Comparison of a joint coordinate system versus multi ...€¦ · 24.05.2007 · Analysis System,...

Comparison of a joint coordinate systemversus multi-planar analysis for equinecarpal and fetlock kinematics

MC Nicodemus1,*, HM Clayton2 and JL Lanovaz31Department of Animal and Dairy Sciences, Mississippi State University, Box 9815, MississippiState, MS 39762, USA2Department of Large Animal Clinical Sciences, McPhail Equine Performance Center, MichiganState University, East Lansing, MI 48824-1314, USA3College of Kinesiology, University of Saskatchewan, PAC 222, 87 Campus Drive, Saskatoon,Saskatchewan, Canada S7N 5B2*Corresponding author: [email protected]

Submitted 24 May 2007: Accepted 13 February 2008 Research Paper

AbstractA kinematic multi-planar analysis (MPA) reduces a subject’s three-dimensional motion to two-dimensional pro-jections onto planes defined by a fixed global coordinate system (GCS). An alternative to this kinematicmethod is a joint coordinate system ( JCS) that describes the three-dimensional orientation of the segmentscomprising the joint with respect to each other so that the JCS moves dynamically with the horse’s anatomy.Therefore, the objectives of this study were to locate where differences may occur between the joint motionmeasurements made using MPA and those made using JCS and to document why these differences in measure-ments may occur. A Peruvian Paso was recorded during six walking trials using 60 Hz video camcorders. Skinmarkers tracked the movements and defined the anatomical axes of antebrachial, metacarpal and proximalphalangeal segments. A JCS was established between the two segments comprising the carpal and fetlockjoints to measure flexion/extension, internal/external rotation and adduction/abduction at each joint. TheMPA model used two markers aligned on the long axis of each segment and measured flexion/extensionangles projected onto the sagittal plane of the coordinate system and adduction/abduction angles projectedonto the frontal plane of the coordinate system. Carpal and fetlock flexion/extension angles for the walkwere similar for the JCS and MPA (peak absolute difference: carpal joint ¼ 7 ^ 48 and fetlockjoint ¼ 7 ^ 28), indicating that sagittal plane analysis using MPA is adequate when flexion and extension arethe only measurements being made provided the horse’s plane of motion is aligned with the plane of cali-bration. There were relatively larger differences in carpal and fetlock adduction/abduction angles measuredusing an MPA compared with a JCS. Peak absolute difference between the JCS and MPA adduction/abductionangles occurred at 53% of the stride for the carpus (17 ^ 48) and at 61% of the stride for the fetlock(123 ^ 258). Analysis of the reasons for these differences indicated that the accuracy of frontal plane analysisto measure adduction/abduction is limited by its inability to correct for out-of-plane rotations along the longaxis of the segments comprising the joint.

Keywords: joint coordinate system; multi-planar analysis; equine kinematics

Introduction

The majority of studies describing the equine gait

measured the movements of the limb segments and

joints of the horse relative to a global coordinate

system (GCS) that is fixed in space and oriented alongthe direction of travel of the subject1–6. Often, segments

are defined by two points placed along the long axis of

the segment with the relative angles between two adja-

cent segments defining the joint angles. The three-

dimensional coordinates of the points are projected

onto the two-dimensional sagittal and frontal planes to

give a multi-planar analysis (MPA) of segmental and

joint motions. MPA has been used to describe verticaldisplacement, flexion/extension and adduction/abduc-

tion of the fetlock joint7 and the stifle and tarsal

Comparative Exercise Physiology 5(1); 43–55 DOI: 10.1017/S1478061508945965

qCambridge University Press 2008

joints8. A limitation to MPA occurs when the segmental

motion is not parallel to the projection plane, causing

the angular measurements in that plane to be distorted.

An alternative to MPA is to use an anatomically

based coordinate system established on each rigid seg-

ment. The joint angle can then be expressed as relative

rotations between two adjacent segments. One

method used to accomplish this is known as a jointcoordinate system ( JCS)9. Equine studies have applied

a JCS to describe three-dimensional joint angles of the

limbs during the walk10,11 and trot12,13. Results were

expressed in terms of flexion/extension, adduction/

abduction and internal/external rotation. However, a

limitation to three-dimensional analysis using JCS is

that the three-dimensional marker placement require-

ments are restrictive and prone to marker motionerror, requiring bone-fixed markers using bone pins,

which is not practical outside of the research environ-

ment. These limitations promote those researchers

doing performance-based studies to utilize MPA. There-

fore, the objectives of the study were to locate where

differences may occur between the joint motion

measurements made using MPA and those made

using JCS, and to document why these differences injoint motion measurements may occur.

Materials and methods

SubjectThe subject was a sound, registered Peruvian Paso

ridden at the walk. This breed was chosen because it

is noted for a characteristic outward motion of the

forelimbs originating from the shoulders during theswing phase of all its gaits, which is known as a ter-

mino14. Limb motion during termino offers an ideal

opportunity to study three-dimensional limb motion

of the sound horse. The subject was selected for its

ability to perform a distinctive termino during the

walk that represented breed standards15.

JCS segmental axesThe origin of the antebrachial coordinate system waslocated on the lateral styloid process on the distal

radius. The antebrachial x-axis was directed laterally

from the origin perpendicular to the antebrachial sagit-

tal plane and on the same vector as a line from the

medial to lateral styloid processes. The antebrachial

z-axis, contained in the antebrachial frontal plane,

ran from the origin proximally along the long axis of

the segment at 908 to the x-axis. The antebrachialy-axis ran dorsally from the origin, perpendicular to

the antebrachial frontal plane.

The origin of the metacarpal coordinate system was

located on the lateral aspect of the distal end of the

third metacarpus. The metacarpal x-axis was directed

laterally from the origin perpendicular to the

metacarpal sagittal plane and on the same vector as a

line from the medial to lateral distal aspect of the third

metacarpus. The metacarpal z-axis, contained in the

metacarpal frontal plane, ran from the origin proximally

along the long axis of the segment at 908 to the x-axis.

The metacarpal y-axis ran dorsally from the origin,

perpendicular to the metacarpal frontal plane.

The origin of the proximal phalangeal segment waslocated on the lateral aspect of the distal end of the

proximal phalanx. The z-axis of the proximal phalan-

geal segment ran from the origin proximally along

the long axis of the segment. Similar to the other seg-

mental axes, the proximal phalangeal x- and y-axes ran

laterally and dorsally from the origin, respectively.

MPA segmental axesThe axes of the antebrachial, metacarpal and proximal

phalangeal segments used for the MPA corresponded

to the z-axis of the segments as defined by the JCS.Therefore, the MPA segmental axes passed along the

long axes of the antebrachial, metacarpal and proximal

phalangeal segments.

Data collectionKinematic data were collected at 60 Hz using four

Super VHS camcorders (Panasonic AG-450, Matsushita

Electric Corp., Secaucus, NJ, USA), arranged around

the data collection area in increments of 608. One cam-

corder was located towards the front of the horse, one

towards the back of the horse and two towards thelateral aspect of the right side of the horse. A 500 W

lamp was placed behind each camcorder to illuminate

the retroreflective markers during recording. The cali-

bration volume (342 £ 190 £ 210 cm) was defined

using 30 equally spaced points.

The horse was filmed while standing and while

being ridden along the runway at a walk for six strides.

A trial was considered acceptable if the horse travelledat a consistent velocity (2.50–2.56 m s21) along a

straight line that was parallel to the plane of calibration

with all markers being clearly visible in the two lateral

camcorder views. The walk needed to be a symmetri-

cal, four-beat stepping gait with a lateral footfall

sequence and regular rhythm14. Gait performance

followed breed standards15.

Marker placementReflective markers were attached to the skin overlying

well-defined bony landmarks on the proximal and

distal ends of the antebrachial, metacarpal andproximal phalangeal segments on the lateral aspect

of the limb (Fig. 1). The tracking markers were located

at sites that have been shown to have relatively little

skin displacement during the walk16.

Segmental coordinate systems for the JCS were defined

by the two lateral markers used for MPA together with a

MC Nicodemus et al.44

third marker attached on the medial styloid process of the

radius, the metacarpal attachment of the medial collateral

ligament of the fetlock and the medial aspect of the distal

end of the proximal phalanx. The medial markers were

tracked using a virtual targeting system17,18 utilizing a

fourth marker located on the cranial aspect of each seg-

ment, midway between the two lateral markers. The

horse was filmed standing before the walking trials toestablish a relationship between the tracking markers

and the segmental coordinate system. A transformation

matrix was applied to transform the tracking markers

locations in the standing file taken from the global

coordinate system (GCS) to their respective segmental

coordinate systems12.

Two reflective markers were placed along the spi-

nous processes of the vertebral column (along thethoracic vertebrae, approximately along the sixth thor-

acic spinal process and the tuber sacrale at the point of

the croup) to determine the horse’s angle of travel rela-

tive to the GCS during the trials. This information

was used to align the horse’s plane of motion with

the GCS using a simple two-dimensional rotational

transformation (Fig. 2).

Data reductionThe skin markers were automatically tracked and

digitized using a video analysis system (Ariel Performance

Analysis System, Ariel Dynamics, Inc., Trabuco Canyon,

CA, USA). Three-dimensional locations of markers wereobtained using direct linear transformation19. The

three-dimensional coordinates were filtered using a

fourth-order Butterworth digital filter at a cut-off fre-

quency of 6 Hz. The joint motion in the MPA was

described for the distal segment relative to the proximal

segment20. The MPA was used to measure flexion/exten-

sion in the sagittal plane and adduction/abduction in the

frontal plane. The JCS was established based on the

system described by Grood and Suntay9. For the carpaljoint, flexion/extension was measured around the

antebrachial fixed x-axis, internal/external rotation

was measured around the metacarpal fixed z-axis and

adduction/abduction was measured around a floating

axis perpendicular to both the flexion/extension and

internal/external rotation axes. For the fetlock joint,

flexion/extension was measured around the meta-

carpal fixed x-axis, internal/external rotation wasmeasured around the proximal phalangeal fixed z-axis

and adduction/abduction was measured around a

floating axis perpendicular to both the flexion/extension

and internal/external axes. Therefore, flexion occurred

when the angle on the caudal/palmar aspect of the joint

decreased, internal rotation occurred when the distal

segment rotated medially relative to the proximal seg-

ment and adduction occurred when the angle on the

FIG. 1 Marker placement during the standing file as seen in thelateral (left) and frontal (right) views. Virtual targeting system mar-kers on the lateral aspect of the limb (black circles) for establish-ing the joint coordinate systems (JCS) were placed over bonylandmarks on the proximal and distal ends of the antebrachial,metacarpal and proximal phalangeal segments. A third virtualmarker (open circles) was placed on the medial aspect of the dis-tal extremity of each segment. Two of the JCS markers used fortracking the limb during locomotion (black circles) were the sameas the lateral virtual markers. The third tracking marker (greycircles) was placed on the dorsal surface, midway between theother two tracking markers. The tracking markers (black circles)on the proximal and distal ends of each segment were used formulti-planar analysis (MPA)

FIG. 2 Calculation used to correct sagittal plane measurementsusing MPA for the horse’s angle of travel determined from theback markers (black spheres), where Q is the angle that thehorse is out-of-plane, x0 and z0 are the uncorrected marker coor-dinates and x and z are the corrected marker coordinates

Joint coordinate system versus multi-planar analysis 45

medial aspect of the floating axis decreased. Zero joint

angle was defined as alignment of the proximal and

distal segments. Flexion, adduction and internal rotation

were assigned negative values. Extension, abduction

and external rotation were assigned positive values.

The mean (^ standard deviation) curves for flexion/

extension, adduction/abduction and internal/external

rotation of the carpal and fetlock joints at the walkmeasured by the JCS were plotted. The mean (^ stan-

dard deviation) curves for flexion/extension and

adduction/abduction of the carpal and fetlock joints at

the walk measured using MPA were plotted. Joint

angle–time variations between strides were normalized

to percentage of stride time. The mean (^ standard

deviation) absolute difference between the flexion/

extension joint angles measured using JCS and MPAwas calculated, and so was the mean (^ standard

deviation) absolute difference between the adduction/

abduction joint angles measured using both the

methods. Statistical comparisons between joint angles

measured by MPA and a JCS were not made as the use

of only one subject did not give sufficient power.

The anatomical orientation and the location of the seg-

mental axes relative to the GCS were explored in anattempt to understand the differences between the two

methods. Segmental angles were determined in the sagit-

tal view using the raw x-coordinates for the proximal

and distal markers that were aligned along the long axis.

Segmental angles in the sagittal plane were measured

from an axis perpendicular to the ground so that the seg-

ment rotated around its proximal end. An angle of 08

indicated that the segment was vertical and an angle of908 indicated that it was horizontal. In addition, the

mean difference between the raw z-coordinates of the

proximal and distal markers of the segmental long

axis was plotted. A difference of zero between the two

markers indicated that the segment was aligned along

the x-axis of the GCS (longitudinal axis of the horse).

The mean (^ standard deviation) angle between the

JCS adduction/abduction axis and the x-axis of the GCS(longitudinal axis of the horse) was calculated to

demonstrate the relationship between the axes of the

JCS and the GCS during locomotion, and this relation-

ship was further explored by the calculation of the

mean (^ standard deviation) angle between the JCS

flexion/extension axes and the z-axis of the GCS (med-

iolateral axis of the horse). The y-displacement of the

joint centre during a single stride was plotted to facili-tate visualization of the motion of the joint in the sagit-

tal plane during locomotion. The y-displacement was

plotted along the vertical axis, and for every 3% of

the stride, the orientation of the JCS adduction/abduc-

tion axis relative to the joint centre was plotted.

To determine any error that was made in the

adduction/abduction measurements using MPA due to

axial rotation of the proximal segment combined with

retraction of the distal segment, the following calculation

wasperformedand a three-dimensional surface graphwas

plotted from the resulting calculations. Figure 3 is used to

illustrate the calculation for adduction/abduction

measurement errors (Qe) due to rotation. Figure 3

shows the angle and orientation of two lines viewed

from the side, the front and the top before (solid line)

and after (dotted line) a rotation of the top line. Theangle of flexion (Qf) is determined from the side view by

subtracting the known flexion angle between the two

lines from 1808. The rotation angle (Qr) is determined

from the top view and is a given value for the calculation.

The length (L) of the lower line in the sagittal view is

known, which is used with Qf to determine the distance

(S) from the location of the lower line before the

rotation to a line perpendicular to the ground.

S ¼ sinQf L ð1Þ

This distance (S) represents the length of the lower

line before and after rotation in the top view. Using the

determined S value and the known rotation in the top

view, the distance the line rotates (D) is calculated.

D ¼ sinQr S ð2Þ

When S from equation 1 is substituted into equation

2, D becomes a function of Qf and Qr

D ¼ sinQr sinQf L ð3Þ

The height (H) of the line in the front view is deter-

mined from Qf and the length of lower line as

measured from the side view.

H ¼ cosQf L ð4Þ

From the determined distance (D) and the height

(H), the angle of adduction/abduction error (Qe) is

determined.

tanQe ¼D

Hð5Þ

FIG. 3 Illustration used for the calculation of adduction/abductionmeasurement errors (Qe) using multi-planar analysis (MPA) dueto axial rotation of the proximal segment (Qr) and retraction of thedistal segment creating joint flexion (Qf)


tanQe ¼sinQr sinQf L

cosQf Lð6Þ

tanQe ¼ sinQr tanQf ð7Þ

Results

Carpal jointFlexion/extension patterns were similar when

measured using MPA and JCS (Fig. 4). During stance

until the start of breakover at 39% of stride, both

methods showed the carpus adducting and extending

during which measurements made using the JCS indi-

cated a period of internal rotation (Fig. 5). The peak

flexion angles ( JCS: 291 ^ 28 and MPA: 294 ^ 38),which occurred during mid-swing, showed minimal

differences between the two techniques (Fig. 4).

Absolute difference (7 ^ 48) between the JCS and

MPA flexion/extension measurements peaked at 85%

of stride (Fig. 6). The adduction/abduction curves

showed differences in the general pattern and timing

of peaks (Fig. 7). Peak abduction for both methods

was similar in values ( JCS: 23 ^ 48 and MPA:21 ^ 98), with both methods showing peak abduction

occurring at mid-swing during which the JCS method

demonstrated a peak of external rotation (7 ^ 48)

(Fig. 5). However, peak values demonstrated differ-

ences between the MPA and JCS during the transition

from stance to swing, with peak absolute difference

(17 ^ 48) between the JCS and MPA adduction/abduc-

tion angles occurring around the time of lift-off at 53%of the stride (Fig. 8). At this point in the stride, MPA

measured the minimum abduction angle (2 ^ 88) for

the stride, while the JCS measured an increasing

abduction angle (Fig. 7). Minimum abduction angle

(6 ^ 28) for the JCS occurred during stance at 36% of

stride. Peak absolute difference between the adduc-

tion/abduction measurements occurred while the

carpus was flexing and externally rotating (Fig. 5).

This scenario is illustrated with the three-dimensional

surface graph (Fig. 9), which demonstrates the adduc-tion/abduction error in the MPA measurements

resulting from different combinations of axial rotation

of the proximal segment in conjunction with

joint flexion created by distal segment retraction.

When axial rotation of the proximal segment occurred

as the distal segment was retracting, it created

an adduction/abduction error in the frontal plane

measurements.At 53% of stride, where peak absolute difference

between the JCS and MPA adduction/abduction

measurements occurred, the radial segment was

FIG. 4 Mean (thick line) ^ standard deviation (thin lines) carpalflexion (2)/extension (þ ) of the Peruvian Paso walk for JCS(black line) and MPA (grey line)

FIG. 5 Mean (thick line) ^ standard deviation (thin lines) carpalflexion (2)/extension (þ) (black line), adduction (2)/abduction(þ) (dark grey line), internal (2)/external (þ ) rotation (light greyline) of the Peruvian Paso walk using JCS

FIG. 6 Mean (thick line) ^ standard deviation (thin lines) absolutedifference between carpal flexion (2)/extension (þ ) of the Peru-vian Paso walk measured using JCS and MPA


determined to be vertical in the GCS (sagittal plane

angle 08) (Fig. 10). The difference in the z-coordinates

between the proximal and distal markers along the

radial long axis was zero at 53% of the stride, indicating

that the long axis was aligned along thex-axis of the GCS.

The angle between the JCS adduction/abduction axis

and the GCS x-axis demonstrated a peak (53 ^ 28) at

58% of stride (Fig. 11) following peak absolute diffe-rence between the JCS and MPA adduction/abduction

measurements. A relatively smaller peak (27 ^ 48) in

the angle between the flexion/extension axis of the

JCS and the z-axis of the GCS occurred at 47% of stride.

Figure 12 illustrates the orientation of the JCS adduc-

tion/abduction axis as seen from the sagittal view in

which the axis did change orientation as the joint

centre moved vertically throughout the stride. Whenthe orientation of the JCS adduction/abduction axis

was not parallel with the corresponding GCS axis as

seen during breakover, there were greater differences

measured between the JCS and MPA adduction/abduc-

tion angles (Fig. 8). Around 20% of stride (Fig. 12) the

JCS and MPA adduction/abduction axes were almost

parallel, resulting in less adduction/abduction diffe-

rence (Fig. 8).

Fetlock jointFetlock flexion/extension patterns were similar when

measured using MPA and JCS (Fig. 13), with extension

during stance and flexion during swing. Peak exten-

sion occurred before breakover with both methods

demonstrating similar values ( JCS: 74 ^ 28 and MPA:

71 ^ 18). Peak flexion angles were similar betweenmethods ( JCS: 241 ^ 68 and MPA: 246 ^ 68) and

both methods showed the peak occurring early in

the swing phase during which there was a peak in

external rotation (3 ^ 118) as measured by the JCS

(Fig. 14). Flexion/extension absolute difference

(7 ^ 28) between the JCS and MPA peaked at 68% of

stride (Fig. 15). Both MPA and JCS showed adduction

during stance with similar peak adduction values( JCS: 227 ^ 108 and MPA: 228 ^ 58) (Fig. 16). Peak

abduction occurred in early swing for both methods,

but the peak abduction angles were visibly different

between JCS (2 ^ 18) and MPA (125 ^ 238). The

peak absolute difference (123 ^ 258) between the

JCS and MPA adduction/abduction angles occurred at

61% of the stride (Fig. 17), which coincided with

peaks of flexion and a small amount of externalrotation (Fig. 14).

As the absolute difference in adduction/abduction

between the JCS and MPA increased, the pastern

segment angle decreased reaching 2908 (Fig. 18)

around the time the adduction/abduction absolute diffe-

rence peaked (Fig. 17). The difference in z-coordinates

FIG. 7 Mean (thick line) ^ standard deviation (thin lines) carpaladduction (2)/abduction (þ) of the Peruvian Paso walk for JCS(black line) and MPA (grey line)

FIG. 8 Mean (thick line) ^ standard deviation (thin lines) absolutedifference between carpal adduction (2)/abduction (þ ) of thePeruvian Paso walk measured using JCS and MPA

FIG. 9 Contour surface showing adduction (2)/abduction (þ )error measured by MPA in the frontal plane as a result of differentcombinations created by axial rotation and flexion when the proxi-mal segment is vertical in the sagittal plane. The shading alongthe contour indicates the error (deg), the vertical line along thecontour indicates axial rotation of only the proximal segment (deg)and the horizontal line indicates the joint flexion angle (deg). Theerror is based on measured differences between JCS and MPA


of the proximal and distal markers along the pastern long

axis was zero at this point, indicating that the pastern

long axis was aligned with the x-axis of the MPA global

coordinate system (Fig. 18).The angle between the JCS adduction/abduction

axis and the GCS x-axis demonstrated a peak

(89 ^ 38) at 61% of stride (Fig. 19), coinciding with

peak absolute difference between the JCS and MPA

adduction/abduction measurements. A relatively smal-

ler peak (24 ^ 48) in the angle between the flexion/

extension axis of the JCS and the z-axis of theGCS occurred at 71% of stride. The JCS adduction/

abduction axis as seen from the sagittal view

(Fig. 20) demonstrated a shift in orientation starting

FIG. 10 Top graph: carpal flexion (2)/extension (þ) (solid line) and internal (2)/external (þ ) rotation (dotted line) of the Peruvian Pasowalk measured using JCS. Middle graph: segmental angles in GCS sagittal plane for antebrachium (solid line) and metacarpus (dottedline). Bottom graph: z-displacement between the proximal and distal tracking markers for the antebrachium (solid line) and metacarpus(dotted line). The thick vertical line at 53% of the stride is the time when carpal adduction/abduction angles measured using JCS andMPA have the maximal difference

FIG. 11 Mean (thick line) ^ standard deviation (thin lines) anglebetween the JCS adduction/abduction axis and the x-axis of theGCS for the carpal joint (black line) and mean (thick line) ^ stan-dard deviation (thin lines) angle between the JCS flexion/exten-sion axis and the z-axis of the GCS for the carpal joint (grey line)

FIG. 12 y-displacement of the carpal joint centre throughout asingle stride of the Peruvian Paso walk and the correspondingorientation of the JCS carpal adduction/abduction axis within thesagittal plane shown at intervals of 3% of the stride


at breakover, which coincided with an increase in the

absolute difference between the JCS and MPA adduc-tion/abduction angles (Fig. 17).

Discussion

Since the JCS is an anatomically based coordinatesystem, it moves dynamically with the horse’s anatomy

so that the axes of the coordinate system make adjust-

ments for the subject’s angle of travel, conformation

and unique limb motion patterns, such as the termino

of the Peruvian Paso. Since the MPA is based upon a

global coordinate system, the axes are not dynamic

and do not adjust to the changes in anatomical orien-

tation as the horse moves. This is the basis for thedifferences in adduction/abduction angles measured

using JCS and MPA. Understanding the movement of

the horse during locomotion will assist in locating

where this limitation for MPA creates error in the

measurements of adduction/abduction.

FIG. 15 Mean (thick line) ^ standard deviation (thin lines) abso-lute difference between fetlock flexion (2)/extension (þ ) of thePeruvian Paso walk measured using JCS and MPA

FIG. 13 Mean (thick line) ^ standard deviation (thin lines) fetlockflexion (2)/extension (þ ) of the Peruvian Paso walk for JCS(black line) and MPA (grey line)

FIG. 14 Mean (thick line) ^ standard deviation (thin lines) fetlockflexion (2)/extension (þ ) (black line), adduction (2)/abduction(þ) (dark grey line), internal (2)/external (þ) rotation (light greyline) of the Peruvian Paso walk using JCS

FIG. 16 Mean (thick line) ^ standard deviation (thin lines) fetlockadduction (2)/abduction (þ ) of the Peruvian Paso walk for JCS(black line) and MPA (grey line)

FIG. 17 Mean (thick line) ^ standard deviation (thin lines) abso-lute difference between fetlock adduction (2)/abduction (þ) of thePeruvian Paso walk measured using JCS and MPA


During the stance phase, the horse’s body moved

laterally over the supporting forelimb as shown by

the z-displacements. Figure 10 shows the proximal

end of the radius is lateral to its distal end throughoutstance. The antebrachium and metacarpus were

retracted with approximately equal angular velo-

cities until breakover, when the antebrachium reve-

rsed its direction of rotation reaching a vertical

position at lift-off, while the metacarpus continued tobe retracted. This combination of segmental motions

FIG. 18 Top graph: fetlock flexion (2)/extension (þ) (solid line) and internal (2)/external (þ) rotation (dotted line) of the Peruvian Pasowalk measured using JCS. Middle graph: segmental angles in GCS sagittal plane for metacarpus (solid line) and pastern (dotted line).Bottom graph: z-displacement between the proximal and distal tracking markers for the metacarpus (solid line) and pastern (dotted line).The thick vertical line at 61% of the stride is the time when carpal adduction/abduction angles measured using JCS and MPA have themaximal difference

FIG. 19 Mean (thick line) ^ standard deviation (thin lines) anglebetween the JCS adduction/abduction axis and the x-axis of theGCS for the fetlock joint (black line) and mean (thick line) ^ stan-dard deviation (thin lines) angle between the JCS flexion/exten-sion axis and the z-axis of the GCS for the fetlock joint (grey line)

FIG. 20 y-displacement of the fetlock joint centre throughout asingle stride of the Peruvian Paso walk and the correspondingorientation of the JCS fetlock adduction/abduction axis within thesagittal plane shown at intervals of 3% of the stride


resulted in carpal joint flexion. During breakover, the

horse’s body moved medially until at lift-off the proxi-

mal and distal ends of the antebrachium were aligned

vertically when viewed in the frontal plane, as seen in

Fig. 10. Therefore, at lift-off, the antebrachium was

vertical when viewed in either the sagittal or frontal

plane. Under these conditions, any axial rotation of

the antebrachial segment would have a large effecton the adduction/abduction angle of the carpal joint

as seen in the frontal plane using MPA. This orientation

of the proximal segment and the error that is created

under the conditions described above is illustrated

in Fig. 9.

At lift-off (55% of the stride), the pastern segment

had rotated beyond the horizontal (sagittal plane

angle 2928). At this time, the difference betweenthe z-coordinates of the proximal and distal markers

on the pastern long axis was zero so that the distal

aspect of the segment was not visible in the frontal

view during the time that the pastern segment was

rotated beyond 2908. Under these conditions, the

angle of the pastern segment was difficult to measure

in the frontal plane because the pastern segment was

behind the metacarpal segment when the sagittalplane angle was 2908 or the pastern segment was

hidden behind the metacarpal segment when it rotated

beyond 2908. Under these circumstances, any axial

rotation of the metacarpal segment, even without flex-

ion/extension or adduction/abduction of the fetlock

joint, allowed the pastern segment to become visible

in the frontal view. Thus, axial rotation of the metacar-

pus was misinterpreted as adduction/abduction of thefetlock joint in MPA. The sudden large peak in MPA fet-

lock adduction/abduction just after lift-off as seen in

Fig. 16 occurred during a period of axial rotation of

the metacarpus, while the pastern segment angle

was past the horizontal, but the adduction/abduction

angle of the fetlock was not changing. Axial rotation

of the metacarpus created an artificial adduction/

abduction angle of the fetlock in the frontal view asmeasured by MPA.

While the application of a JCS allows for measure-

ment of axial rotation and the detection of coupling

between the three types of joint motion, the JCS set-

up for tracking an anatomical coordinate system is

quite lengthy and may not be practical in most clinical

applications. Using MPA, Back et al.8 studied the effect

of heel wedges on frontal plane motion of the hindpastern relative to the metatarsus. The problems

described above for frontal plane measurements of

the fetlock joint were avoided by measuring the

changes in adduction/abduction with and without

heel wedges during the stance phase only. The hind

fetlock joint was found to show increased abduction

with the heel wedges during stance. The findings pre-

sented in this study show that smaller differences were

found between the JCS and MPA adduction/abduction

measurements during stance. Additionally, the greatest

angle between the JCS adduction/abduction axis and

GCS x-axis occurred during the swing phase where

absolute difference between the JCS and MPA adduc-

tion/abduction angles peaked. Therefore, limiting

frontal plane measurements to the stance phase as

described by Back et al.8 reduced some of the pro-

blems in using MPA.

Based on in vitro studies of distal joint loading,

Degueurce et al.21 described the three movements of

the joint as being always associated. The relationship

between the different types of movement affects the

axes of the JCS in that the JCS adduction/abduction

axis is perpendicular to both the flexion/extension and

internal/external rotational axes. Therefore, flexion/extension and/or internal/external rotation will cause

a shift in the orientation of the adduction/abduction

axis. This dynamic quality of the JCS axes allows for

the depiction of the relationship or coupling between

the three joint movements. On the other hand, the

MPA axes do not move with the anatomy nor do they

account for the relationship between the three

joint movements. Therefore, comparing measurementsmade using the dynamic axes of the JCS with those

made using the static axes of MPA will undoubtedly

show differences, particularly during the period of

the stride where orientation of the JCS axes is actively

changing, as seen in Figs 12 and 20 during breakover

and into the swing phase.

The angle at which the horse travelled through the

data collection area was transformed during post-pro-cessing of the data so that it was aligned with the

z-axis of the JCS, and thus differences between the

flexion/extension measurements for the two tech-

niques were minimized. However, even after correct-

ing for the horse’s angle of travel, there were small

differences in the flexion/extension measurements

due to the fact that axial rotation was not accounted

for when using MPA. The angular differences betweenthe JCS and MPA flexion/extension axes at the fetlock

were fairly consistent throughout the stride, with peak

differences occurring before mid-swing following

immediately after peak adduction/abduction axes

differences. The fact that a peak in angular difference

in the JCS and MPA carpal flexion/extension axes

occurred during stance, whereas the peak angular

difference in adduction/abduction axes occurredduring swing, may be due to the lateral movement of

the body over the limb during stance and the accompa-

nying change in radial orientation due to this rotation.

Differences due to radial orientation may be minimized

by choosing appropriate axes for establishing the

radial segmental coordinate system.

A limitation to the JCS used in this study was the

selection of the primary axis used for establishing


the segmental coordinate system. In order to facilitate

direct comparisons with the measurements made in

this study using MPA, the axis created by the two vir-

tual markers along the segmental long axis was used

as the base axis for establishing the segmental coordi-

nate system. However, the radius is wider proximally

than distally and the proximal marker is somewhat

lateral to the distal marker. This affects the orientationof the coordinate system so that it may not truly rep-

resent the load-bearing axis of the antebrachium. The

greater the discrepancy, the greater will be the offset

of the base axis from the load-bearing axis of the seg-

ment. This also explains why the peak absolute diffe-

rence between the JCS and MPA carpal adduction/

abduction is slightly offset from the point at which

the radius is vertical, as seen on the bottom graph inFig. 10. In future studies, it may be preferable to use

the axis through the medial and lateral markers

on the distal aspect of the segment as the base axis

of the antebrachial coordinate system so that differ-

ence in size of the proximal and distal radial epiphyses

has less effect on measuring the load-bearing axis of

the segment.

Recent three-dimensional studies have utilizedbone-based markers attached to bone segments using

bone pins to measure equine limb motion, as bone

pins alleviate the concern for errors related to skin

displacement10,12,13,15,22,23. Limited three-dimensional

equine studies have used skin-based markers for

measuring limb motion during locomotion11,24, in

which Khumsap et al.24 applied three-dimensional

skin correction algorithms determined by Lanovazet al.

25 for the tibia and third metatarsus of the trotting

Quarter Horse. Corrections for skin displacement were

not performed in this study, since three-dimensional

correction algorithms and even two-dimensional cor-

rections16 are not available for the Peruvian Paso

breed or for Peruvian gaits. The application of bone

pins is an invasive method in which care needs to be

taken to minimize pain and associated unsoundnessfor the research subject, as unsoundness has been

determined to influence normal equine three-dimen-

sional limb motion26. Invasive methodologies limit

the use of high-quality performance horses for

research purposes, and thus restrict the type of

horses used for kinematic research. Reducing the

need for bone-based markers requires further research

to determine three-dimensional skin correction algori-thms for various equine gaits and breeds for the

joints of the equine fore and hind limbs.

Description of the Peruvian Paso walk was limited in

this study due to the subject number being restricted

to a single horse. Limited subject numbers are

common in three-dimensional studies10,12,13,24–27 of

live horses. Three-dimensional joint motion during

locomotion has been described using only three

equine subjects12,13,24–27, but this limitation in subject

numbers was done due to the application of invasive

methods to measure three-dimensional joint motion.

However, generalizations from the results of these

studies were made with caution27. For this study,

the subject number was limited to reduce variations

related to how different horses perform the same

gait25. Intersubject variation, particularly concerningout-of-sagittal plane motion, has been reported in

horses as a limitation to three-dimensional analysis.

This intersubject variation may have hidden some of

the differences between the adduction/abduction

measurements made in this study using MPA and JCS,

making it difficult to locate the exact points where

adduction/abduction measurement errors can occur.

However, application of the results from this studyto other equine subjects of both the same breed and

other equine breeds is apparent as the scenarios rep-

resented in this study as to reasons why adduction/

abduction measurement errors occurred would not

be uncommon for other equine subjects. Nevertheless,

further three-dimensional equine studies using JCS and

larger sample sizes are encouraged for measuring and

describing the locomotion of the Peruvian Paso horse.Subject number was limited to one to avoid inter-

subject variation that could have created difficulties

in determining the causes of measurement differences

between MPA and JCS. Intersubject variation in

three-dimensional analysis has been reported in pre-

vious equine research27 and was attributed to confor-

mational differences and variations in the marker

placements used in establishing an anatomicallybased coordinate system in which measurements out-

side of the sagittal plane were reported to be more

impacted by these differences than the flexion/exten-

sion measurements. Intersubject variation due to

marker placement of an anatomically based coordinate

system can potentially be reduced using the same

researcher for marker placement and easily palpable

bony landmarks. Nevertheless, due to only one subjectbeing measured in this study, statistical comparisons

between the joint angles measured using MPA and

those using JCS were restricted. Although statistical

comparisons were not performed, the flexion/exten-

sion measurements for an MPA and a JCS were deter-

mined to be similar as the joint motion patterns

were similar, with peak angles occurring at the same

point in the stride and with peak angles fallingwithin the standard deviation curves of the opposing

measurement technique. However, comparisons

between the MPA and JCS carpal adduction/abduction

measurements found the joint motion curves of the

two measurement techniques did not share similar pat-

terns, particularly during breakover and into the swing

phase. At the point of peak absolute difference for

the carpal adduction/abduction measurements, joint


angles measured using JCS showed the carpal joint

actively increasing the abduction angle, while

measurements using MPA showed the carpal joint

moving in the opposite direction reaching minimum

carpal abduction. As for the fetlock joint, measure-

ments using MPA and a JCS found the joint actively

abducting, but the peak absolute difference between

the MPA and JCS measurements was relatively large.The MPA fetlock abduction angle was much larger

than what has been reported by previous equine

three-dimensional studies10,11,13,22,28 and anatomically

would not be possible in a sound horse.

A Peruvian Paso was selected as the subject for this

study due to its natural ability to perform three-dimen-

sional joint motion according to breed association

guidelines15. While other equine breeds may not demon-strate the same amount of out-of-sagittal plane

joint motion, even small amounts of axial rotation

combined with retraction of the distal limb will create

adduction/abduction errors in MPA measurements, as

seen in Fig. 9. Clayton et al.27 measured three-dimen-

sional carpal joint range of motion for three Quarter

Horses at the trot, finding 15 ^ 68 of flexion/extension,

5 ^ 18 of adduction/abduction and 4 ^ 18 of internal/external rotation during stance. During swing, the

carpal joint range of motion was 76 ^ 138 for flexion/

extension, 13 ^ 68 for adduction/abduction and

11 ^ 78 for internal/external rotation. Carpal joint

motion patterns were comparable between the Quarter

Horse and the Peruvian Paso measured in this study.

Additional research on the Quarter Horse reported

three-dimensional fetlock joint range of motion at thetrot, finding 42 ^ 98 of flexion/extension, 8 ^ 38 of

adduction/abduction and 17 ^ 78 of internal/external

rotation during stance, and during swing, the fetlock

joint range of motion was 45 ^ 68 of flexion/extension,

10 ^ 38of adduction/abduction and 18 ^ 98of internal/

external rotation7. While the range of joint motion for

the trotting Quarter Horse was smaller than for the

Peruvian Paso measured in this study, the amount ofout-of-sagittal plane joint motion reported for the

Quarter Horse would allow for MPA adduction/abduc-

tion measurement errors, as depicted in Fig. 9.

Summary and conclusions

Previous three-dimensional analysis of the horse

has reported joint motion outside of the sagittal plane

in both the walk and trot of various horse

breeds10,12,13,24–27. As reported in this study, the pre-sence of axial rotation combined with joint flexion as

measured at the time of breakover and the initiation

of the swing phase leads to adduction/abduction

measurement errors when joint motion measurements

are made using MPA. The inability of the GCS to dynami-

cally adjust with the orientation of the limb segments

was shown in this study to influence adduction/abduc-

tion measurements using MPA. Nevertheless, flexion/

extension measurements using MPA were found to be

similar to JCS measurements, despite evidence of axial

rotation when corrections were made for the horse’s

plane of travel. Therefore, if research objectives are to

measure only flexion/extension of the joint, MPA

would be appropriate for joint measurements, but ifmeasurements are to include joint motion outside of

the sagittal plane it is recommended that more than

two tracking markers are used per segment and a JCS

or a similar anatomically based coordinate system be

used.

Acknowledgements

This research was supported by the McPhail Endow-

ment and Michigan State University College of Veterin-

ary Medicine and College of Agriculture and Natural

Resources. The authors would like to thank the

gaited horse owner that took part in the study.

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