Page 163 - Adams and Stashak's Lameness in Horses, 7th Edition

Examination for Lameness 129

Mild induced lameness Mild induced lameness increased
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Mild induced lameness increased more Severe induced lameness increased more
Figure 2.119. Overall vertical head trajectory patterns for a horse increases, characteristic downward and then upward head movement
with gradually increasing forelimb lameness from sole pressure. Least changes (reverses) to upward and then downward head movement
amount of induced lameness shown in upper left plot and most until lameness amplitude increases to the point that greatest observed
amount of induced lameness shown in lower right plot. Gray bars movement direction is upward during lame or more lame forelimb
indicate stance phase of one forelimb. As amplitude of lameness stance and downward during not or less lame forelimb stance.

The amplitude of lameness, which affects both the dif­ WITHERS MOVEMENT
ference in the low and high positions of the limb, is pro­
portional to some combination of these differences. The Vertical movement of the withers may also be used to
better maxim, or definition of “head nod,” to determine detect and measure amplitude of forelimb lameness, and
side of lameness, is “lowest on sound.” However, the there is evidence that the character of withers vertical
amplitude of this explanation of “head nod” will not movement may help to differentiate compensatory from
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always be proportional to the true amplitude of fore­ primary lameness in the multiple limb lameness situation.
limb lameness, because a difference in low positions of When the asymmetry of withers movement is opposite in
the head, depending on the timing of lameness, may rep­ sign to that of the asymmetry of head movement, for
resent only a portion of the head movement asymmetry. example, if lowest head position is during the stance phase
The side of lameness can be determined by the difference of the left forelimb (a sign of right forelimb lameness) but
in the low positions of the limb, but the amplitude of lowest withers position is during the stance phase of right
lameness must take into consideration both the differ­ forelimb, a primary hindlimb lameness (right) should be
ences in the high and low positions of the head. suspected. Whereas when the asymmetry of withers move­
When horses are trotted in a straight line while ment is the same sign as that of asymmetry of head move­
restrained by a handler or during lunging, both which ment, a primary forelimb lameness should be suspected.
are common activities used by practitioners to evaluate However, in general asymmetric withers movement
horses for lameness, the speed of movement is such that is less sensitive than asymmetric head movement at
most forelimb lameness is measured as greatest during detecting amplitude of forelimb lameness. Because the
the first half of stance (impact to midstance). This is forelimb is attached to and supports the axial skeleton
most likely because at slow speeds the front limb acts by a soft tissue sling mechanism, ground reaction force
mostly as a strut or crutch with little contribution to and body center of mass effects on each other are
active propulsion of the body forward, most of which is damped, and the amplitude of asymmetric movement is
provided by pushoff of the hindlimbs. At higher speeds suppressed.
pushoff‐type forelimb lameness is measured with greater
frequency. Although timing of forelimb lameness is
most likely dependent upon speed of movement and EVALUATION OF VERTICAL MOVEMENT
ground surface qualities, knowledge of whether the OF THE PELVIS FOR HINDLIMB LAMENESS (THE
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lameness is manifested primarily during the first or sec­ VERTICAL PELVIC MOVEMENT [VPM] METHOD)
ond half of stance may be valuable to the practitioner in
helping to localize the cause of lameness within the Evaluation of hindlimb lameness is easier conceptually,
affected forelimb. but more difficult practically, than evaluation of forelimb
Other random causes of vertical head movement, lameness. It is easier conceptually because movement of
which will interfere with subjective evaluation of head the posterior half of the horse’s torso can be modeled as a
nod, must somehow be extracted and ignored by the simple single free body moving down and up in response
examiner when detecting forelimb lameness. This can be to resultant and applied ground reaction forces. It is
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difficult with misbehaving horses when they are ran­ more difficult practically because the amplitude of verti­
domly shaking or jerking the head in the vertical plane. cal movement is small and more difficult to see.