foot. The recoiling DDFT flexes the fetlock and coffin joint causing toe off and the start of the swing phase

1.3 Van Heel’s pressure force analysis work

Figure 3.1 Trajectory of CoP during stance phase of stride (after Van Heel et al 2004) and

percentage timings (after Wilson et al 2001).

The above diagram uses visual trajectory based on Van Heel et al (2004) pressure mat

analysis which is sensitive enough to plot position of first strike relative to the foot, and the

time dependant data based on Wilson’s (2001) force plate analysis.

From this first initial impact the COP advances rapidly to the middle of the hoof as loading

begins with the increasing GRF (15-20% of stance). The theoretical centre of pressure is mid

stance (maximum loading) and stays close to this mid location and doesn’t move rapidly

forward again until 75-80% of stance when the heels start to unload prior to breakover (A.M.

Wilson et al 2001).

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0% - 20% Landing

(Rapid Movement)

20% - 80% Loading

80% - 100% Unloading and Breakover (Rapid Movement)

Van Heel (2004) used her pressure force measuring system to study the effect foot trimming

had on balance. In this study she found the horses preferred way of landing was lateral

asymmetrical in both front feet and hind feet. The duration of landing was greater in the hind

limbs than the fore limbs and trimming reduced landing duration in both front and hind feet.

The horses had a fixed unrollment pattern with a maximum lateral displacement before

returning to the sagittal axis of the hoof. Trimming was found to decrease the individual left

right difference in maximum lateral displacement (Van Heel 2004)

Van Heel (2005) studied the same population of horses to see how the location of the centre

of pressure changed over an eight week shoeing cycle and how a rolled toe optimised hoof

unrollment.(2005) The results indicated that the measured shift in CoP was less than

calculated and the differences were largest in the hind feet. The hoof unrollment pattern in the

front feet stayed basically the same over the eight week cycle, but a substantial lateral shift of

the lateral trajectory of the CoP was found in the hind feet. This she concluded was due to the

horses having a limited ability to compensate for changes in hoof capsule conformation over

time, but this capacity for compensation was less in the forelimbs than in the hinds. Therefore

the relative increase in the loading of these limbs during the shoeing cycle is greater than the

hind limbs.

In van Heels 2005 study into the use of a rolled toe in the shoeing of sound warm bloods

when compared to a flat shoe, the results showed that the kinematics of the limb and temporal

stride pattern were unaffected, this was in agreement with another study (Eliashar et al 2002)

that compared the kinetics of breakover of three horseshoeing styles. However Van Heel

(2005) found the displacement, velocity and trajectory of the CoP were significantly affected

from midstance to toe off (breakover). The flat shoe had a higher single peak in the velocity

of the CoP at the initiation of heel lift whereas the rolled toe shoe had a lower velocity peak at

the initiation of heel lift followed by a second smaller velocity peak prior to toe off. The

higher peak reading with the flat shoe was interpreted to represent a more abrupt breakover

process. After mid stance the flat shoe exhibited a greater lateral displacement than the rolled

toe shoe which had a smoother more linear trajectory towards the toe and point of breakover.

Van Heel (2005) cited that the smoother unrollment of the hoof would lead to less heavy and

less abrupt changes in loading of the internal structures of the digit and therefore reduce the

risk of injury.

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The author of this paper did not have a reliable theoretical method of mapping van Heel’s

CoP measurement onto the lateral radiographs, but van Heel’s extensive works on the subject

cannot and should not be overlooked.

1.4 Duckett’s anatomical CoP

Figure 4 Duckett’s centre of pressure (CoP).

Duckett’s centre of pressure is based on anatomical reference points of the foot (Duckett

1990) and is represented by a vertical line dropped from the termination point of the common

digital extensor tendon at the extensor process down through the semi lunar crest of the distal

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phalanx where the deep digital flexor tendon terminates and down to the ground surface of

the foot (Fig 4)

Duckett believes that as the anatomical structures that are responsible for flexion and

extension of the hoof are in vertical alignment to the ground bearing surface, then this must

be the point at mid stance when the forces in the foot are in equilibrium.

Figure 5 shows where Duckett’s CoP which is often refered to as Duckett’s ‘dot’ appears on

the bottom of the shod foot. Duckett states that in the average sized horse this dot is

aproximately 3/8 inch(9.5mm) palmar to the true apex of the frog, and can be used as a guide

for dorso/palmar balance in the trimming of feet by farriers. Duckett (1990) believed when

the foot has been trimmed correctly the dorsal toe length will be equal to the distance from

the toe to the mapped centre of rotation of the distal interphalangeal joint on the solar surface

of the foot, and that this measrement would also equal the distance from the last weight

bearing point of the heels forward to the ‘dot’.

Figure 5 Duckett’s dot (CoP) hoof mapping method (Duckett 1990)

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Rooney (1969) in agreement with Duckett (1990) acknowledges that the common extensor

and extensor branches of the suspensory exert a moment on the DiP joint. It is interesting to

note that anatomically the common digital extensor tendon is conjoined by the Abaxial

branches of the suspensory ligament a little below the middle of the proximal phalanx which

greatly increases its width. It then passes over the pastern joint and inserts on the proximal

aspect of the middle phalanx and the extensor process of the distal phalanx (Hickman’s

1988).

Wilson however believes that the GRF acting at the CoP extends the DiP joint and cites that

moments from the extensor tendons and navicular ligaments are assumed to be small and are

taken as zero for his calculation (Bartel et al. 1978; Willemen1997).

1.5 Aim

The aim of this study was to define and evaluate three theoretical centres of pressure and

determine whether they had any correlations with anatomical points of interest or angles of

interest in the equine front foot.

2.0 Materials and methods

A population of 23 cadaver limbs cut off above the carpus to preserve suspensory attachment

of varying size and unknown sex were used for this study. The feet were chosen for not

exhibiting any visible foot pathology.

The feet were then mapped and marked up to be radio graphed and photographed to the

protocols as outlined below. Two markers were placed on the feet, the first was to locate

Duckett’s dot, the theoretical centre of pressure (COP) and the second was a marker of known

fixed length so as to be able to calibrate digital x-ray and photographic images using

computerised measuring software (Ontrack). The COP marker would be used by a co worker

(P.Conroy) in a parallel study to assess the accuracy of the external reference point hoof

mapping technique used on the solar ground bearing surface of the foot.

2.1 Equipment used

Hoof trimming kit-Standard farriers’ tool kit

Pneumatic press

Digital cameras Fuji Finepix & Kodak C875 Zoom

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Digital X-ray machine

Ontrack / digital software analysis system

Laptop computers

2.2 Setting up for radiographs

Prior to radiography, the room is prepared by making a cross on the floor using white sticky

tape. Care is taken to ensure that the arms of the cross intersected at 90 degrees to each other.

The press is centred over the middle of the cross and the X-ray machine was placed along the

line of the cross. A laser pen will be rigidly attached to the x-ray machine to drop a beam

distally to a marked point on the floor. This will allow the centre of the cross hairs in the

beam window of the X-Ray machine to be synchronised and accurately aligned with the arm

of the cross on the floor Having achieved correct alignment in this way the head of the X-Ray

machine is locked on its stand and remained un-altered for the duration of the radiography

session.

2.3 Radiographic method

The limbs were all loaded into the mechanical press to the same standard, this was with the

cannon bone being vertical to the floor and the bearing border of the foot in full contact with

the ground plate of the press.

For the lateral view the beam was centred 2cm vertically below the most distal hair follicle at

the mid-point between the last weight bearing point of the foot and the proximo-dorsal aspect

of the hoof wall (after Kummer et al 2006). This was achieved by using digital callipers to

determine the length of the hoof capsule. The measurement was then halved to find the mid-

point. A Wire marker of known length was placed at the dorsal wall from the distal row of

hair follicles at the coronary band. A pin is placed 9.5mm back from the true apex of the frog

(Duckett’s dot) Fig 5. The focal distance for all radiographs is 70cms.

2.4 Foot trim protocol

The trim relies heavily on the initial assessment and identification of anatomical landmarks

and proportions as such it is essential that the co-lateral sulci are clearly visible to their full

depth and that the true apex of frog is identified.

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1. Exfoliating sole is removed to live sole between the 10 and 2 o’clock position.

2. The white line is then exfoliated to reveal the sole and horny wall interface.

3. Removal of the remaining exfoliating solar horn reveals the true solar plane.

4. The bars are trimmed to normal proportions, removing only damaged or week horn.

5. The frog is trimmed back to live frog and proportionate to the foot.

6. Excess hoof wall is removed parallel to the live sole (care is taken not to trim down so

far as to invade the live sole).

7. The heels are trimmed approximately to the widest part of the trimmed frog or the

palmar/plantar aspect of the exfoliated central sulci.

2.5 Data Collection

Using digital measuring software on lateral radiographs that have had the ground bearing

surface of the foot accurately plotted on them vertical lines were used to plot the following

locations and points to be measured to see how their position and proximity at theoretical

mid-stance was related to the ground bearing surface of the foot. As the weight sex and exact

breed of the animals was unknown the limbs could not be loaded to exact forces relevant to

individual body mass.

As a result the limbs were loaded and compressed in the pneumatic press until full contact of

the ground bearing surface was made and the metacarpus was in a vertical position. The hoof

pastern axis was determined by the preserved attachment of the suspensory. Due to the

unknown variables stated above a second position for the CoR MP was mathematically

plotted onto the lateral radiographs to simulate its loaded position, an arbitrary angle of 25

degrees was chosen and applied to all x-rays. See Fig.6

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Figure 6 Shows how CoR MP 25 was plotted.

This produced CoR MP25 which was a constant for all x-rays and was a product of adding

the linear lengths between CoR MP to CoR PiP and CoR PiP to CoR DiP. This combined

linear length was plotted in a straight line from CoR DiP at 25 degrees making the

assumption that both P1 and P2 would be in alignment at this angle, this would give a better

more comparable inter limb position for the fetlock joint relative to the ground bearing

surface.

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2.6 Biomechanical points being measured. (See Figure7)

All measurements were taken from H- The last weight bearing point of the heel and datum

point for all measurements on the ground bearing surface of the foot. The points to be

measured were as follows:-

1. CoR MP- Centre of rotation of metacarpophalangeal joint.

2. CoR MP 25 - Position of CoR MP when P1 has been aligned with P2 and orientated at 25

degrees to the ground bearing surface.

3. CoR PIP- Centre of rotation of proximal inter phalangeal joint

4. CoR DiP – Centre of rotation of distal inter phalangeal joint

5. CoP DA – Duckett’s anatomical centre of pressure

6. CoP AW – Centre of pressure based on Wilsons diagram

7. CoP Roo – Rooney’s centre of pressure

8. GBS- Ground bearing surface length of the foot

9. CoP -CoR D- Distance between CoP DA and CoR DiP.

10. CoP - CoR W- Distance between CoP AW and CoR DiP

11. CoP- CoR R- Distance between CoP Roo and CoR DiP

12. HW- Heel Width

13.TA- Toe angle

14. HA- Heel angle

15. P3E - Palmar angle of distal phalanx relative to ground bearing surface.

All data will be entered into a data spreadsheet for statistical analysis purposes..

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Figure 7.The biomechanical points of interest of the distal limb and foot that have been

measured.

All measurements except those for the three CoP-CoR measurements are taken from the last

weight bearing point of the heel on the ground bearing surface of the foot. All angles are

measured relative to the ground bearing surface.

As well as the raw data analysis the linear length measurements will be converted into

percentage measurements of the GBS to remove the foot size variable and see whether any

proportional correlations exist between the anatomical points of interest and the GBS.

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3.0 Results

Of the three reference points considered Duckett’s CoP is significantly palmar to the other

two. Wilson and Rooney’s CoP are similar in location with Rooney’s being more dorsal of

the two. This pattern was consistent in all feet regardless of the conformation of the hoof

(figure 8)

3.1 Bar graphs of CoPs and CoP CoR scatter graph

E Q B I P J R M N H L O C G V T F D X S U A y0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

Duckett's Wilson's and Rooney's CoPs. CoPs arranged by ascending CoP AW

CoP AWLinear (CoP AW)CoP DALinear (CoP DA)CoP RooLinear (CoP Roo)

Feet

MM

Figure 8 A comparison of the three theoretical CoPs’

The middle value (CoP AW) was arranged into ascending order to give a better visualisation

of the relationship between the three values. As can be seen from the fit of CoP DA’s trend

line it has a slightly lower gradient (s.d7.56) the relationship between CoP AW (s.d.9.78) and

CoP Roo (s.d.10.34) is more uniform with both trend lines near parallel.

17

0 5 10 15 20 250.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

f(x) = 0.295573122529644 x + 17.0761660079051R² = 0.580001167563776

f(x) = 0.721166007905138 x + 29.4125296442688R² = 0.876503833919833

f(x) = 0.827104743083004 x + 37.47604743083R² = 0.603733481193505

CoP-CoR D vs CoP-CoR W vs CoP-CoR R. (Arranged by ascending CoP-CoR W)

C0P-CoR RLinear (C0P-CoR R)CoP-CoRWLinear (CoP-CoRW)CoP-CoRDLinear (CoP-CoRD)

Feet

Dist

ance

mm

Figure 9 A comparison of the three different CoP CoR measurements arranged by ascending

CoP CoR W.

As can be seen in figure 9 from the steeper trend line gradients Wilson and Rooney’s CoP-

CoR measurements are far more variable than Duckett’s. Wilsons CoP-CoR s.d 5.24 and

Rooney’s s.d 7.22 compared to Duckett’s s.d of 2.69. The manipulation of the CoP CoR W

values has given a closer fitting trend line, had CoP CoR R been arranged in ascending order

the effect would be to give a close fitting trend line and a more random appearance to CoP

CoR W. Due to its low s.d of 2.69 CoP CoR D remains close fitting to its trend line no matter

which of the above is manipulated.

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3.2 Table 1 Shows Pearson’s correlations and P values of raw data, correlations

calculated using Excel data analysis program. Negative correlations are shown in red.

Measurements Stronger CORRELATIONS Weaker

CoR MP 25 CoP-CoR R

0.812

P < 0.001

CoR PiP

-0.700

P < 0.001

CoR MP

0.671

P < 0.01

CoP-CoR D

0.664

P < 0.01

CoP-CoR W

0.545

P < 0.05

HA

-0.476

P< 0.05

CoR MP CoR PiP

-0.733

P< 0.001

HA

-0.541

P < 0.05

CoP CoR R

0.486

P < 0.05

C0P CoR D

0.466

P < 0.05

CoR PiP CoR DiP

0.744

P < 0.001

CoP DA

0.603

P < 0.01

CoP AW

0.503

P < 0.05

HA

0.458

P < 0.05

CoR DiP CoP DA

0.933

P < 0.001

CoP AW

0.875

P < 0.001

GBS

0.766

P < 0.001

CoP Roo

0.717

P < 0.001

CoP CoR W

0.478

P < 0.05

CoP DA CoP AW

0.960

P < 0.001

GBS

0.897

P < 0.001

CoP Roo

0.875

P < 0.001

CoP CoR W

0.679

P < 0.001

CoP-CoR D

0.634

P < 0.01

CoP AW GBS

0.978

P < 0.001

CoP Roo

0.934

P < 0.001

Cop-CoR W

0.830

P < 0.001

CoP CoR D

0.650

P < 0.01

TA

-0.525

P < 0.05

CoP CoR R

0.491

P < 0.05

CoP Roo GBS

0.963

P < 0.001

CoP-CoR W

0.905

P < 0.001

CoP-CoR D

0.800

P < 0.001

CoP CoR R

0.739

P < 0.001

TA

-0.648

P < 0.01

P3PE

-0.433

P < 0.05

GBS CoP-CoR W

0.922

P < 0.001

CoP-CoR D

0.724

P < 0.001

CoP CoR R

0.638

P < 0.01

TA

-0.636

P < 0.01

CoP-CoR D CoP-CoR R

0.807

P < 0.001

CoP CoR W

0.793

P < 0.001

HA

-0.492

P < 0.05

TA

-0.470

P < 0.05

CoP CoR W CoP-CoR R

0.834

P < 0.001

TA

-0.732

P < 0.001

HA

-0.535

P < 0.05

CoP-CoR R TA

-0.715

P < 0.001

HA

-0.610

P < 0.01

P3PE

-0.526

P < 0.05

HW

TA P3PE

0.583

P < 0.01

HA

0.535

P < 0.05

HA P3PE

0.577

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P < 0.01

P3PE

A table of critical values for Pearson correlation coefficients (Jeremy Miles download 2011)

was used to calculate the P values for the null hypothesis. As two of the radiographs did not

depict the fetlock joint, CoR MP 25 and CoR MP m were calculated at 19 degrees of freedom

on the critical values table. All other values were calculated at 21 degrees of freedom.

The P values for 19 degrees of freedom; P < 0.05 = Critical value 0.456

P < 0.01 = Critical value 0.575

P < 0.001 = Critical value 0.693

Values higher than 0.456 were significant for a positive correlation, values lower than -0.456

were significant for a negative correlation

The P values for 21 degrees of freedom; P< 0.05 = Critical value 0.433

P< 0.01 = Critical value 0.549

P< 0.001 = Critical value 0.665

Values higher than 0.433 were significant positive correlations and values lower than – 0.433

were significant negative correlations.

Certain correlations were to be expected and unsurprisingly the CoR MP correlates strongly

to the CoR PiP (courtesy of the proximal phalanx) as does the CoR PiP to the CoR DiP

(courtesy of the middle phalanx).

CoR MP 25 Had a total of six correlations the strongest correlation was a positive one with

the CoP CoR R (0.812) this had a P < 0.001. The next strongest correlation was a negative

one with CoR PiP (-0.700) this was still a P < 0.001. In order of strength the next two

correlations were CoR MP (0.671) followed by CoP CoR D (0.664) both had a higher P value

at P< 0.01. The two weakest correlations were a positive one with CoP CoR W (0.545) and a

negative one with HA ( -0.476) these both had the highest P value of P< 0.05.

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CoR MP, Had a total of five correlations and had a strong negative correlation to CoR PiP (-

0.733) with a P value of P < 0.001. The next strongest was HA but this was a negative

correlation (-0.541) with a higher P <0.05. In descending order the two weakest correlations

were CoP CoR R (0.486) and CoP CoR D (0.466) both with a P < 0.05

CoR PiP, This correlated to CoR DiP (0.744) with a P < 0.001. The next strongest correlation

was CoP DA at (0.603) with a P < 0.01. CoP AW at (0.503) and HA at (0.458) were the

weakest correlations both with a P < 0.05.Overall had a total of six correlations.

CoR DiP, The strongest correlation was CoP DA (0.933) with a P < 0.001 this would be as

expected as Duckett’s centre of pressure is an anatomical location using the extensor process

of P3 as a reference point which is an integral part of the DiP joint. The second strongest

correlation was CoP AW (0.875) with a P < 0.001. The GBS was the next strongest

correlation with (0.766) and P < 0.001 CoP Roo correlated at (0.717) and P < 0.001. The

weakest correlation was CoP CoR W at (0.478) with P < 0.05.Had a total of six correlations

overall.

CoP DA, Has a very strong correlation to CoP AW (0.960), and GBS (0.897) as well as a

strong correlation to CoP Roo (0.875) and CoP CoR W (0.679) these all had a P < 0.001, the

weakest correlation was CoP CoR D at (0.634) with a P < 0.01 Overall had a total of seven

correlations.

. CoP AW, GBS (0.978) and CoP Roo(0.934) were very strongly correlated. CoP CoR W

(0.830) was also strongly correlated and all three had a P < 0.001. CoP CoR D (0.650) had a

P < 0.01. TA had a negative correlation (-0.525) and a P < 0.05. The weakest correlation was

CoP CoR R (0.491) P > 0.05 Overall had a total of nine correlations.

CoP Roo, The strongest correlation was to GBS (0.963). It was correlated strongly to all

three CoP CoR measurements; CoP CoR W (0.905), CoP CoR D (0.800) and CoP CoR R

(0.739) all had a P < 0.001. There was a significant negative correlation to TA (-0.648) P <

0.01. The weakest correlation was a negative P3PE (-0.433) with P < 0.05 Overall had a total

of nine correlations.

21

GBS, CoP CoR W (0.922) and CoP CoR D (0.724) correlated with a P < 0.001. CoP CoR R

(0.638) correlated with a P < 0.01. A significant negative correlation with TA (-0.636) was

found with a P < 0.01.Overall had eight correlations the strongest being with CoP AW (see

above)

CoP CoR D, This correlated with the other two CoP CoR measurements CoP CoR R (0.807)

and CoP CoR W (0.793) both with a P < 0.001. The weakest two correlations were negative

HA (-0.492) and TA (-0.470) both had P < 0.05.Overall had ten correlations.

CoP CoR W, Strong correlation with CoP CoR R (0.834) P < 0.001, this was CoP CoR R’s

strongest overall correlation. There was a strong negative correlation to TA (-0.732) with a P

< 0.001. The weakest correlation is also negative HA (-0.535) with a P < 0.05. Overall had

ten correlations (see GBS above for strongest).

CoP CoR R, There is a strong negative correlation with TA (-0.715) with a P < 0.001, and a

significant negative correlation with HA (-0.610) with a P < 0.01. The weakest correlation

was a negative one to P3PE (-0.526) with a P < 0.05. Ten correlations overall (see CoP CoR

W above for strongest.

HW, No correlations were found.

TA, Positive correlations found to P3PE (0.583) giving a P < 0.01 and HA (0.535) giving a P

< 0.05. Eight correlations overall (see CoP CoR W above for strongest). .

HA, A positive correlation was found to P3PE (0.577) giving a P < 0.01.Seven correlations

overall (see CoP CoR R above for strongest).

P3PE, Correlated where mentioned above. Four correlations overall (see TA for strongest).

AS can be seen from table 1 above there are 53 correlations 19 of which involve the plotted

positions of the joints four of these correlations are purely joint related. Seventeen of the

correlations involve angle measurements 8 of which involve TA and 7 which involve HA and

4 which involve P3PE. Three of the correlations are angle to angle. All three CoP CoR

measurements correlate 10 times each which is the highest number of correlations achieved

three of these are with each other. CoP CoR R has a strong negative correlation to all three

angular measurements. CoP CoR D and CoP CoR W both negatively correlate to TA and HA.

22

3.3 Table 2 Pearson correlations and P values of percentage data. Linear measurements

are converted into percentages of GBS to remove foot size variable.

Measurements Stronger CORRELATIONS Weaker

CoR MP 25 CoP CoR

R

0.844

P < 0.001

CoR PiP

-0.799

P < 0.001

CoP CoR

W

0.718

P < 0.001

CoP CoR

D

0.711

P < 0.001

CoR MP

0.693

P < 0.001

CoR DiP

-0.681

P < 0.001

CoP AW

-0.642

P < 0.01

CoP Roo

0.577

P < 0.01

HW

0.500

P < 0.05

CoR MP CoR PiP

-0.782

P < 0.001

CoP CoR

R

0.548

CoP CoR

D

0.534

CoP CoR

W

0.471

CoR PiP CoP AW

0.832

P < 0.001

CoP CoR

W

-0.821

P < 0.001

CoP CoR

R

-0.806

P < 0.001

CoR DiP

0.802

P < 0.001

CoP DA

0.706

P < 0.001

HA

0.624

P < 0.01

CoP CoR

D

-0.500

P < 0.05

TA

0.452

P < 0.05

CoR DiP CoP AW

0.940

P < 0.001

CoP CoR

W

-0.915

P < 0.001

CoP DA

0.872

P < 0.001

CoP CoR

R

-0.825

P < 0.001

HA

0.641

P < 0.01

TA

0.510

P < 0.05

CoP DA CoP AW

0.899

P < 0.001

CoP CoR

W

-0.839

P < 0.001

TA

0.652

P < 0.01

CoP CoR

R

-0.586

P < 0.01

HA

0.580

P < 0.01

CoP AW CoP CoR

W

-0.862

P < 0.001

CoP CoR R

-0.764

P < 0.001

TA

0.593

P < 0.01

HA

0.589

P < 0.01

CoP Roo CoP CoR D

0.619

P < 0.01

CoP CoR R

0.615

P < 0.01

CoP-CoR D CoP CoR R

0.678

P < 0.001

CoP CoR

W

0.480

P

CoP-CoR W CoP CoR R

0.822

P < 0.001

HA

-0.563

P < 0.01

CoP-CoR R

HW TA

0.435

P < 0.05

TA HA

0.535

P < 0.05

HA P3PE

23

As can be seen from table 2 there are fewer correlations overall when the foot size variable is

removed by converting the raw data into percentages of the GBS, although some

measurements have had significant increases to their number of correlations.

CoR MP25, CoP CoR R correlated the strongest (0.844) this was stronger than its linear

correlation. CoR PiP once again had a negative correlation but was also stronger than its

linear value (-0.799). CoP CoR W was much stronger than it’s linear value (0.718). CoP CoR

D was stronger than linear value (0.711) as was CoR MP (0.693). CoR DiP had not

previously correlated but had a strong negative correlation (-0.681), all these correlations had

a P < 0.001. CoP AW was also a new and negative correlation (-0.642), a new but positive

correlation was CoP Roo (0.577) these both had a P < 0.01. Finally HW (0.500) which did not

correlate at all in the raw data set became significant with a P < 0.05. CoR MP25 had nine

correlations overall (an increase of three)

CoR MP, CoR PiP was the strongest with a stronger negative correlation than in table 1 (-

0.782) and a P < 0.001. CoP CoR R (0.548) was slightly stronger than its linear value, as was

CoP CoR D (0.534) a corresponding increase now made CoP CoR W significant (0.471). All

three had a P < 0.05. CoR MP had five correlations overall.

CoR PiP, This correlated strongest with CoP AW (0.832) this was much stronger than its

linear value. CoP CoR W (-0.821) and CoP CoR R (-0.806) were both negatively correlated

and hadn’t been significant in the raw data. CoR DiP (0.802) correlated stronger as a

percentage and had been the strongest correlation in the raw data. CoP DA (0.706) was also

stronger as a percentage. All the above correlations had a P < 0.001. HA correlated stronger

than previously in raw data (0.624) with a P < 0.01. CoP CoR D (-0.500) had become

significant with a negative correlation and TA had a positive correlation (0.452), both had a P

< 0.05. CoR PiP had ten correlations overall (an increase of four).

24

CoR DiP, Strongest correlation was CoP AW (0.940) this was stronger than its previous

value in table 1. CoP CoR W (-0.915) had changed from a positive weak correlation in table 1

to a strong negative one here. CoP DA (0.872) was slightly weaker than in table 1 where it

had been the strongest correlation. CoP CoR R (-0.825) had become significant with a strong

negative correlation. All the above correlations had a P < 0.001. HA (0.641) had become

significant with a positive correlation P < 0.01. TA (0.510) had a positive correlation with a P

< 0.05. Overall CoR dip had eight correlations (an increase of two).

CoP DA, The strongest correlation was with CoP AW (0.899) but this was not as strong as its

correlation in table 1. CoP CoR W (-0.839) had been transformed from a positive correlation

in table 1 to a strong negative correlation here. These first two correlations had a P < 0.001.

TA (0.652) had become a significant correlation. CoP CoR R (-0.586) had become a

significant negative correlation, HA (0.580) had become a significant correlation. These last

three correlations had a P < 0.01. Overall CoP DA had six correlations (a decrease of one).

CoP AW, The strongest correlation was with CoP CoR W (-0.862) this had transformed from

a positive correlation in table 1 to a slightly stronger negative one here. CoP CoR R (-0.764)

had also transformed from a positive value in table 1 to a negative stronger value here, both

had a P < 0.001. TA (0.593) was slightly stronger but now a positive correlation, HA (0.589)

had become significant, both had a P < 0.01. Overall CoP AW had eight correlations (a

decrease of one)

CoP Roo, Strongest correlation was with CoP CoR D (0.619) this was weaker than its value

in table 1. CoP CoR R (0.615) was also weaker than its raw data correlation, both had a P <

0.01. Overall CoP Roo had three correlations (a decrease of six).

25

CoP CoR D, Correlated strongest with CoR MP 25 (see above). CoP CoR R (0.678) positive

correlation that was not as strong as its value in table 1 with a P < 0.001. CoP CoR W (0.480)

was also weaker, P < 0.05. Overall CoP CoR D had six correlations (a decrease of four).

CoP CoR W, A very strong correlation with CoP CoR R (0.822) not as strong as previous

value in table 1. P < 0.001.HA (-0.563) had a negative correlation that was stronger than its

value in table 1. Overall CoP CoR W had seven correlations (a decrease of three)

CoP CoR R, All correlations listed above. Overall CoP CoR R had 9 correlations (a decrease

of one)

HW, Strongest correlation was with CoR MP 25 (see above). TA (0.435) with a P < 0.05.

Overall HW had two correlations (an increase of two).

TA, A significant but not strong correlation to HA (0.535) this gave a P < 0.05. TA had five

correlations (a decrease of three).

HA, Is correlated to P3PE (0.577) P < 0.02 .Overall HA had six correlations (a decrease of

one).

P3PE Correlated to TA and HA and as the angles cannot be transformed into a percentage the

correlation values are the same as for table 1.

26

4.0 Discussion

4.1 The effects of transforming raw data into percentages.

As can be seen from data Tables one and two transforming raw data into percentages can

cause correlations to change from positive to negative and an example of how this can happen

is given below. In the raw data, CoP AW has strong positive correlations with CoR DiP and

CoP CoR W see 3.2 Table 1. This is because CoP AW is the linear distance from the heel to

the midpoint between CoR DiP and the toe and CoP CoR W is the linear measurement from

CoR DiP to CoP AW. It is easy to see how an increase in size in one will increase the size of

the others. The bigger the foot the bigger the measurements and the strength of the correlation

will be down to how proportional the measurements stay to each other across the range of

foot sizes. In the raw data all foot A measurements are greater than foot T measurements.

Raw data

CoR DiP Foot A= 62.01mm Foot T=40.48mm

CoP AW Foot A= 104.76mm Foot T=87.93mm

CoP CoR W Foot A= 42.74mm Foot T=41.45mm

GBS Foot A= 145.16mm Foot T=129.38mm

When transformed into percentages;

CoR DiP Foot A = 42% Foot T = 32%

CoP AW Foot A = 71% Foot T = 66%

CoP CoR W Foot A = 29% Foot T = 34%

GBS Foot A = 100% Foot T = 100%

As can be seen because CoR DiP in foot A is in a more dorsal location relative to the GBS

there is only 58% of the ground bearing surface left to bifurcate to give the CoP CoR W and

CoP AW measurements (CoR DiP + CoP CoR W = CoP AW) the CoP CoR W measurement

27

for foot A is 5% smaller than foot T whose more palmar location of CoR DiP left 68% of the

GBS to bifurcate giving the higher percentage value of 34%. To put another way CoP AW +

CoP CoR W must = 100% so CoP AW can only increase at the expense of CoP CoR W. The

percentage data is useful in comparing the proportional positions of the anatomical structures

such as the centres of rotation relative to the ground bearing surface of the foot, but due to the

situation highlighted above the raw data is more significant for the moment arms and their

comparison to the angle measurements.

4.2 Heel angle correlations

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.000.00

10.00

20.00

30.00

40.00

50.00

60.00

HA vs CoR MP

HALinear (HA)

COR MP mm

HA d

egre

es

Figure 10 Negative correlation between HA and CoR MP

The above correlation (figure 10) shows the raw data negative correlation between heel

angle (HA) and the fetlock joint (CoR MP) P< 0.05. It would appear that the relative position

of the fetlock joint to the ground bearing surface has a significant effect on heel angle. The

further the fetlock is behind the last weight bearing point of the heels then the lower the heel

angle.

28

30.00 35.00 40.00 45.00 50.00 55.00 60.00 65.000.00

10.00

20.00

30.00

40.00

50.00

60.00

Ha vs CoP CoR R

HALinear (HA)

CoP CoR R mm

HA d

egre

es

Figure 11 Shows negative correlation between HA and CoP CoR R.

Not only does heel angle seem to be influenced by the remote force of the fetlock joint but

also by the extending moment arm of the distal interphalangeal joint. All three CoP CoR

measurements (moment arms) were negatively correlated to heel angle. CoP CoR R (Figure

11) was the strongest raw data correlation of HA (negative), P< 0.01. CoP CoR D the weakest

of the three. This was consistent with the pattern that the moment arms exhibited on all feet,

this being that Duckett’s moment arm was always the shortest and Rooney’s always the

longest. HA was positively correlated to TA so this would explain its strong correlation to

CoP CoR R as this was plotted by taking a line from the centre of rotation of the distal

interphalangeal joint and running it down to the ground parallel to the dorsal wall.

However this was not the case with the CoP CoR measurements and heel angle when the raw

data was transformed into percentages of the ground bearing surface, and shows the

importance of working with the raw data before it is transformed as information can be lost.

CoP CoR D was weakened to become insignificant (-0.492 to -0.206) as was CoP CoR R (-

0.610 to -0.425). Interestingly CoP CoR W became stronger (-0.535 to -0.563 and P< 0.05 to

P < 0.01). When the CoP CoR percentage data was plotted on scatter graphs with HA data

organised in ascending order, the graph with the best fit trend lines was the CoP CoR W

graph.(see Figure 12).

29

0 5 10 15 20 250.00

10.00

20.00

30.00

40.00

50.00

60.00

CoP CoR's % vs HA degrees (arranged by ascending HA)

CoP-CoRDLinear (CoP-CoRD)CoP-CoRWLinear (CoP-CoRW)C0P-CoR RLinear (C0P-CoR R)HALinear (HA)

Feet

CoP

CoR'

s % /

HA

degr

ees

Figure 12 CoP CoR percentages of GBS compared to HA.

As can be seen from Figure 12 above as a percentage the CoP CoR W value is always

approximately 30% hence the smooth fit trend line ,this appears to be the reason for the slight

increase in correlation, the CoP CoR R percentage data is more randomly arranged around its

trend line. As the foot size variable is not as significant when comparing lever arm

measurements to angle measurements as was shown by the other two CoP CoR measurements

correlations becoming insignificant and the fact that an angle cannot be transformed into a

percentage, the author does not regard this percentage CoP CoR W (see Figure 13) correlation

as significant as the raw data CoP CoR correlations.

30

20.0025.00

30.0035.00

40.0045.00

50.0055.00

25.0026.0027.0028.0029.0030.0031.0032.0033.0034.0035.00

CoP-CoRW% vs HA

CoP-CoRWLinear (CoP-CoRW)

Heel Angle degrees

CoP

CoR

W %

Figure 13 CoP CoR W % negatively correlated to HA, P< 0.01.

4.3 Mechanics of the fetlock joint

As the spatial positions of the metacarpophalangeal joint (CoRMP25 and CoR MP) relative to

the ground bearing surface are influential on both heel angle and moment arms of the distal

interphalangeal joint (CoP CoR), then the mechanical effects of the fetlock joint on the hoof

cannot be overlooked.

In the introduction it was explained how the basic mechanics of the foot worked and how the

linear forces were in equilibrium (Figure 1), and that when they were not in equilibrium the

effects would be substantial. The example given was the foal with flaccid tendons. The reason

why the extending moment, when greater than the flexing moment can actually lift the toe off

the ground is because of the oblique alignment of the phalangeal bones and the palmar

position of the fetlock joint relative to the ground bearing surface of the foot.This causes the

heels to become a fulcrum to the remote force of descending body weight. The author

theorises how the mechanics of the fetlock joint may account for the results found (see Figure

14).

31

Figure 14 Theoretical equilibrium diagram of metacarpophalangeal joint. (R.J. Mather 2011)

Descending body weight – GRF = 0

DDFT clockwise flexing moment – Anti-clockwise extending moment = 0

Unsupported component of CoR MP anti-clockwise extending moment (UCEM) +

Descending body weight = - R (Resultant remote compressive force on heels)

Unsupported component of CoR MP (fully loaded) anti-clockwise extending moment

(UCEM fully loaded) + Descending body weight = - R1 (Increased resultant compressive

force on heels).

As the CoR MP descends in an arc tension in the DDFT increases, at the same time a

corresponding and proportional increase occurs in the length of the unsupported component

32

of the anti-clockwise extending moment (UCEM). In this study the last weight bearing point

of the heel was the datum point for all measurements relative to the GBS, therefore CoR MP

and CoR MP25 are both linear measurements of UCEM. Given the negative correlations in

this data set between heel angle and CoR MP and CoR MP25 it would appear that UCEM

acts as a lever arm for the remote force of descending body weight travelling down the leg

that has a compressive or in extreme cases a crushing effect on the heels. If this theory is

correct then there would be a measureable and physical effect on the trajectory of the CoP.

4.4 Fetlock joint and CoP trajectory

In Figure 3.1 the trajectory of the CoP is plotted on the solar surface of the foot (after Van

Heel et al 2004) as can be seen the CoP advances forward rapidly to just dorsal of the apex of

the frog but then makes a palmar regression to a location at mid stance just palmar and medial

to the apex of the frog. Van Heel gave no explanation for this regression in the trajectory of

the CoP, and as the horses were trotted over the pressure force measuring system in a straight

line and constant velocity then the centre of mass of the horse would have been moving

smoothly forward in relation to the foot once it had landed and loaded, so a more linear

progression of CoP would be expected.

Tensile forces in the DDFT are trying to flex the foot against the GRF and are responsible for

the advancement of the CoP to its position just before mid stance in front of the frog apex. As

stated above the fetlock moves in a palmar distal arc as it loads with descending body weight

relative to the GBS and, because a greater lever arm can exert a greater load from an applied

force, as the UCEM increases to its maximum length (mid stance, fetlock fully descended) it

momentarily pulls the CoP rearward (Figure 3.1). These interacting and opposing

mechanisms of the fetlock and foot may also explain why after rapid advancement the CoP

occupies a mid location in the foot for so long (55% - 60% of stance phase), before rapidly

advancing again at the onset of breakover (Figure 3).

4.5 HA and CoR DiP

As can be seen from the negative correlation between HA and CoR MP (figure 10) indicates

higher loading forces or prolonged loading forces causing the heels to collapse and run

forwards. This was further backed up with data from this work by the percentage correlation

of HA against CoR DiP (figure 15) Although this is an angle being compared to a

transformed linear measurement the removal of the foot size variable is important in

33

establishing the position of CoR DiP in relation to the GBS. The CoR DiP when calculated as

a percentage of the GBS (figure 16) to remove the foot size variable was found to have a

mean of 38.12% and a median value of 38.67% the standard deviation was 3.77. This

aproximates with the 60:40 split found in previous study (Mather 2009)

30.00 35.00 40.00 45.00 50.000.00

10.00

20.00

30.00

40.00

50.00

60.00

HA vs CoR DiP%

HALinear (HA)

CoR DiP % of GBS

HA d

egre

es

Figure 15 Positive correlation of HA against percentage CoR DiP

In the previous study using Duckett’s hoof mapping technique (Mather 2009) the CoR DiP

was found to divide the ground bearing surface of the foot in a 60:40 dorso/palmar split and

not bi-sect the ground bearing surface of the foot in a 50:50 split as is so commonly claimed

in many farriery and veterinary texts, one such example being No foot No Horse (Williams,

Deacon 1999). This finding did concur with another study (Craig et al 2005), in not

supporting this 50:50 split. Their study found an average of 67.06% of the ground bearing

surface in front of the COR but had a higher s.d of 5.41% although their study was, like this

one based on radiographic evidence and not external reference points. They did not state what

foot trimming protocol was used in their study.

34

A B C D E F G H I J L M N O P Q R S T U V X y0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

CoR DiP as % of GBS

CoR DiP

FEET

MM

Figure 16 Shows the variability of the CoR DiP as a percentage of the ground bearing surface

of the foot.

The above graph shows that as the heel angle reduces, the percentage distance from the last

weight bearing point of the heels to the CoR DiP also reduces indicating under run heels

causing the GBS to migrate forward relative to the CoR DiP. The CoR DiP distance varied

between 31.29 % - 45.15 % of the GBS (Figure 16).

The three theoretical CoPs all correlate strongly with each other as can be seen in figure 8 this

is to be expected as they were all plotted or calculated using the distal interphalangeal joint as

a reference point although CoP Roo also used the dorsal wall angle as a reference point, and

as a result in the raw data all three correspondingly have a strong positive correlation with

CoR DiP. All three also correlate strongly with GBS, CoP AW being the strongest (Figure

17) this is because of their proportional consistency regardless of foot size. Surprisingly in the

percentage data CoP Roo no longer correlated with the other two CoPs or CoR DiP this is

believed to be because the CoP Roo position is mainly dictated by the dorsal toe angle.

35

80.00 100.00 120.00 140.00 160.00 180.000.00

20.00

40.00

60.00

80.00

100.00

120.00

CoP AW vs GBS

CoP AWLinear (CoP AW)

GBS mm

CoP

AW m

m

Figure 17 Shows positive correlation of raw data CoP AW vs. GBS

The above graph shows how the CoP AW remains proportional to the GBS across the range

of foot sizes. The CoP CoR measurements are the moment arms of the distal interphalangeal

joint according to Duckett, Wilson and Rooney. In the raw data all three are negatively

correlated to toe and heel angles (figure 18), so as the CoP CoR measurements increase the

toe and heel angles decrease (long toe low heel) as these are extending moments then it can

be assumed that a corresponding increase in the tensile forces of the DDFT will also be

required to keep the corresponding flexor moment in bio mechanical equilibrium.

20.00 30.00 40.00 50.00 60.00 70.0040.0042.0044.0046.0048.0050.0052.0054.0056.0058.0060.00

CoP CoR R vs TA

TALinear (TA)

CoP CoR R mm

TA d

egre

es

4.6 Figure 18 CoP CoR R had the second strongest negative correlation to TA, P<0.001.

36

CoP CoR R was the only CoP CoR measurement to correlate with P3PE (Figure 19) as well

as TA, HA.

-2.00 0.00 2.00 4.00 6.00 8.00 10.000.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

C0P-CoR R vs P3PE

C0P-CoR RLinear (C0P-CoR R)

P3PE degrees

Cop

CoR

mm

Figure 19 CoP CoR R negatively correlated to P3PE – 0.526 P<0.05

37

The three CoP CoRs are positively correlated to CoR MP 25 (unsupported component of

fetlock extending moment arm) so the greater the CoR Mp 25 the greater the CoP CoR

measurement (figure 20). The implication of this is that the direct and remote leverage forces

acting on the lower limb and foot are directly responsible for and reflected in the physiology

and morphology of the hoof capsule.

30.00 35.00 40.00 45.00 50.00 55.00 60.00 65.000.00

10.0020.0030.0040.0050.0060.0070.0080.0090.00

100.00

CoR MP25 vs CoP CoR

CoR MP25Linear (CoR MP25)

CoP CoR R mm

CoR

MP2

5 m

m

Figure 20 The positive correlation between CoR MP25 and CoP CoR R.

4.7 Study limitations and future studies.

In this experiment the superficial and deep digital flexor tendons were not clamped when the

limbs were loaded into the press so the orientation of the pastern under load was maintained

purely by the attachment of the suspensory ligament. In the live horse overextension of the

proximal interphalangeal joint during loading is resisted by contraction of the superficial

digital flexor muscle, this increases the tension in the superficial digital flexor tendon which

tightens against its insertions on the distal end of the proximal phalanx and proximal end of

the middle phalanx, preventing the pastern joint from buckling (Stashak 2002).

This lack of tension from the superficial digital flexor tendon may have had an effect on the

pastern alignment of the feet when loaded in the press; this would influence their relative

position to the GBS. Furthermore the body weight of the horses was not known so the limbs

could not be loaded to 30% of body weight as would be the case when standing still and

square. As the assumption was made that P1 and P2 would be in alignment when CoR MP25

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was plotted the position of the proximal interphalangeal joint at this angle (CoR PiP25) was

not plotted. The author feels that, with hindsight, CoR PiP25 should also have been plotted

given the variables mentioned above. As P1 and P2 were aligned at the same angle then CoR

PiP 25 and CoR MP25’s positions relative to the GBS would have been dictated purely by

their linear lengths, so any correlation between them and other anatomical features and

measurements would have been more relevant. The proportional lengths that P1 and P2 are,

of their own combined length, may be influential to the posture of the hoof pastern axis in the

live horse at mid stance. As has been shown by the results a longer moment arm (CoP CoR)

leads to a lower toe and heel angle and a larger unsupported component of the fetlock

extending moment, so it is safe to assume that a very short CoP CoR measurement would

give a very upright toe and heel angle and very short unsupported component of the fetlock

extending moment. It is widely regarded in both farriery and veterinary professions that

neither extreme is desirable, horses with long toes and sloping pasterns are more prone to

tendon injuries and short toed upright pastern horses are more prone to concussive injuries of

the hard tissue. Therefore for any given foot size there will be an optimum range of moment

arm lengths that will allow the lower limb and foot to function to their maximum potential

while remaining within their safe stress strain limit. With more research this could lead to the

possibility of a scientific method for scoring the conformation of the lower limb and foot

radiographically as opposed to the more subjective methods that are generally used. Clearly

more research is needed and an ideal future study on live horses using a pressure force

measuring system to measure the actual, as well as theoretical CoP on three groups with

alternative characteristics in foot conformation would be desirable. The first group being

horses that are long toe low heel and the second group being horses with “normal” feet, the

third group would have upright feet. This would help highlight the relationships between the

mechanics, ground reaction forces and posture of the horse.

4.8 Conclusion

Some of the correlations although strong are to be expected as they are between two

anatomical structures (joints) that are directly connected by a bone, even so these correlations

would be more relevant in a live horse study as they are a product of the angle of the bone

and its linear length and would give an indication of the posture of the horse and how the

distal limb orientation affects the hoof capsule. These spatial positions of the distal three

joints of the horse’s forelimb, relative to the ground bearing surface of the foot and its

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associated ground reaction forces and resulting moment arms , given the results of this study,

may affect and influence the conformation of the hoof capsule.

In this study I have shown the strong correlations and theorized about possible relationships

between biomechanical lever arms, spatial positions of joints and angulations’ of toe and heel.

However I cannot determine the causal relationships between the lever arms and hoof capsule

morphology because of the experimental design, as all the data was collected from cadaver

limbs. If the structural integrity of a functional hoof is overwhelmed and fails (heels collapse)

the lever arms will be increased further exasperating the condition, or does the skeletal

architecture of the lower limb dictate the lever arms which then determines the mechanical

efficiency of the hoof and therefore its ability to resist stress and accommodate strain. How

these relationships exist can only be determined from a prospective live animal study. This

would need an immature population of horses to be followed over an extended period of time

to see how their hooves, conformation and posture developed.

5.0 Manufacturers’ addresses

Ontrack Equine Software, c/o Lameness Solutions LLC, PO Box, 152 Lake Elmo, Minnesota

55042, USA. www.ontrackequine.com

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