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Head, trunk and pelvic kinematics in the frontal plane in un-mounted
horseback riders rocking a balance chair from side-to-side
Article in Comparative Exercise Physiology · November 2018
DOI: 10.3920/CEP170036
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Comparative Exercise Physiology, 2018; 14 (4): 249-259
Head, trunk and pelvic kinematics in the frontal plane in un-mounted horseback riders
rocking a balance chair from side-to-side
M.T. Engell1*, E. Hernlund2, A. Byström2, A. Egenvall3, A. Bergh3, H. Clayton4 and L. Roepstorff2
1Swedish University of Agricultural Sciences, Faculty of Veterinary Medicine and Animal Science, Unit of Equine Studies, Box 7046, 750 07
Uppsala, Sweden; 2Swedish University of Agricultural Sciences, Faculty of Veterinary Medicine and Animal Science, Department of Anatomy,
Physiology and Biochemistry, Box 7011, 750 07 Uppsala, Sweden; 3Swedish University of Agricultural Sciences, Faculty of Veterinary Medicine
and Animal Science, Department of Clinical Sciences, Box 7057, 750 07 Uppsala, Sweden; 4Sport Horse Science, 3145 Sandhill Road, Mason,
MI 48854, USA; mariaterese.engell@slu.se
Received: 20 November 2017 / Accepted: 1 July 2018
© 2018 Wageningen Academic Publishers
OPEN ACCESS
RESEARCH ARTICLE
Abstract
For efficient rider-horse communication, the rider needs to maintain a balanced position on the horse, allowing
independent and controlled movements of the rider’s body segments. The rider’s balance will most likely be negatively
affected by postural asymmetries. The aims of this study were to evaluate inter-segmental symmetry of movements
of the rider’s pelvis, trunk, and head segments in the frontal plane while rocking a balance chair from side to side
and to compare this to the rider’s frontal plane symmetry when walking. Frontal plane rotations (roll) of the pelvis,
trunk and head segments and relative translations between the segments were analysed in twenty moderately-skilled
riders seated on a balance chair and rocking it from side to side. Three-dimensional kinematic data were collected
using motion capture video. Principal component analysis and linear regression were used to evaluate the data. None
of the riders displayed a symmetrical right-left pattern of frontal plane rotation and translation in any of their core
body segments. The intersegmental pattern of asymmetries varied to a high degree between individuals. The first
three principal components explained the majority of between-rider variation in these patterns (89%). A significant
relationship was found indicating that during walking, when foot eversion was present on one side, pelvic/trunk
roll during rocking the chair was asymmetric and larger to that same side (P=0.02, slope=0.95 in degrees). The
inter-individual variation in the rider’s intersegmental strategies when rocking a balance chair was markedly large.
However, there was a significant association to the rider’s foot pattern while walking, suggesting consistent intraindividual patterns over multiple situations. Although further studies are needed to confirm associations between
the findings in this study and rider asymmetry while riding, riders’ postural control can likely be improved and this
may enhance their sport performance.
Keywords: posture, pelvic symmetry, motor control, riding
1. Introduction
Riding is the most popular female (fourth overall) sport
in Sweden based on the number of registered active
athletes. In order to perform optimally in this sport, the
rider needs to communicate precisely with the horse,
using predominantly physical cues that are transmitted
to the horse via the rider’s seat (pelvis), legs and hands.
A prerequisite for efficient rider-horse communication is
the rider’s ability to maintain a position on the horse that
ISSN 1755-2540 print, ISSN 1755-2559 online, DOI 10.3920/CEP170036
allows independent and controlled movements of the limbs
and core body segments (pelvis, trunk and head). The
horse’s back movements involve gait-specific translations
(vertical, longitudinal, transverse) and rotations (yaw-,
roll-, pitch-rotations around the vertical, longitudinal and
transverse axes) (Faber et al., 2001; Van Weeren, 2006),
which determine the movement control strategies required
from the rider (Hobbs et al., 2014). The rider’s movements
are generated by perturbations (motion pattern disturbance)
arising from the movements of the horse (Münz et al., 2014;
M.T. Engell et al.
Wolframm et al., 2013). Expert riders solve this task with
a lower variability of sagittal plane movements in relation
to the horse (Lagarde et al., 2005) and a lower phase-shift
(vertical displacement) relative to the horse’s movements
(Lagarde et al., 2005; Olivier et al., 2017) compared to less
experienced riders. Expert riders are also able to maintain a
more consistent movement pattern in their mount (Lagarde
et al., 2005; Peham et al., 2004).
The term balance is defined as the state of an object when
the resultant of the loads acting upon it is zero (Pollock
et al., 2000). The ability of a human to balance in a static
situation is related to the position of the centre of mass
(COM) within the area of the base of support. Balance
control involves the visual, vestibular and somatosensory
systems (Olivier et al., 2017; Pollock et al., 2000). Athletes
strive to develop good responses to multiple sensory
information sources (proprioceptive, tactile, auditory). This
process is refined over many years of training. For, example,
studies have shown that the contribution of vision in the
regulation of balance and posture tends to decrease with
expertise (Stambolieva et al., 2011). However, the relative
contributions of different types of sensory information to
postural control differ according to the level of practice, the
type of physical activity and the degree of focus on posture
and skilled movement (Olivier et al., 2017). Inter-segmental
position, stability and movement of the pelvis trunk and
head in both the frontal and sagittal planes, are central
to determining the rider’s effectiveness; if the core body
segments (pelvis, trunk, head) are not adequately balanced,
stabilised and controlled, it will adversely affect the rider’s
balance and coordination (Roussouly et al., 2005).
Symmetry is suggested to be confirmed when no statistical
difference is noted on kinetic or kinematic parameters
measured bilaterally (Hesse et al., 1997), or when both sides
of the body behave identically (Olney and Richards, 1996).
The degree of symmetry in both the horse and the rider is
suggested to be highly relevant to excel in the sport (Symes
and Ellis, 2009; Hobbs et al., 2014). In many equestrian
disciplines, such as dressage and show-jumping, the aim is
for the horse to be equally dynamic, balanced and flexible
when being ridden to the left as it is when being ridden
to the right (Blokhuis Zetterqvist et al., 2008; Münz et
al., 2014). The symmetry of the rider is therefore highly
relevant, and the equestrian community generally agrees
that a symmetrical rider has a better possibility to influence
the horse in an optimal way (Hobbs et al., 2014). Rider
asymmetry, is on the other hand, has been suggested to be
associated with under-performance and injury to both the
horse and the rider (Symes and Ellis, 2009). Even so, several
studies have documented systematic asymmetries in the
riders’ movement pattern while riding. Experienced riders
have been reported to sit in an asymmetrical posture with
the pelvis rotated and twisted to the right (Alexander et al.,
2015), the trunk twisted to the left (Symes and Ellis, 2009),
250
and with greater external rotation of the right hip (Gandy
et al., 2014). Postural and functional asymmetries in riders
have also been documented through various unmounted
tests (Hobbs et al., 2014; Guire et al., 2016), and unmounted
physiotherapy exercise programs that focus on symmetry
have been developed to improve the rider’s symmetry
during riding (Nevison and Timmis, 2013).
There are purpose-made stools with a mobile seat
constructions that can be tilted in various directions and
these are commonly called balance chairs. Several brands
are found around the world, but one type known to be
used by riders is the Balimo chair (www.balimo.info). The
principle of balance chairs is to challenge the subject’s
ability to control the direction and amount of centre of
mass (COM) displacement when sitting. Because of these
properties, a balance chair can be used as a tool to evaluate
a person’s inter-segmental coordination of the head, trunk
and pelvis, and movement symmetry during lateral leftright rocking movements. The use of an unmounted test
situation, such as a balance chair, to evaluate riders’ postural
strategies offers an advantage over the mounted situation
by isolating the rider’s movements from the movements
of the horse.
Foot posture is known to influence pelvic alignment both
in the sagittal and frontal planes. Studies performed have
shown that during standing with one foot showing excessive
foot eversion, the pelvis will drop (roll) towards the same
foot (Khamis and Yizhar, 2007; Pinto et al., 2008; Resende
et al., 2015; Rothbart and Estabrook, 1988). The pelvic
roll may affect spinal posture, and studies have shown
that different degrees of scoliosis might be the result
(Gurney, 2002; Legaye et al., 1998; Levine and Whittle,
1996). Research has shown that certain degrees of scoliosis
might be associated with low back pain (Aebi, 2005; Gurney,
2002; Pinto et al., 2008). Riders competing at advanced
levels have been shown to have a high risk of developing
morphological asymmetries and, potentially, chronic
back pain, rather than improving their symmetry during
training (Hobbs et al., 2014). In the equestrian community,
movements of the pelvis are thought to offer the optimal
means of communication for higher performance (Blokhuis
Zetterqvist et al., 2008). If there is a correlation between
foot eversion during walking, and asymmetry when rocking
a balance chair from side to side, then it may validate the
use of a balance chair to evaluate postural asymmetries.
The primary objective of this study was to evaluate the
rider’s ability to perform a symmetric movement with
regards to the frontal plane rotational and translational
movements of the pelvis, trunk, and head segments when
rocking a balance chair from side to side. The second
objective was to investigate if there were any similarities
between movement symmetry when walking and when
rocking the balance chair. Based on previous studies of
Comparative Exercise Physiology 14 (4)
Frontal plane kinematics in un-mounted horseback riders
symmetry in riders (Alexander et al., 2015; Gandy et al.,
2014; Symes and Ellis, 2009) it was hypothesised that
the riders` movement pattern would be asymmetrical in
general, and that asymmetries detected in individual riders
might display a relationship with left-right differences in
foot eversion during walking.
2. Materials and methods
Experimental design
The riders were 24 female moderately skilled riders (age
mean: 22 years, range: 21-25 years; mean body mass: 68 kg,
range: 61-75 kg) who were students in equine studies, and
were competing in show jumping (115-130 cm) and/or
dressage (Intermediate A or B). Subjects with previous
orthopaedic injury of the pelvis or the lower limbs, or
obvious foot abnormalities other than eversion were
excluded (Engell et al., 2015).
Kinematic measurements
Spherical markers, 8 mm diameter, were fixed to the
rider’s skin according to a full body-marker model. Once
attached, all markers remained on the skin throughout the
calibration trials and the dynamic trials. The markers used
for this analysis were positioned bilaterally on the following
anatomical points: acromial edge, spinous process of C7
and T10, anterior and posterior superior iliac spine, greater
trochanter, lateral and medial malleoli, heads of the first,
second and fifth metatarsi, and the distal aspect of the
Achilles tendon insertion on the calcaneus. Clusters of
markers were attached bilaterally to the participant’s mid
shanks and mid thighs.
Figure 1. Subject seated on the reference position chair that
was used to define the neutral zero position for the kinematic
analysis.
Kinematic data were collected in 3D (250 Hz) using eight
motion capture cameras (Qualisys Oqus, AB, Gothenburg).
The participants were positioned in a custom designed
chair (Figure 1) that provided a reference position for the
kinematic recordings, before they were introduced to the
dynamic balance chair (Figure 2). In the reference position,
each subject’s acromial edges and iliac crests were aligned
horizontally so that they were symmetrical in the frontal
plane. In the sagittal plane, the head, trunk and pelvis
were aligned vertically, in a neutral spine position. For the
walking trials, the baseline marker positions were recorded
in a stance trial with each participant in a neutral standing
position on a marked area on the floor. The feet were placed
10 cm apart, toes pointing forward. Shoulders and hips
were aligned along the walking direction.
Data collection
The riders were seated on a balance chair, constructed
with a stable base, and an adjustable height element with a
seat on top. This chair had a metal rod that tilted around a
Comparative Exercise Physiology 14 (4)
Figure 2. Subject seated on the balance chair. The movement
performed on this chair was rocking from side to side at 40
beats per minute.
251
M.T. Engell et al.
pivot point located 0.3-0.7 m below the seat (adjusted to the
height of the rider). The seat could be tilted and rotated in
all directions, forward/backwards and sideways, around a
rotation and tilt element positioned about midway between
the seat and the floor, incorporating a spring element giving
progressive resistance towards the outer endpoint on each
side, and helping the rider, to a small degree, to return the
seat to a neutral position (Figure 2). The chair was moved
and controlled by the rider. The riders were instructed
to rock the chair by placing more weight alternately on
their left and right seat bones (t. ischii), as they would do
during different exercises on horseback (De Cocq et al.,
2010). When doing this, the chair would rock sideways
from right to left and left to right. They were told to follow
a frequency of 40 beats per minute, which was defined by
a metronome. The riders were allowed to try the chair
for 2 min to get comfortable with the situation before the
measurement was performed. Three to seven complete
movement cycles per subject were used for the analysis (this
large variation was due to technical difficulties during data
collection). In addition, four trials were recorded for each
subject as the riders walked barefoot along a straight 10
m walkway. Kinematic data were collected with the same
measurement technique as for the balance chair, but with
additional markers placed on the feet. Details of the walking
data have been reported previously (Engell et al., 2015).
Data analysis
Data processing and model building were performed in
Visual 3D™ (C-Motion, Germantown, MD, USA) (Cappozzo
et al., 1995). Marker data were gap-filled and signals were
filtered with a low-pass Butterworth filter at 15 Hz. Data
from four riders could not be used because of technical
problems (markers fell off (n=1), or markers not correctly
detected by cameras (n=3)), therefore leaving complete
data from 20 riders for further data analysis.
For each segment, the X-axis was oriented mediolaterally
and was positive towards the right; the Z-axis was oriented
vertically and was positive cranially and the Y-axis was
oriented in the posterior-anterior direction and was
positive anteriorly. For the pelvic segment, the origin of
the coordinate system was located mid-way between the
left and right anterior and posterior superior iliac spines,
and the left and right greater trochanters (Standard human
model in Visual 3D) (Figure 3). Definition of the given
rotations are: rotation around Y = roll, rotation around X
= pitch and rotation around Z = yaw. The rotations were
positive clockwise.
Segment rotations were calculated using a Cardan x-y-z
sequence (Cappozzo et al., 1995) of rotations. In the seated
data, the segment angles were expressed relative to the
reference position (reference-chair). Head, trunk and pelvic
translation was measured along the transverse axis of the
252
Figure 3. Coordinate system of the pelvic segment, Standard
human model in Visual 3D. The markers shown are used to
produce the segment ‘pelvis’ in Visual 3D. PSIS = posterior
superior iliac spine, ASIS = anterior superior iliac spine. The
mid-(ASIS/PSIS) markers are produced by the model.
laboratory-based coordinate system, which was aligned with
the balance chair. Relative head and trunk translations were
calculated with the pelvic position as reference.
Data for roll of the head, trunk and pelvis and relative
mediolateral translations of the head and trunk were
exported to MATLAB (The Math Works Inc., Natick, MA,
USA) for further processing. The symmetry of rotation
and translation to the right and left sides was evaluated as
differences between positive (larger degree of rotation or
translation to right) and negative (larger degree of rotation
or translation to left) areas under the curve (AUC).
The foot model and its specific terminology during walking
have been described previously (Engell et al., 2015). A
positive value for foot roll difference indicated greater
foot eversion on the right foot. A negative value indicated
greater foot eversion on the left foot.
Statistical analysis
To identify intersegmental asymmetry strategies, a principal
component analysis (PCA) was performed in MATLAB.
Asymmetry variables were normalised to have a zero
mean and unit variance. Only riders with complete data
were included. Principal component analysis rearranges
a multidimensional variable space (in this case our body
segment rotation and translation asymmetry) so that as
much as possible of the variation is described in the first
principal component. The second principal component is
constructed so that it is orthogonal to the first principal
component, and explains as much of the remaining variance
as possible, etc., until the total variance is explained. For
each principal component, the variable weights quantify
Comparative Exercise Physiology 14 (4)
Frontal plane kinematics in un-mounted horseback riders
the relative contribution of each original variable to the
principal component. Looking at the sign of the weight the
relation between two variables can be studied. This means
that if two weights have opposite signs those variables have
a negative correlation. Technically, the PCA transforms
the normalised data (X5×n) into principal components,
eigenvectors, using an eigenvector decomposition method
on the input’s covariance matrix. The eigenvectors (V5×5)
and eigenvalues (L5×1) were produced and used to compute
the principal component scores (Z5× n) by multiplying the
normalised input data matrix (X5×n) by the eigenvector
matrix (V5×5). The principal components were then sorted
by the percentage of variance explained by each.
Regression analysis (PROC REG, SAS version 9.4, SAS
Institute Inc., Cary, North Carolina) was utilised to compare
intersegmental coordination while rocking on the balance
chair, with foot eversion during walking. The dependent
variable (y axis) designates the difference in degree of
eversion between the left and right foot and the independent
variables (x axis) were translations and rotational symmetry
variables as well as sum of the pelvic and thoracic symmetry
values, six models in total. The dependent variable was
found to be normally distributed after applying the ShapiroWilks test. Independent variables were plotted versus the
dependent variable to evaluate departure from linearity.
In this analysis, only riders with an eversion asymmetry
were included. The cut-off for determining if eversion
asymmetry was present was set at 1.5 degree because it
was judged by the authors that asymmetries below this
level would not be accurately detected. This established
cut-off level excluded the data of one rider (no 3) for the
analysis. A P-value of <0.05 was used to determine if a
significant relationship existed. The trunk and pelvic roll
data were summed to include both riders that compensated
for COM displacement more with the pelvis or more with
the trunk, to eliminate the variation of individual segmental
strategies. Since the main aim was to evaluate if there was
any correlation between torso roll asymmetry and the foot
with the highest degree of eversion.
3. Results
Examples of the raw kinematic data series are shown in
Figure 4. Symmetry values for each rider’s translational and
roll movements along or around the Y axis of the head, trunk
and pelvic segments are shown in Table 1. Some riders, e.g.
(11, 6, 8) and (1, 7, 17), could be grouped together based on
having the same pattern of deviation from baseline, across
the five variables (marked with the same grey shade), but in
general there was considerable variation between the riders.
In spite of displaying individual characteristics, the data
reveal some tendencies to common strategies as well: when
one or two segments were rotated or translated more to
one side, another body segment automatically compensated
for the COM displacement by rotating or translating to the
Comparative Exercise Physiology 14 (4)
opposite side. The trunk was the segment with the most
pronounced tendency for roll (a higher range of motion].
The pelvis showed only a small degree of roll but more
translation. The head had a more pronounced tendency
for translation compared to the trunk (both translations
relative to the pelvis).
Table 2 (from n=17 riders with complete data in Table 1)
displays the first 4 principal components (eigenvectors)
that accounted for 98% of the explained variance. The
first 3 principal components alone explain the majority
of different strategies between riders (89%). For the first
principal component, the major effects (having the largest
weights) are the trunk and head translations, which are
in the same direction and, to compensate for the COM
translation, the pelvis rolls to the opposite side. For the
second principal component the pelvis, trunk and head
roll have the major effect, with the head and pelvis rotating
to the same side and the trunk to the opposite side. The
third principal component also has roll rotation of head,
trunk and pelvis as the major effect but, in this principal
component, the pelvis and trunk rotate to the same side
and the head towards the opposite side.
When comparing segmental coordination while sitting
with foot eversion during walking, the cut off for the
eversion (excluding subjects with absolute eversion values
<1.5 degrees because such low asymmetry values were
considered to have less biological significance and accurate
detection within the study measurement setup) excluded
one rider (no 3). Of the six regression models only one
regression model produced a significant result. Significance
(P=0.0232) was found for the sum of the trunk and pelvis
roll asymmetry values during rocking a balance chair
modelled as the independent variable (x-axis), while the
dependent variables (y-axis) was the degree of eversion
during walking. The intercept was -2.71 (standard error
1.47) and the slope was 0.95 (standard error 0.37), in degree
units. The data suggest an association between eversion on
one foot and a higher degree of rotation in either the pelvis
or trunk, or a combination of the two, to the opposite side of
the more everted foot. A plot of the data, and a regression
line, for the regression of pelvis and trunk roll asymmetry
added, versus left-right difference for foot eversion (n=16)
is shown Figure 5.
4. Discussion
The frontal view kinematics of the pelvis, trunk and head
segments in twenty riders have been analysed in a seated
position while rocking a balance chair from side to side
in rhythm with a metronome (40 bpm). It is not within
the scope of this paper to evaluate how well this balance
chair is suited for evaluating rider-specific skills, but our
goal with the chair was rather to use it in a test situation
in order to evaluate specific postural skills for a sitting
253
M.T. Engell et al.
A
Rider 1
0.4
30
0.3
20
10
0.1
0
0
Angle (degrees)
Distance from lab center (m)
0.2
-0.1
-10
-0.2
-20
-0.3
-0.4
B
0
2
4
6
8
10
Time (sec)
12
14
16
18
Rider 4
0.4
-30
20
30
0.3
20
10
0.1
0
0
Angle (degrees)
Distance from lab center (m)
0.2
-0.1
-10
-0.2
-20
-0.3
-0.4
Pelv trans
Trunk rel trans
Head rel trans
0
2
4
6
8
10
Time (sec)
12
14
16
Pelv roll
Trunk roll
Head roll
18
-30
20
Figure 4. (A) Raw data series from two (a and b) individual riders. The graph displays head, trunk and pelvis roll (degrees) with
interrupted lines; and head, trunk and pelvis translation (distance in meter) with solid lines. (B)
254
Comparative Exercise Physiology 14 (4)
Frontal plane kinematics in un-mounted horseback riders
Table 1. Individual level symmetry values for translations (mm) relative to pelvis and roll (degrees) variables for 20 riders.1
Test person Head roll
Head
translation
Trunk roll
Trunk
translation
Pelvis roll
Eversion
left foot
Eversion
right foot
Eversion
diff
Pelvis and trunk
roll added
11
6
8
3
10
15
18
1
7
17
2
4
9
12
13
14
16
20
5
19
4.4
4.3
-0.7
2.9
7.4
0.1
3.3
3.3
5.3
11.2
-3.1
-1.6
-1.5
-4.9
-4.0
-2.6
4.5
24
42
-30
-12
40
-11
-9
11
78
38
27
20
36
8
-1
-32
15
0
1.8
-1.9
4.2
1.3
5.2
-2.6
-0.2
-4.4
-3.1
-4.0
0.0
-7.6
-1.5
-0.2
0
-1
-0.3
3.2
0.2
26.0
10.9
8.9
13.4
16.3
21.3
10.9
30.6
23.0
-8.5
-4.3
2.8
-1.1
-3.1
-4.5
13.8
-15.9
-2.3
15.9
11.5
13.6
19.0
18.4
21.0
18.8
26.8
-2.5
-9.5
-5.2
-7.8
20.9
11.7
9.6
16.9
10.0
11.4
4.1
1.6
-1.8
-3
11
7
-11
1
15
1
-19
4
22
2
10
9
10
9
2
-20
1
3
-5
28
17.4
6.7
11.7
12.4
13.2
16.7
24.7
14.7
20.7
-0.8
2.9
0.0
-1.2
4.2
-4.6
1.1
-5.5
2.4
9.8
2.9
0.0
4.3
3.4
1.9
1.2
0.9
-2.8
-2.5
5.5
0.8
13.2
9.5
3.7
2.9
1.8
-3.1
8.3
-3.4
6.3
-8.1
2.1
5.4
-0.1
-4.1
4.3
-4.2
0.4
1.0
0.8
-3.4
-2.8
8.7
1.0
1
Values are averages over available motion cycles (3-7 per subject). Negative value = offset to left side, positive = offset to right side. Eversion diff is
the difference between left and right foot eversion. Negative value = left foot more everted, positive = right foot more everted. The grey shades indicate
similar strategies among individuals with regard to direction of asymmetry between segments. The light-grey-marked riders have the same sign throughout,
either only positive or only negative values; light light-grey riders are similar to dark blue, but have one upper segment variable with the opposite sign;
dark-grey riders has pelvis roll to one side and all upper segment variables to the other side and light dark-grey coloured riders are similar to those with
dark grey colour, but have head roll to the same side as pelvis roll.
Table 2. The relative contribution (weights) of 5 variables to the principal components analysis (PCA) (n=17). The variance explained
by each of the first 4 principal components is shown.
Principal components
Pelvis roll
Trunk translation
Trunk roll
Head translation
Head roll
Explained variance (%)
1
2
3
4
-0.27
0.57
0.40
0.62
0.26
47.90
0.47
-0.01
-0.39
0.14
0.78
22.96
0.78
0.26
0.51
-0.15
-0.19
18.05
-0.17
-0.54
0.66
-0.19
0.46
9.95
athlete. The riders were instructed to alternately place more
weight on their left or right seat bone, just as they should
do during many ridden exercises, such as on circles, in
canter transitions, and during lateral movements. Because
the subjects were educated riders, it was expected that they
would accomplish this weight shift primarily by displacing
Comparative Exercise Physiology 14 (4)
their pelvis (translation and/or rotation). It is generally
accepted that the rider should communicate with the horse
mainly by the use of the pelvis (Blokhuis Zetterqvist et
al., 2008; Münz et al., 2014), while avoiding large lateral
deviations of the head and trunk.
255
M.T. Engell et al.
15
10
Eversion
5
0
Observations 16
Parameters 2
Error DF 14
MSE 33,602
R-Square 0.3302
Adj R-square 0.2824
-5
-10
Data
Fit
Confidence bounds
-15
-20
-8
-6
-4
-2
0
2
4
Pelvis + trunk roll
6
8
10
Figure 5. Fit plot for eversion (Y) versus trunk and pelvis roll
added (X).
To tilt the balance chair to the side, the rider needs to create
a lateral movement of one or more body segments from
baseline, resulting in a lateral displacement of the rider’s
COM. The effect will be dependent on the segments’ relative
mass. A larger mass in the upper segments (head, trunk,
arms) requires less translation and/or rotation to create the
necessary leverage (lever arm * mass). But the rider could
also create the desired effect by moving both upper and
lower segments to one side simultaneously, by initiating
a small translation of the trunk before translating the
pelvis to apply a force onto the chair and move it laterally.
Moving the upper or lower segments simultaneously will
require a smaller lateral displacement to move the chair,
likely making it easier to stop the chair before it reaches
its endpoint where the rider might lose control over the
chair movement. We anticipated that total movement of the
pelvis, translation and roll, would be at least as large as in
the upper segments, but this was true only for a minority of
the riders. Almost all riders displayed either a high degree
of trunk roll (in general more than 1.5 times the degree
of pelvic rotation, Table 1) or trunk translation combined
with a very low degree of translation and roll of the pelvis.
A reason for this unexpected pattern could be that the
balance chair presents an artificial situation that does not
replicate the typical movements performed on horseback,
which are driven by the horse and followed by the rider.
On the other hand, the chair is constructed so that, as the
rider feels the progressive resistance in one direction, the
force applied in that direction is reduced and transferred
towards the opposite side, which mimics how the weight is
transferred on the horse during midstance of each hind limb
(von Peinen et al. 2009). Perhaps the strategy of rolling the
trunk and head laterally is the simplest and most intuitive
way of rocking the chair since it takes advantage of the long
lever arms of these segments. But when riding a horse,
it is assumed that the optimal way of communication
should be through the pelvis (Blokhuis Zetterqvist et al.,
2008; Münz et al., 2014). This implies that if the riders
performed the lateral movements similar to changing seat
bone when riding, the rider should initiate, control, and stop
the movements (range of motion in rotation/translation)
from the pelvis while the upper segments (trunk and head)
are maintained more stationary, so the rider COM is not
on the outer limit of the base of support.
If the riders are initiating the movement from side to side
on the chair with the trunk or head, resulting in a large
range of rotation or translation of these segments, the
Figure 6. Snapshots of a rider on the balance chair. This rider displays a higher left pelvic roll during moving from left towards
right (B) compared to right pelvic roll during moving from right to left (A). This rider had a higher degree of eversion on the right
foot, compared to the left foot, during walking (not shown in figure).
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Comparative Exercise Physiology 14 (4)
Frontal plane kinematics in un-mounted horseback riders
pelvis will need to compensate with the opposite rotation/
translation and this will cause a higher degree of phase
shift (vertical displacement) between segments (Leirdal et
al., 2006) and probably also between horse and rider. It is
important to keep the phase shift at a low level in order to
maintain the rhythm, which is one of the main goals during
riding (Lagarde et al., 2005; Olivier et al., 2017). The riders
were instructed to put weight on each seat bone assuming
that they would be able to create movements similar to
movements they would do when riding. We suggest that
these riders may use more or less the same strategy on
horseback as on the chair. Future studies will clarify the
relationship between movement patterns on the balance
chair and during riding.
The data reported here are part of a series of studies
designed to investigate associations between postural
asymmetries during walking, sitting and riding. We have
previously reported that during walking the majority of
riders had significantly greater contralateral pelvic drop
when the foot with the higher degree of eversion was in
early stance (Engell et al., 2015). For the task of rocking
a balance chair from side to side, the data display an
association between eversion on one foot and a higher
degree of rotation to the opposite side in either the pelvis
or trunk, or a combination of the two segments. The reason
for this observation might be that over time walking with
the same postural asymmetry (e.g. contralateral pelvic drop
due to a highly everted foot) eventually leads to a change
in the organisation of movement representations in the
primary motor cortex (Jensen et al., 2004; Lakhani et al.,
2016). Future studies will investigate if the same postural
asymmetries present during walking and rocking a balance
chair are also present during riding.
The findings of this study showed that educated riders did
not have a symmetrical right-left pattern of frontal plane
rotation and translation in any of their core body segments
while rocking the balance chair (i.e. motion cycle mean
position was not zero, Table 1). The specific pattern of
asymmetries varied between individuals (Table 1), but the
data revealed some common tendencies as well. Generally,
when one or two segments were rotated or translated
significantly more to one side, another body segment
compensated for the COM displacement by increased
rotation and/or translation to the opposite side. Only two
riders displayed a larger range of movement to the same
side for all segments in both translation and rotation. This
compensatory pattern is further reflected in the weights of
the principal components, with each principal component
having negative weight for at least one variable (Table 2).
Roll asymmetry in two of the core body segments (head,
trunk, pelvis) to one side was typically compensated by
roll asymmetry to the contralateral side in the remaining
segment. This was evident in the first three principal
components. Less often predominant roll to one side was
Comparative Exercise Physiology 14 (4)
compensated by translation to the contralateral side as
indicated by opposite signs for the weights of these two
movements for the head and pelvis in the fourth principal
component (explained 10% of the variance, Table 2). One
reason might be that the human motor cortex has a low
ability for precise repetition of a given movement and a
remarkably high flexibility with regards to motor control
(Sanes, 2000), unless it is specifically trained through
skilled learning/deliberate training (training at different
speeds with high precision) (Jensen et al., 2004). We might
assume that the more advanced level rider, like athletes
performing at a high level in other sports (Jensen et al.,
2004), needs to refine their skill training and define what
type of movement strategy is optimal for superior sports
performance. If the same postural asymmetries that are
present during walking and sitting are also present during
riding, the riders in our study could potentially benefit from
balance control training, since asymmetry is recognised
as a negative trait with regards to equestrian performance
(Hobbs et al., 2014; Symes and Ellis, 2009). Further studies
are needed to define this in detail. The practice of training
on different instruments to develop an athlete’s skill or
precision is well accepted in many other sports (Anderson
et al., 2005; Leirdal et al., 2006). It has been shown that the
rider’s balance and symmetry during riding can be improved
through unmounted training (Nevison and Timmis, 2013),
but more studies are needed to clarify if this correlates
to improved rider-horse communication efficiency and
competition results.
Some riders had a significant, persistent lateral rotation of
the head towards one side. As discussed in Olivier et al.,
2017, it is well known that vision is of great importance
for postural control. Other studies have shown that the
visual contribution to the regulation of postural balance
tends to decrease with expertise, while somatosensory and
vestibular information become more important (Olivier
et al., 2017; Stambolieva et al., 2011). If the head is out
of vertical alignment, however, the other segments might
need to compensate, which could explain some of the
asymmetrical patterns seen. It is generally accepted among
riders that, in order to develop symmetric movements in the
horse, the rider should exhibit a high degree of symmetric
communication from the left and right seat-bones, as well
as from the hands and legs (Alexander et al., 2015; Hobbs
et al., 2014; Peham et al., 2004). It is, therefore, somewhat
intriguing that in a seated test condition, which is not too
unlike sitting on a saddle, the riders’ performances were
asymmetric. This also included the head movements.
One of the limitations of this study is the use of skinmounted markers. It has been shown that skin markers
move in relation to the underlying bone position, but
the possibility of use of bone markers was not an option.
Another limitation is the fact that the leg position on the
balance chair is somewhat different to that of sitting on a
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M.T. Engell et al.
saddle with the feet supported in the stirrups. To determine
whether these limitations present major differences when
comparing with the mounted situation, these data need to
be compared with mounted data. In addition a limitation is
that the population only comprised women. The riders in
the study were deemed as moderately skilled based on their
competition level. Unambiguous determination of rider skill
level is problematic. Unambiguous determination of rider
skill level is problematic. Considerable variation between
evaluators performing subjective assessments of rider
skills has been shown (Blokhuis Zetterqvist et al., 2008).
Currently there are no objective methods available that
are sufficiently validated. Therefore we cannot determine
if moderate variation in rider skill level has confounded
our results.
5. Conclusions
The frontal view kinematics of pelvis, trunk and head
segments in twenty riders have been analysed in a seated
position, while rocking a balance chair from side to side
in rhythm with the beat of a metronome. Our findings
show that the riders positioned themselves and moved
asymmetrically on the chair. None of the riders displayed a
symmetrical pattern of right-left rotations and translations
in the frontal plane. A significant relationship (P=0.02)
was found between the within subject difference in of foot
eversion during walking and the rotation of the pelvic and
trunk segments during sitting. The side of the body that had
a higher degree of foot eversion displayed greater trunk and
pelvic roll to the opposite side when sitting and rocking the
chair. It needs to be confirmed whether riders in general,
and specifically those performing at higher levels, maintain
this asymmetrical movement pattern while riding. We
suggest that the riders’ postural control can be improved
and that this possibly would have a positive effect on sport
performance.
Acknowledgements
We express our gratitude to Ulla Håkansons Stiftelse for
funding the study. The authors thank Håvard Engell for
expertise on the analysis on the rider’s posture, and the
riders for their participation.
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