=WM= IN-DEPTH: LAMENESS IN MOTION
Head Movement Pattern in Horses With Forelimb
and Hindlimb Lameness
Kevin G. Keegan, DVM, MS, Diplomate ACVS
The vertical head movement pattern in horses with forelimb lameness may contain information useful
for determining the instant of peak pain within the stride cycle. This information may be helpful to
the practitioner in isolating lameness within the affected limb. Author's address: E. Paige Laurie
Endowed Program in Equine Lameness, College of Veterinary Medicine, University of Missouri,
Columbia, MS 65211; e-mail: [email protected]. © 2005 AAEP.
1. Introduction
Most equine practitioners look at how the horse's
head moves during a trot to help them diagnose
lameness." It is a common and well-accepted
maxim that horses with forelimb lameness will show
a "head nod" (or "head bob"), but descriptions of the
"head nod" are incomplete, varied, and sometimes
conflicting. Objective and precise measurements of
head movement in lame horses have been made in
experimental studies." The observations from
these studies tell much about what the "head nod"
really is and how close evaluation of it may tell us
something about the specific lameness being evaluated. This paper will first try to document what is
written in the most popular equine lameness textbooks about the "head nod" and then compare
these descriptions to what has been determined by
controlled objective analysis of the head movement patterns in sound and lame horses. In the
latter part of this paper, I will comment on head
movement patterns in horses with primary hindlimb lameness, and finally, I will introduce a hypothesis that the specific head movement patterns
observed help the evaluator determine the type of
NOTES
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2005 / Vol. 51 / AAEP PROCEEDINGS
lameness (i.e., when the pain is occurring during
the stride).
2. What the Textbooks Say
"As a result of lameness in a forelimb, the head will
drop when the sound foot lands and rise when
weight is placed on the unsound foot or limb." 1
"The head and neck elevate or rise when the lame
forelimb is bearing weight or hits the ground and nod
down or fall when the sound forelimb hits the ground
. . . it is immediately obvious that the elevation of the
head and neck is much easier to see than the head nod
down . . . the horse appears to be elevating the head
and neck just before the lame limb hits the ground,
and then, during the later part of the support or stance
phase, the head and neck nod down." 2
"Forelimb lameness is usually evident as a head
bob: the head rises immediately prior to and during weight bearing of the lame limb. Conversely,
the head drops as the sound limb contacts the
ground and bears weight." 3
Each of these descriptions are similar in that they
state clearly that the head moves upward during the
weight-bearing phase of the lame limb and downward during the weight-bearing phase of the sound
IN DEPTH: LAMENESS IN MOTION
-
Sound in Forelimbs
-10- Head height -4-Right forelimb foot height_
timo
Fig. 1. Pattern of vertical head movement in a sound horse.
limb. Indeed, this is what actually happens in
horses with severe forelimb lameness and what appears to happen to the naked eye during the trot in
horses with mild to moderate forelimb lameness.
The rapid movement of the limbs in a horse even at
a slow trot (4 m/s) and the limited temporal resolution of the human eye cause it to appear as such.
However, they are also somewhat different in their
varied emphases on whether it is the upward or
downward movement of the head that is easiest to
see. In general, they are also somewhat simplified
and, in so being, incomplete. In the next sections, I
will describe the objective evidence for how the head
moves during forelimb lameness using data generated with high-speed cameras and computer-assisted kinematic evaluation during controlled
conditions. The more complete description of head
movement with lameness allows one to see that
sometimes it is the downward movement and sometimes the upward movement of the head that is
important for the detection of forelimb lameness.
Quite possibly, a more complete and discriminating
evaluation of head movement can help us isolate
lameness within the lame limb.
3. Objective Analysis of Head Movement in the Sound
Horse and in the Horse With Unilateral Forelimb
Lameness
In a sound horse at the trot, the head moves up and
down twice during one complete stride (Fig. 1).
At the beginning of stance, the head is already
moving down along with the rest of the torso and
following the contralateral suspension or swing
phase. It continues to move down, reaching a nadir
or minimum position at full weight bearing when
the limb is perpendicular to the ground at midstance. It then begins to move upward during the
caudal one-half of stance, continuing its upward
movement during breakover and pushoff and into
the beginning of the swing phase of the stride when
the horse is suspended in mid-air with no limbs on
the ground. It reaches a maximum position a short
time before impact of the contralateral limb. After
impact of the contralateral limb, this same sequence
of downward and then upward head movement occurs again. The key observations in this sequence
of events are the downward followed by upward
head movement during stance and upward followed
by downward head movement during the swing
phase of the stride. In the sound horse, the maximum and minimum vertical he.Fd posAicas-durifig
each one -h alf e_ _of _the . stride. r e_equ al.
In the horse with a mild to moderate unilateral
forelimb lameness, this sequence of downward and
upward head movement is the same. There is downward and then upward movement of the head during
the stance phases of both the lame and sound limbs,
and there is upward and then downward movement of
the head after pushoff of both the lame and sound
limbs. The difference is in the relative head heights
at the midstance positions and after pushoff of the two
forelimbs. This relationship breaks down in horses
with severe lameness when the movement of the head
during stance may be first upward and then downward or when the head maximum position is reached
before the end of stance. The purpose of this manuscript is not to describe the head movement patterns in
horses with severe forelimb lameness; accurate detection of this lameness is not difficult.
Therefore, the relative height of the head during
midstance (the downward movement) or after pushoff (the upward movement) determines the side of
lameness, but which one is more important? It is
not so simple.
AAEP PROCEEDINGS / Vol. 51 / 2005
115
IN-DEPTH: LAMENESS IN MOTION
Right Forelimb Lameness - pattern 1
-a- Head height -4-Right forelimb foot height
303
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Fig, 2. Pattern 1. The head moves down during the stance phase of the lame limb, but the absolute height is higher than during
the stance phase of the sound limb. The head moves up after pushoff of the lame limb, but the absolute height is lower than after
pushoff of the sound limb.
After evaluating many horses with mild to moderate
unilateral forelimb lameness, both natural and induced, we have observed the following patterns of
movement: (1) The head moves down during the
stance phase of the 1wib
l ut the absolute height
is higheran during the stance prase the kind
limb- the head moves up after pushoff of thilaTne limb,
but the absolute height is lower than after pushoff of
the so=l_limb-(Fig. 2). (2) The head moves down
during the stance phase of the lame limb, but the
absolute height is higher than during the stance phase
of-the inab; the head moves upward after pushoff of the lame limb, but the-abbulutelreightii the
same as after pushoff of the sound limb (Fig. -13). (3)
Thse-head7moves_down during-the_ stsn re phase_ of the
laMe limb, but theabscAute_heigia-is-higherthaadurMg the stance phase of the sound limb; the head moves
up after pushoff of the—Ta—
me limb, but the absolute
height is high-6r Than after pushoff of the sound limb
(Fig 4). (4) -The1iead moves downduring-the stance
,
Right forelimb lameness - pattern 2
--s- Head height
Rig_ht forelimb foot height
'GT
EE
0
time
Fig. 3. Pattern 2. The head moves down during the stance phase of the lame limb, but the absolute height is higher than during
the stance phase of the sound limb. The head moves upward after pushoff of the lame limb, but the absolute height is the same as
after pushoff of the sound limb.
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2005 / Vol. 51 / AAEP PROCEEDINGS
IN-DEPTH: LAMENESS IN MOTION
Right forelimb Lameness - pattern 3
-
Right forelimb foot height
a Head height
-
- 0
125,
time
Fig. 4. Pattern 3. The head moves down during the stance phase of the lame limb, but the absolute height is higher than during
the stance phase of the sound limb. The head moves up after pushoff of the lame limb, but the absolute height is higher after pushoff
of the sound limb.
phase of the_lame_limb, but the absolute height is the
same as _dining the--stice _phase of the sound limb;
the hea_d_moyes_upafteip
- ushOfrortfie1
the absolute height is higher than after pushoff of the
sokmdlififF-Wig. 5).
There is an old adage "down on sound," which
means that the head moves down to a lower height
during the stance phase of the sound limb. Judging
from the patterns that we have seen, this statement
is not always correct. "Down on sound" would pick
the correct limb in the first three patterns described
above, but pattern four would be evaluated as
sound. The head never moves down to a lower position during stance phase of the lame limb cOrnpaivitYlEe sound limb, so if this is observed, the
correct
-ori
'Rt-Timip-can always be identified. "Down on
sound" is generally a good way to observe forelimb
lameness.
Evaluation of the head movement upward is more
difficult but, as we will discuss later, potentially
Right forelimb lameness
-a- Head height
-
pattern 4
-•-• Right forelimb foot height
time
Fig. 5. Pattern 4. The head moves down during the stance phase of the lame limb, but the absolute height is the same as during ,
the stance phase of the sound limb. The head moves up after pushoff of the lame limb, but the absolute height is higher after pushoff
of the sound limb.
AAEP PROCEEDINGS / Vol. 51 / 2005
117
IN-DEPTH: LAMENESS IN MOTION
very informative for isolating forelimb lameness.
Sometimes the head reaches a maximum position
after pushoff of the lame forelimb that is sometimes
higher, sometimes lower, and sometimes the same
as that after pushoff of the sound forelimb.
One could argue that it is the relative excursion of
the head rather than the absolute height that one
should be watching when evaluating for forelimb
lameness. If this is the case, then the four patterns
described above are the same but can be rephrased
in excursion terminology. (1) The head moves
down the same amount during the stance phases of
the lame and sound limbs, but it moves up less after
pushoff of the lame limb compared with pushoff of
the sound limb. (2) The head moves down less during the stance phase of the lame limb than during
the stance phase of the sound limb, and it moves up
less after pushoff with the lame limb compared with
pushoff of the sound limb. (3) The head moves
down less during the stance phase of the lame limb
than during the stance phase of the sound limb, but
it moves up the same amount after pushoff of the
lame and sound limbs. (4) The head moves down
less during the stance phase of the lame limb than
during the stance phase of the sound limb, and the
head moves up more after pushoff of the lame limb
than after pushoff of the sound limb. These descriptions are more difficult to understand and, as
we shall see later, are more problematic for detection of lameness, which may result because of variations in lameness severity and time of occurrence.
4. Head Movement in Horses With Primary Hindlimb
Lameness
It has been well described in textbooks and in objective, controlled studies that horses with primary
hindlimb lameness can appear to have ipsilateral
forelimb lameness."'6'7-16 This is because the horse
uses its head movement to help reduce load on the
lame hindlimb, moving it down to a lower position
when the lame hindlimb is in stance. Using the
"down_ and sound" maidm, it seems—that the horse has
a forelimb lameness ipsilateral to the true hindlimb
lameness. The que§tiofilS-hol,y -6M-1-x10-68-this happen and how likely is it for a practitioner to "mistake"
a forelimb lameness for an ipsilateral hindlimb lameness. In a recent study of 17 horses with induced
hindlimb lameness, it was apparent that rather mild,
primary hindlimhlameness frequently caused a more
apparent and severe but false ipsilateral forelimb
lameness_16 This effect was significant when evaluated over the entire group of 17 horses, but it was also
apparent that this effect was variable when looking at
the horses individually. Some horses with primary
hindlimb lameness had
little change in head
movement, whereassome
some had dramatic he movement asymmetry. An individual horse's body conformation may be a discriminating factor. Watching out
for ipsilateral, false forelimb lameness is an important
consideration during lameness evaluation in horses.
This author on occasion has spent days blocking out
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2005 / Vol. 51 / AAEP PROCEEDINGS
suspected forelimb lameness only to find the true primary hindlimb lameness.
5. Types of Forelimb Lameness? Can Head
Movement Help Us to Further Localize Lameness
Within the Forelimb? A Hypothesis
A sound horse, with no extraneous, random, vertical
head movement, will move its head up and down
twice per stride in a symmetrical pattern such that
frequency analysis of the vertical head movement
signal will yield a single frequency; this will be
equivalent to twice the stride rate.' Frequency
analysis of the vertical head movement signal in a
horse with unilateral forelimb lameness will yield
two prominent harmonic frequencies: one representing the lameness, occurring at the stride rate,
and one at twice the stride rate representing the
natural, inertially driven, up and down movement of
the head, as in the sound horse. 6 Each of the
above, previously described four different head
movement patterns, although they seem quite different, represent a continuum in the temporal relationship between the two harmonic components that
make up vertical head movement in a trotting horse
with unilateral forelimb lameness. The frequency
amplitude spectrums of each pattern are identical,
but the phase differences between the two predomjnant harmonic frequencies vary.
,--Pa-t-i-6-riT-T4•:_,) the head moves down during the
itatitse-ThiSe of the lame limb, but the absolute
height is higher than during the stance phase of the
sound limb; the head moves up after pushoff of the
lame limb, but the absolute height is lower than
after pushoff of the sound limb. This pattern results when the two predominant harmonic frequencies are in phase (i.e., their phase difference is 0°).
When the phase difference is 0°, the peak or maximum of the lameness component, which can be
thought of as the time of maximum pain or discomfort during the stride, coincides with the beginning
of stance. Thus, this is the_nattern_Pf_head-movement ex_pect
ieness is "felt! by_the horse
primarily
1 _ du ring impact of the forelimb (Fig. 6).
(attern th-e-held moves down during the
stance phaie of the lame limb, but the absolute
height is higher than during the stance phase of the
sound limb; the head moves upward after pushoff of
the lame limb, but the absolute height is the same as
after pushoff of the sound limb. This pattern results when the two predominant harmonic frequencies are out of phase by 90°. When the phase
difference is 90°, the peak or maximum of the lameness component coincides with midstance. Thus,
this is the pattern of head movement expected when
lameness is "felt" by the horse primarily during full
weight bearing on the limb, which occurs at midstance (Fig. 7).
The difference in the downward positions of the
head increases and the difference in the upward
positions of the head decreases as the phase difference varies from 0-90° or from impact to full weight
IN DEPTH: LAMENESS IN MOTION
-
Fig. 6. Head-movement components in phase (phase difference = 0°). The peak of the lameness component occurs at the
instant of the right forelimb hoof impact (arrows). Note the less
downward and less upward movement of the head during and
after stance of right forelimb. Black horizontal bars, right forelimb stance; blue signal, lameness component; red signal, natural
vertical head movement component; purple signal, sum of blue
and red signals.
Fig. 8. Head-movement components out of phase (phase difference between 90° and 180°). The peak of the lameness component occurs at the end of stance (arrows). Note the less
downward movement of the head during stance but greater upward movement of the head at the end of stance. Black horizontal bars, right forelimb stance; blue signal, lameness component;
red signal, natural vertical head movement component; purple
signal, sum of blue and red signals.
bearing in the deceleratory part of the stance phase
of the stride.
ittern 3: ---jhe head moves down during the
stakes-- e of the lame limb, but the absolute
height is higher than during the stance phase of the
sound limb; the head moves up after pushoff of the
lame limb, but the absolute height is higher than
after pushoff of the sound limb. This pattern results when the two predominant harmonic frequencies are out of phase by 90-180°. When the phase
difference is between 90-180°, the maximum of the
lameness component occurs between midstance and
the end of stance. Thus, this is the pattern of head
movement expected when lameness is "felt" by the
horse primarily during theiaiidarone-half or acceleratory
-o-f_ftgt'ance phase of the strid-e,
des the_ entirety of breakover(Fig. 8).
the head- moves down during the
stance phase of the lame limb, but the absolute
height is the same as during the stance phase of the
sound limb; the head moves up after pushoff of the
lame limb, but the absolute height is higher than
after pushoff of the sound limb. This pattern results when the two predominant harmonic frequencies are out of phase by 270°. When the phase
difference is between 180° and 270°, the maximum
of the lameness component occurs after breakover-at
the end of the stance phase of the stride. Thus, this
is the patternOf -field movement expected when
lameness is "felt" by the horse primarily during the
initial swing part of the stride when thislimb is not
weight bearing (Fig 9).
The difference in the downward position of the
head decreases and the difference in the upward
position of the head increases as the phase difference varies from 90-270° or from full weight bearing
through pushoff (in the acceleratory part of the
stance phase of the stride) and into the beginning of
the swing phase of the stride.
The two-component model of vertical head movement for diagnosis of lameness is plausible, because
the front end of the horse can be thought of as two
linked segments (the torso-leg and the head-neck
segments). Movement in the head-neck segment
Fig. 7. Head-movement components out of phase (phase difference = 90°). The peak of the lameness component occurs at
midstance (arrows). Note the less downward movement of the
head during stance but the same upward movement at the end of
stance. Black horizontal bars, right forelimb stance; blue signal,
lameness component; red signal, natural vertical head movement
component; purple signal, sum of blue and red signals.
Fig. 9. Head-movement components out of phase (phase difference = 2701. The peak of the lameness component occurs after
the end of stance (arrows). Note that there is no difference in
downward movement of the head but greater upward movement
of the head after the end of the stance. Black horizontal bars,
right forelimb stance; blue signal, lameness component; red signal, natural vertical head movement component; purple signal,
sum of blue and red signals.
AAEP PROCEEDINGS / Vol. 51 / 2005
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IN-DEPTH: LAMENESS IN MOTION
will always try to reduce force on the torso-leg segment. Because much is already known about the
dynamic behavior of many anatomic structures
within the equine limb, each a potential focus of
lameness, knowledge of the instant of occurrence of
peak lameness within the stride cycle may help further isolate the lameness within the limb. For example, limb vibration, as opposed to a total force
applied to it at any one instant, is known to cause
specific arthritic and other musculoskeletal disorders in humans." In the horse at limb impact, a
significant proportion of the vibration occurring to
the limb is attenuated or damped within the joint
structures. 12 Therefore, arthritic conditions would
be expected to a show pattern 1 head motion. Figure 1 depicts the head-motion pattern in a horse with
experimentally induced carpal arthritis. Similarly, it
is known that at the instant of hoof impact at the
beginning of the stance phase, the force on the hoof is
small, but it experiences high-frequency vibration.
This vibration is damped predominantly by the lamina
of the foot. Laminar pathology could, therefore. also
be expected to cause maximum pain res onse at hoof
impact; TeariTo
ii
head-movement pattern .
Another example is that the naviciiTa-f-§ii§pensory
ligament is known to be strained maximally during
breakover. 13 Injury or lameness to the navicular
suspensory ligament could be expected to cause pain
duking breakover, leading to head movement like
patterh-3. Figure 3 is an actual example of the
head-iii-ovement pattern of a horse with calcification
of the navicular suspensory ligament seen on radiographic examination of the foot. This horse also
"blocked out" (i.e., the vertical head movement pattern became temporally symmetric) after a palmar
digital nerve block. Similarly, the inferior check
strained maximally duringligamentskowb
breakav_er, 14 and injury to the inferior check ligament could be expected to result in a similar headmovement pattern. Other anatomic structures _are
known to be maximally stressed at full weight bearing or activated during the swin
le
d
stride,
leadin-g-to headnovement patte s 2 an
respectively. Knowledge of when pain is maximally occurring during the stride, obtained from close
analysis of the specific head movement pattern,
would be immeasurably beneficial to equine practitioners attempting to narrow the differential diag-
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2005 / Vol. 51 / MEP PROCEEDINGS
nosis in specific lameness cases. With practice,
close visual analysis of the specific head-movement
patterns can potentially ascertain these different
head movement patterns.
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