Pressure mat analysis of naturally occurring lameness in young pigs after weaning
BMC Veterinary Research
Pressure mat analysis of naturally occurring lameness in young pigs after weaning
Ellen Meijer 0
Maarten Oosterlinck 2
Arie van Nes 0
Willem Back 1 2
Franz Josef van der Staay 0
0 Department of Farm Animal Health, Faculty of Veterinary Medicine, Utrecht University , Yalelaan 7, NL-3584 Utrecht, CL , The Netherlands
1 Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University , Yalelaan 112-114, NL-3584 Utrecht, CM , The Netherlands
2 Department of Surgery and Anaesthesiology, Faculty of Veterinary Medicine, Ghent University , B-9820 Merelbeke , Belgium
Background: Lameness is a common problem in modern swine husbandry. It causes welfare problems in affected pigs as well as financial problems for farmers. To minimize these negative consequences of lameness, new treatment and prevention strategies need to be developed and validated using objective and quantitative measurement techniques. An example of such a putative diagnostic tool is the use of a pressure mat. Pressure mats are able to provide both objective loading (kinetic) as well as objective movement (kinematic) information on pig locomotion. In this study, pressure mat analysis was used to assess compensatory force redistribution in lame pigs; in particular a predefined set of four pressure mat parameters was evaluated for its use to objectively distinguish clinically lame from sound pigs. Kinetic data from 10 clinically lame and 10 healthy weaned piglets were collected. These data were analyzed to answer three research questions. Firstly the pattern of compensatory weight distribution in lame animals was studied using the asymmetry indices (ASI) for several combinations of limbs. Secondly, the correlation between total left-right asymmetry index and visual scores of lameness was assessed. Thirdly, by using receiver-operated curve (ROC) analysis, optimal cutoff values for these ASIs were then calculated to objectively detect lame pigs. Results: Lame animals generally showed a shift in loading towards their diagonal and contralateral limbs, resulting in a clear left-right asymmetry. The degree of lameness as graded by visual scoring correlated well with the total left-right ASIs. Lame pigs could be objectively distinguished from sound pigs based on clear cutoff points calculated by ROC analysis for the complete set of four evaluated parameters. Conclusions: The gait of lame pigs is asymmetric, due to the unloading of the affected limb and concomitant weight redistribution towards other limbs. This asymmetry objectively expressed as total left-right asymmetry, correlates well with the subjective visual lameness scoring and can be used to objectively distinguish lame from sound pigs. Pressure mat gait analysis of pigs, therefore, appears to be a promising and useful tool to objectively quantify and possibly early detect lameness in pigs.
Kinetics; Gait analysis; Porcine; Symmetry; Redistribution; Loading
Lameness in pigs is a common problem in modern swine
husbandry, affecting up to 19% of finishing pigs [
]. It has
negative consequences both from an animal welfare as well
as from an economic point of view. Lameness seriously
impairs welfare and may have an effect on all of Brambell’s
“five freedoms” [
]. The economic impact of lameness is
caused by lower productivity in lame animals, the cost of
treating affected animals and the cost of premature culling
of animals [
]. The aforementioned welfare and economic
issues can be minimized if veterinarians provide
evidencebased advice on treating and preventing lameness.
Detection of lameness needs to be sensitive and reliable, and
treatment and prevention strategies need to be assessed
using objective, repeatable methods to quantify lameness.
As already proven in other animals including man,
pressure mats are a noninvasive and objective tool to
quantitatively assess gait, providing kinetic and
temporospatial data. Their use has increased lately as they seem
to have some distinct advantages over other, ‘classical’
methods to analyze gait, like force plates and infrared
high-speed camera systems. When using pressure mats,
it is possible to measure consecutive, overlapping and
even simultaneous footfalls, thus enabling the collection
of several footfalls in one run. Moreover, an elaborate
calibration setup that some other, aforementioned
methods would require for a 3D kinetic and kinematic
analysis is not needed. In addition, pressure plates with
high sensor density provide information on the pressure
distribution between different regions, for example
between different claws within the foot.
Pressure mats have been used to study gait in several
other quadruped species, such as horses [
], cattle [
], dogs [
], and cats [
]. Most of these studies
focused on sound animals. Considerably less information,
however, is available on the sensitivity and reliability of
pressure mats to objectively distinguish lame from sound
animals. Earlier studies in dogs and cattle indeed reported
that it is possible to distinguish lame from sound
]. However, hardly any information on the
use of pressure mat analysis to detect lameness in pigs is
available yet. Only one single study was identified that
described lameness in sows using pressure mat analysis,
but only peak vertical pressure symmetry was used to
objectively quantify lameness .
Therefore, the objective of this study was to evaluate
the ability of four kinetic pressure mat parameters (peak
vertical force: PVF, load rate: LR, vertical impulse: VI,
and peak vertical pressure: PVP) to distinguish clinically
lame from sound weaned piglets.
We assessed compensatory weight redistribution by
calculating the asymmetry-indices (ASI) for several
combinations of limbs and for each of the pressure mat parameters.
In order to compare this new method to the method that is
currently used most often in lameness research in pigs
(visual scoring), correlations of total left-right ASIs for the
four pressure mat parameters with visual scores of
lameness were assessed as well. We used receiver-operated
characteristic (ROC) curve analysis to determine the
optimal cutoff values of total left-right ASI to identify
Mean body mass was 9.5 ± 2.0 kg for the lame pigs and
9.1 ± 1.7 kg for the sound pigs and did not differ between
groups (t(18) = 0.462, p = 0.650). Mean velocity was
1.3 ± 0.5 m/s in the sound pigs and 0.6 ± 0.4 m/s in
the lame pigs. The velocity was lower in the lame pigs
compared to the sound pigs (t(18) = 4.146, p = 0.001).
The visual gait score (0 = sound, 5 = non-weigh bearing
lameness) in the lame pigs ranged from 2 to 4 with a
mean of 3.0 ± 0.8.
Details of the gross pathological examination and the
visual lameness scoring of the lame animals are shown
in Table 1.
Pressure mat analysis yielded prints for each claw
(See Figure 1 for an example of pressure mat recordings).
Redistribution of pressure
Figures 2, 3, 4, 5, 6 and 7 present the ASIs of three groups
of animals: sound animals, animals that were lame on a
limb that was assessed by that particular ASI and animals
that were lame on a limb that was not assessed by that
ASI. Full test statistics and exact p-values are provided as
Additional file 1.
Contralateral forelimb ASI for PVF differed between
groups (χ2(2, N = 20) = 11.61, p = 0.003) and was higher in
the animals that were lame on a forelimb compared to
animals that were lame on a hind limb or animals that
were not lame (see Figure 2). Load rate CFL differed
between groups as well (χ2(2, N = 20) = 11.09, p = 0.004)
and animals that were lame on a forelimb had higher
CFL than animals that were lame on a hind limb and sound
animals. A difference between all groups was present in the
CFL of VI (χ2(2, N = 20) = 12.19, p = 0.002). Although CFL
of PVP also differed between groups (χ2(2, N = 20) = 7.01,
p = 0.030), the Mann-Whitney U test only showed a
difference between sound animals and animals that were lame
on a forelimb.
Contralateral hind limb ASI (Figure 3) was different
between groups for all parameters (PVF (χ2(2, N = 20) = 15.48,
p = 0.000), LR (χ2(2, N = 20) = 14.58, p = 0.001), VI (χ2
(2, N = 20) = 15.48, p = 0.000) and PVP (χ2(2, N = 20) = 14.29,
p = 0.001)). The posthoc Mann-Whitney U test showed
a difference between all groups for PVF and VI. Sound
animals had lower ASIs than animals that were lame on a
forelimb or animals that were lame on a hind limb for LR
and PVP, but there was no difference between animals
that were lame on a forelimb and animals that were lame
on a hind limb for these parameters.
The ASI of the ipsilateral left limbs (Figure 4) differed
between the three groups for PVF (χ2(2, N = 20) = 11.09,
p = 0.004), LR (χ2(2, N = 20) = 9.07, p = 0.011) and VI (χ2
(2, N = 20) = 8.73, p = 0.013), but not for PVP. In the
posthoc analysis of ILL of PVF, pigs that were lame on the
left limb had higher ILLs than pigs that were lame on
the right limb and sound pigs. ILL of LR and VI were
higher in pigs that were lame on the left limb compared
to sound animals.
Ipsilateral right limb ASI (Figure 5) differed between
groups for PVF (χ2(2, N = 20) = 6.32, p = 0.042) and PVP
(χ2(2, N = 20) = 9.53, p = 0.009), and the posthoc
MannWhitney U test showed the animals that were lame on
a right limb had higher ASIs than sound animals. The
difference between groups was also found for LR (χ2(2,
N = 20) = 8.22, p = 0.016), with sound animals having
lower ASIs than lame animals, regardless of which limb
was lame. VI (χ2(2, N = 20) = 7.84, p = 0.020) differed
between groups with the animals that were lame on the
right limbs having higher ASIs than other animals.
For all parameters, diagonal LF/RH ASI (Figure 6)
was different between groups (PVF (χ2(2, N = 20) = 9.52,
Visual lameness score
p = 0.009), LR (χ2(2, N = 20) = 10.94, p = 0.004), VI (χ2(2,
N = 20) = 9.15, p = 0.010), PVP (χ2(2, N = 20) = 13.91,
p = 0.001)). Animals that were lame on the left fore or
right hind limb had higher DLL than animals than animals
that were lame on the right fore or left hind limb and had
higher DLL than sound animals for PVF and VI. LR DLL
was lower in sound animals than in other animals. For
PVP, all groups differed from each other, with animals that
were lame on the left fore or right hind limb having higher
DLL than animals that were lame on the right fore or left
hind limb and sound animals.
Diagonal ASI of right front limb and left hind limb
(Figure 7) differed between groups for all parameters (PVF
(χ2(2, N = 20) = 9.14, p = 0.010), LR (χ2(2, N = 20) = 14.66,
p = 0.001), VI (χ2(2, N = 20) = 9.33, p = 0.009), PVP (χ2(2,
N = 20) = 13.01, p = 0.001)). Pigs that were lame on the
right fore or left hind limb had higher DLR for PVF and
VI compared to the sound group. For LR, all groups
differed from each other. Sound animals had lower PVP
DLR than lame animals.
CLT (Figure 8) was higher in lame animals compared
to sound animals for all parameters (PVF: t(18) = -6.11,
p = 0.000, LR: t(18) = -4.22, p = 0.002), VI: t(18) = -6.80,
p = 0.000, PVP: t(18) = -4.60, p = 0.001).
Correlation with visual scoring
The visual score of the 20 pigs correlated highly with
CLT of PVF (r = 0.82, p < 0.05), CLT of VI (r = 0.83,
p < 0.05), CLT of LR (r = 0.80, p < 0.05) and CLT of PVP
(r = 0.77, p < 0.05).
The ROC analysis showed distinct cutoff values with
100% sensitivity and specificity for all four parameters.
The cutoff value calculated from the ROC analysis was
33.1 for CLT of PVF, 24.0 for CLT of LR, 39.8 for CLT of
VI and 25.6 for CLT of PVP.
Data collection from both lame and sound pigs appeared
relatively simple, as both groups showed exploratory
behavior and a strong response to the treats. Lame pigs
walked at a slower mean velocity than sound pigs. This is
in agreement with findings in sows [
] and cattle [
Velocity itself may provide an indicator for lameness.
However, there is some overlap between lame pigs and
sound pigs on this measure. Also, in practice, velocities
can only be measured and compared under standardized
conditions. It may be difficult to distinguish the different
velocities by visual judgment. Lastly, velocity per se does
not provide any clue as to which limb is the lame one.
Directional ASI’s can provide this information. Velocity,
however, may be a first indicator to direct a farmer’s or
veterinarian’s attention to certain pigs; a pig moving
exceptionally slow may warrant further examination.
Incorporation of “walking speed” as a parameter into lameness
scoring protocols therefor may be advisable.
The lame pigs in this study were selected from a
commercial breeding farm without prior knowledge of
the cause of the lameness. As a result, the group with
the lame pigs was rather heterogeneous, with several
pathomorphological diagnoses on different locations in
the limbs. Since the kinetic parameters we used were
representing the forces in the entire limb, we did not
expect location of the lesion (proximal or distal in the
limb) to influence the magnitude of ASIs. However, the
study group was not large enough to make any
definitive statements on this subject.
Redistribution of pressure mat parameters
The contralateral asymmetry was assessed from the CFL
and CHL parameters. CFL compares the two forelimbs
to each other. If a pig is lame on a forelimb, it will
reduce the amount of force put on that limb by shifting it
towards the other forelimb, and to a lesser extent to the
ipsilateral and diagonal hind limbs. Thus, it is possible
that in a pig that is lame on a forelimb, the hind limb
asymmetry (CHL) also increases due to a mechanical
compensation, as there is no pain related lameness in
these hind limbs. In our experiment, as expected the
CFL was increased in forelimb lame pigs compared to
that recorded in sound pigs. The CHL of pigs that were
lame on a forelimb was also higher than that of the
sound animals. The increased asymmetry in the
forelimbs was also found in studies on lame horses [
] for PVF and VI, and for PVP in lame pigs
. In both the aforementioned equine and canine
studies, the increased asymmetry was mainly due to a
significant decrease in PVF and VI in the lame limb. The
PVF and VI in the contralateral limb did not increase
significantly in lame dogs at walk, but did increase in
horses at higher lameness grades.
In agreement with our study, also in forelimb lame
dogs and horses the CHL similarly increased for both
PVF as well as VI. This may be due to a significant force
shift from the lame forelimb to the diagonal hind limb.
Moreover, this has been attributed to unloading of the
limb ipsilateral to the lame limb (i.e., when RF is lame
RH is also unloaded) [
]. We also saw a shift towards
the diagonal limb in our lame animals, which would
cause asymmetry due to a mechanical lameness also in
the hind limbs, even though there is no true pain related
lameness present in these limbs.
The load distribution between the two hind limbs is
quantified by the CHL. Indeed, the CLH of the hind
limb lame pigs in our experiment increased, similar as to
earlier findings in hind limb lame dogs, horses and pigs
]. The increased asymmetry may be due
to a decreased loading of the lame limb, which was
found in horses [
] and dogs [
] and additionally due
to an increased loading of the contralateral hind limb. The
latter mechanism was observed in the study by Fischer
et al. [
], whereas in the study by Weishaupt et al. [
was only observed for VI and not for PVF.
The CFL of animals that were lame on a hind limb was
significantly higher than that of sound animals only for VI.
This may be because the lameness mainly affects stance
time rather that the PVF. This adaptation of timing rather
than loading may be a more subtle manifestation of
compensation in lame pigs. A significant increase of stance
time was found using kinematics in lame sows [
increased CFL in walking dogs that were lame on a hind
limb was found both for PVF and VI [
], but in trotting
horses that were lame on a hind limb no increased
asymmetry was found [
]. In the dogs, the ipsilateral
forelimb had an increased PVF and the diagonal forelimb
had an increased VI, thereby influencing forelimb
The fore-hind symmetry is assessed by the ILL and ILR.
Generally, we expected that front-hind asymmetry would
increase on the side of the lesion, as was shown previously
in dogs [
] and pigs [
]. Again, the main reason for this
asymmetry was unloading of the lame limb [
With the exception of the study by Fischer [
found an increased PVF in the ipsilateral limb of dogs, no
increase in loading of the ipsilateral limb was found in
these studies. In our experiment, we also found an
increase in asymmetry on the ipsilateral side of the lameness
with the exception of ILL of PVP. It may be that the same
mechanism that cause lameness in the forelimbs also
caused contralateral asymmetry in the hind limbs, namely
unloading of the ipsilateral limb [
], causes the difference
between the ipsilateral limbs to become smaller, resulting
in an increase in asymmetry that is not statistically
There was no increase in ipsilateral symmetry on the
side opposite the lesion, except for LR. This is in contrast
with the study on walking dogs [
], where a significant
increase in ipsilateral asymmetry of PVF and VI was seen
on the sound side as well as on the lame side. This change
only in load rate may represent a subtle manifestation of
Lameness in the left fore or right hind limb caused an
increase in DLL. Lameness in the right fore or left hind limb
caused an increase in DLR. This increase was expected, as
several studies on horses and dogs have shown
redistribution of force away from the lame limb and towards the
diagonal limb [
]. Lameness on a diagonal limb pair,
however, did not always influence the other diagonal. The
only observable change in ASI of diagonal limbs in pigs
that were lame on the limbs outside the diagonal limb
pair, was seen for LR and PVP. Bockstahler et al. [
not find a similar significant change in diagonal ASIs for
the non-lame limbs for PVF and VI, like was found in our
study. This may be due to the relatively high degree of
lameness of the pigs in our study, which may have caused
a larger effect on other limbs compared to the study by
Bockstahler et al.
Correlation with visual scoring
Correlations between visual scoring of lameness and CLTs
of all parameters were high. This finding is in agreement
with the study of Oosterlinck et al. in dogs [
], who also
found good correlations between visual scoring and PVF,
VI, and PVP, as recorded using a pressure mat. In that
study, PVP was the parameter that correlated lowest with
visual gait scores, in contrast to the findings in our study.
Oosterlinck et al. [
] hypothesized that the decrease in
limb loading of the lame limb combined with a concurrent
decrease in contact area resulted in a “falsely” low pressure
value (which is force per unit of area), rendering PVP as a
less reliable parameter. A study in horses [
compared visual lameness scoring to force plate, also found
significant correlations, particularly with PVF and VI.
Quinn et al. [
], however, did not find correlations
between PVF measured by force plate and numerical
rating scale scores of three observers scoring lame dogs. Only
one out of three observers had a significant correlation
between the numerical rating scale score and VI as recorded
using a force plate. In that study, the highest correlation
was only found when dogs were considerably lame. Our
study did not include pigs that had a lameness score of 1
(subtle lameness. This may be a reason for the discrepancy
between our findings and those of Quinn et al. [
]. It is
also possible that ASIs of kinetic parameters provide
better correlation with visual scoring than absolute values, as
a visual evaluation is an interpretation of symmetry of
locomotion rather than a measure of absolute limb
loading. Often, compensatory load redistribution to the
contralateral limb occurs, thus enhancing asymmetrical
limb kinetics and kinematics [
In the present study, we assessed the use of CLT of a set
of only four pressure mat parameters to distinguish lame
from sound pigs. Lameness has often been described in
terms of asymmetry. A previous study by Karriker et al.
] found a substantial decrease in symmetry of PVP in
an experimental lameness model in sows. However, no
cutoff values were estimated yet for these ASIs to
distinguish lame pigs from sound pigs. In dogs, however, these
cutoff values already have been established. Fanchon et al.
] used an instrumented treadmill to compare ASIs for
PVF, VI and LR and found PVF to be the only ASI with a
high accuracy. Oosterlinck et al. [
] found ASIs of PVF,
VI and contact area to be highly accurate, in contrast to
the ASI of PVP. In both studies, it was known that the
dogs were lame on the hind limbs. However, in a clinical
setting it may not always be clear which is the lame limb.
In our study, the pigs were lame only on one of the four
limbs. Therefore, we used the CLT as a potential tool to
determine the presence of lameness.
A certain degree of asymmetry was present in sound
animals as well as in lame animals. No animal was
completely symmetrical, which is in agreement with findings
in dogs [
] and horses [
]. Using ROC analysis, optimal
cutoff points were found for all four overall left-right
ASIs, with a 100% sensitivity and specificity. In future
studies, it would thus be interesting to assess the
performance of the pressure mat as a tool to detect very
subtle lameness. In this study, there were no pigs with a
lameness score of 1 (very subtle lameness) used, as such
pigs are not easily identified on a farm in a practical
In conclusion, lameness in walking pigs causes an increase
in contralateral, ipsilateral and diagonal asymmetry, due
to unloading of the lame limb combined with
redistribution of this load to the non-lame limbs. However, the six
ASI’s that were studied only took into account two limbs
at a time. Animals that were lame on a limb that was not
assessed by that ASI did not always have a higher
asymmetry than sound animals. In situations where it is
unknown which is the lame limb, a method that takes into
account all four limbs is needed. An example of such a
method is the use of overall left-right ASIs.
Using the overall left-right ASIs we were able to
distinguish lame pigs from sound pigs with 100% sensitivity and
specificity, which correlated well with subjective lameness
scores. They may provide a future objective method to
assess lameness and the effect of interventions on the
presence of lameness.
The study was reviewed and approved by the local ethical
committee of Utrecht University (DEC no 2012.III.05.04),
The Netherlands, and was conducted in accordance with
the recommendations of the EU directive 86/609/EEC. All
effort was taken to minimize the number of animals used
and their suffering.
A group of n = 10 clinically lame and of n = 10 sound
Topigs 20 × Tempo pigs (6 male, 4 female per group) were
selected by a veterinarian at a commercial breeding farm.
Only pigs that were clinically lame on one limb, that could
stand and walk unaided and that did not show signs of any
other disease were selected. The group of sound pigs was
assessed as one total batch, while due to logistics the lame
pigs were evaluated in 3 batches 2 weeks apart. All pigs
were clinically examined at the farm by a veterinarian to
make sure they fitted the inclusion criteria for this study,
before they were transported to the research facility of the
Veterinary Faculty, Department of Farm Animal Health,
The pigs were housed at the research facility of Utrecht
University. They were kept in small subgroups of 3-5
pigs in pens with closed concrete floors measuring
153 cm × 256 cm that were covered with sawdust. The
pigs were provided with 11 hours of light per day (from
7 a.m. to 6 p.m.) from both daylight and artificial
lighting. They were housed in a stall with an ambient
temperature between 22 and 24°C and one extra
heatlamp per pen was provided. The animals had ad libitum
access to food (Groeiporco, De Heus Animal Nutrition,
Ede, The Netherlands) and water, and were provided
with enrichment toys (plastic ball, metal chain) during
the entire experiment.
Upon arrival at the research facility, the lame pigs (first
batch n = 4 pigs, second batch n = 3 pigs, third batch n = 3
pigs) were put together in one single pen, without any
sound animals in the same pen. The sound pigs were
grouped according to size in two subgroups of 5 pigs. All
pigs were allowed to acclimatize for one day before
entering the experiment. At the starting day of the experiment,
piglets were weighed and clinically examined (breathing
rate, heart rate, rectal temperature, assessment of skin,
mucous membranes and lymph nodes).
The pressure mat recordings were performed using a
Footscan® 3D Gait Scientific 2 m system (RSscan
International, Olen, Belgium) with an active sensor surface of
1.95 m × 0.32 m containing 16384 sensors (2.6 sensors
per cm2), with a sensitivity of 0.27-127 n/cm2 and a
measuring frequency of 126 Hz. The pressure mat was
connected to a laptop with dedicated software (Footscan
Scientific Gait 7 gait 2nd generation, RSscan International,
Olen, Belgium). The mat was placed in a custom-built
runway as used by Meijer et al. [
]. The pressure mat
was calibrated according to the manufacturer’s
instructions by a person weighing approximately 70 kg.
Visual scoring was performed according to the method
described by Main et al. [
]. This scoring system yields
a score from 0 to 5, with 0 being a sound individual and
5 an extremely lame pig. Observations were first made
in the home pen of the piglets without disturbing them.
Posture, behavior and gait were marked. After that, the
observer approached the pigs and opened the pen to
note the behavioral response to this stimulus. Finally, if
pigs had not risen yet, the observer encouraged the pig
to stand up so that locomotion could be scored.
For the pressure mat analysis, one pig at a time was let
out of its pen and was allowed to walk freely to the
holding area. Very lame pigs that were reluctant to leave the
pen were carried. Once inside the holding area, the pig
was allowed to acclimatize for one minute. After this,
the door to the runway was opened and the pig could
walk to the holding area at the other side. All pigs
eventually started to explore their surroundings and crossed
the runway. Every time they did, they were rewarded
with candy. Exploratory behavior together with rewards
was sufficient to collect three correct runs per pig. A
run was considered correct when the pig walked (gait
confirmed by duty factor) across the runway without
stopping at a steady velocity and looking straight ahead.
After all data had been collected, the pigs were
euthanized. They were sedated using a 2 mg/kg intramuscular
injection of Azaperone (Stresnil, Elanco Animal Health,
Greenfield, USA) and subsequently euthanized by
intracardial injection of 200 mg/kg Pentobarbital (Euthanimal,
Alfasan, Woerden, The Netherlands). Gross pathology was
performed at the Department of Pathobiology of the
Faculty of Veterinary Medicine of Utrecht University, with
specific attention paid to their limbs, and in particular the
The collected footprints from their claws in the three runs
were manually assigned to left fore (LF), right fore (RF),
left hind (LH) and right hind (RH) limb using the software
provided by the pressure plate manufacturer. PVF (N),
LR (N/s), VI (Ns) and PVP (N/cm ) were normalized
for body mass.
ASIs were calculated for each variable using
modifications of the formula introduced by Oomen et al. [
The total left-right ASI (CLT) compared the left limbs
to the right limbs and was calculated using the formula:
CLT ¼ 0:5
ðLF þ LHÞ−ðRF þ RHÞ
ððLF þ LHÞ þ ðRF þ RHÞÞ
These formula’s for ASIs yield a score between -200 and +
200. Both extreme values indicate very severe (non-weight
bearing) lameness. The direction of the extreme (negative or
positive) indicated the direction of the weight redistribution.
An ASI of 0 indicates perfect symmetry.
To determine redistribution, the ability of ASIs to identify
lame pigs, and the correlation between visual scoring and
ASIs, the absolute value of the ASIs was used, removing
the distinction between right- or left-sided asymmetry. This
yields a score between 0 and 200, with higher values
indicating relatively higher loading of the left or right limb and
0 indicating perfect symmetry. Mean ASIs were calculated
from the 3 ASIs per pig.
The body mass, velocity and CLT of the lame and sound
groups were compared using an independent-samples
t-test. The data for body mass and velocity had equal
variances in both groups according to Levene’s test, but
the data from the CLT did not, therefore the
nonparametric Mann-Whitney U test was used.
To assess whether redistribution was taking place, the
animals were divided into 3 groups for the six ASIs
comparing two limbs (CFL, CHL, ILL, ILR, DLL, DLR): sound
animals, animals that were lame on a limb that was
assessed by that particular ASI and animals that were lame
on a limb that was not assessed by that ASI. For example,
for the CFL there were three groups: sound animals,
animals that were lame on the left front limb or the right
front limb (the limbs that are assessed by CFL) and
animals that were lame on the left hind limb or the right hind
limb (the limbs that are not assessed by the CFL). Since
the data were not distributed normally (confirmed by
Kolmogorov-Smirnov test), a Kruskall-Wallis test was
used to compare the three groups. A Mann-Whitney test
with Bonferroni correction was performed as a post-hoc
test to assess the differences between groups.
Spearman’s rank correlation coefficient was used to
evaluate correlations between visual scores and CLT’s.
Receiver-operated curve analysis [
] was performed to
assess the performance of each of the CLT’s as diagnostic
test in the diagnosis of lameness. The sensitivity (y axis)
was plotted against 1-specificity (x-axis) for each possible
cutoff value. A diagonal line where sensitivity is equal to
1- specificity represents a discriminating ability of the test
that is no better than chance. The top left corner
represents 100% sensitivity and specificity. The resulting area
under the curve (AUC) was used to assess the
performance of each of the CLT’s.
All data are presented as means ± SD. Statistical
significance was set at p < 0.05.
Additional file 1: Full test statistics and exact p-values for all ASI's.
LF: Left forelimb; RF: Right forelimb; LH: Left hind limb; RH: Right hind limb;
PVF: Peak Vertical Force; LR: Load Rate; VI: Vertical Impulse; PVP: Peak Vertical
Pressure; ASI: Asymmetry index; CLT: Asymmetry of both left limbs (LF and
LH) vs. both right limbs (RF and RH); CFL: Asymmetry of contralateral forelimbs;
CHL: Asymmetry of contralateral hind limbs; ILL: Asymmetry of left ipsilateral
limbs; ILR: Asymmetry of right ipsilateral limbs; DLL: Asymmetry of diagonal
fore- and hindlimb (LF vs. RH); DLR: Asymmetry of diagonal fore- and hindlimb
(RF vs. LH).
The authors declare that they have no competing interests.
EM contributed to study design and data collection, analyzed the data and
drafted the manuscript. MO provided advice on pressure mat data collection
and analysis, and critically revised the manuscript. AvN critically revised the
manuscript. WB contributed to study design, provided advice on automated
gait analysis and critically revised the manuscript. FJS aided in writing the
first draft of the manuscript, provided advice on data analysis and critically
revised the manuscript. All authors read and approved the final manuscript.
The authors are grateful to STW for providing funding for leasing the pressure
mat equipment. We would like to thank Hans Vernooij for help with the data
analysis, Leo van Leengoed for providing both practical and in-depth advice on
the design of the experiment, Matthijs Schouten for selecting the pigs on the
farm, Josette Bonhof, Bart van der Hee and Christian Bertholle for their help in
collection the pressure mat data and Arjan Stegeman for his support on
conducting the experiment and writing the manuscript.
This work was supported in part by the Science and Technology Foundation
of the Netherlands Organization for Scientific Research (NWO-STW, grant
number 11116), with co-financers Institute for Pig Genetics BV (IPG BV) and
Product Board Animal Feed (PDV).
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