Sports Eng (2016) 19:213–218
DOI 10.1007/s12283-016-0209-7
ORIGINAL ARTICLE
The influence of sitting posture on mechanics and metabolic
energy requirements during sit-skiing: a case report
K. B. Hofmann1 • M. L. Ohlsson2 • M. Höök2 • J. Danvind3,4 • Uwe G. Kersting2,5
Published online: 4 June 2016
International Sports Engineering Association 2016
Abstract Several different sitting postures are used in
Paralympic cross-country sit-skiing. The aim of this study
was to evaluate the impact of sitting posture on physiological and mechanical variables during steady-state double-poling sit-skiing, as well as to determine how seat
design can be improved for athletes without sufficient trunk
control. Employing a novel, custom-designed seat, three
trunk positions were tested while performing double-poling
with submaximal oxygen consumption on an ergometer.
Cycle kinematics, pole forces, and oxygen consumption
were monitored. The athlete performed best, with longer
cycle length and less pronounced metabolic responses,
when kneeling with the trunk resting on a frontal support.
For this case, a forward leaning trunk with knees below the
hip joint was interpreted as most optimal, as it showed
lower oxygen consumption and related parameters of performance during cross-country sit-skiing. Further investigations should examine whether such improvement is
& Uwe G. Kersting
uwek@hst.aau.dk
1
Otto Bock Healthcare Products GmbH, Vienna, Austria
2
Department of Health Sciences, Swedish Winter Sports
Research Center, Mid Sweden University, Östersund,
Sweden
3
Department of Quality Technology and Management,
Mechanical Engineering and Mathematics, Mid Sweden
University, Östersund, Sweden
4
Parasport Sweden and Swedish Paralympic Committee,
Stockholm, Sweden
5
Department of Health Science and Technology, The Faculty
of Medicine, Center for Sensory-Motor Interaction, Aalborg
University, Fredrik Bajers Vej 7D3, 9220 Aalborg Ø,
Denmark
dependent on the level of the athlete’s handicap, as well as
whether it is also seen on snow.
Keywords Sit-ski Oxygen consumption Biomechanics
Poling force Seat
1 Background
After being introduced at the Paralympic Winter Games
in Innsbruck 1988, cross-country sit-skiing (XCSS) has
become an integral part of cross-country skiing competitions organized by the International Paralympic Committee (IPC). When performing this sport, athletes sit on a
seat mounted on a pair of skis and generate forward
propulsion employing the double-poling (DP) technique,
i.e., using the right and left poles simultaneously. To
divide them into classes with similar physical limitations,
the Test-Table-Test is utilized to categorize these athletes
on the basis of their degree of trunk control and leg
impairment into five divisions (LW10, LW10.5, LW11,
LW11.5, and LW12) defined by the IPC for Nordic Skiing [1–3].
In connection with optimizing performance, energy
transfer between the athlete and his/her equipment is of
considerable importance [4, 5]. At submaximal levels of
_ 2 ), the relative VO
_ 2 is higher
oxygen consumption (VO
during arm than leg exercise [6] and, therefore, metabolic
stress on the arms should be minimized when XCSS. At the
Paralympic Winter Games in Vancouver 2010, it was
found that competitors in XCSS with sufficient trunk
control prefer to place their knees lower than the hip joint,
which allows for flexion of the trunk and effective forward
propulsion with a large pole angle during poling [7].
214
To enable this preferred sitting posture also for athletes
with insufficient trunk control, a new seat with frontal
support was designed here, and the biomechanics (kinetics
and kinematics) and metabolic output associated with three
different sitting positions were compared. Our hypotheses
were that a forward lean of the trunk both requires less
metabolic energy and enables lower poling frequency, both
longer absolute pushing and return times, than a backward
lean.
2 Case description and methods
2.1 Subject
One male athlete (body height, 1.50 m; body mass,
66.1 kg; age, 40 years) volunteered for this study who had
participated in XCSS competitions in earlier Paralympic
Winter Games, and was preparing to compete in sledge
hockey in the 2014 Sochi Paralympic Winter Games. With
a growth defect in his legs, but adequate sitting stability,
the athlete was placed in the LW 12 class [2]. Prior to the
testing, he provided written informed consent, which was
approved by the Regional Ethical Review Board in Umeå,
Sweden (2013/412-31).
2.2 Testing procedures
K. B. Hofmann et al.
trunk leaning backward onto a back rest with knees in level
with hip joints (TB KinLevel ).
Testing procedures were performed on 2 days, separated
by an 18-h resting period. The subject was instructed to
only perform light exercise (45–60 min low intensity) the
day before the testing procedures and eat and drink normally. On the first day of testing, the subject was instructed
to refrain from food and caffeine ingestion for at least 2 h
prior testing on both days. On the second day, the subject
was instructed to eat a light meal not later than 1 h before
each test. Drinking was limited to water from 1 h before
and during all tests as the subject wished. In a separate
session prior to the testing, the subject was familiarized
with the ergometer to find a preferred sitting posture for the
maximum effort trial, to become accustomed to the testing
environment and being introduced to the Borg Rating of
Perceived Exertion (RPE) 6–20 scale [8]. On test day 1, a
10-min warm-up with DP was performed followed by
_ 2 max) in the
5-min maximum oxygen uptake test (VO
preferred sitting posture, the TF Kbelow posture, during
which he was strongly encouraged and provided with
feedback each minute. The criteria for maximum effort
were a respiratory exchange ratio (RER) C1.1 and RPE
(breathing and arms) C18. On test day 2, the subject per_ 2 max, separated by
formed three 7-min trials at 80 % VO
2 h of rest to avoid fatigue [9]. Each trial began with a
10-min warm-up and involved one of the three different
sitting postures. Time of preparation and testing took
approximately 6 h on the test day 2.
All DP trials were carried out on a commercial skiing
ergometer (ThoraxTrainer, ThoraxTrainer ApS, Kokkedal,
Denmark) equipped with a custom-made adjustable seat.
The following trunk positions were tested (Fig. 1): trunk
leaning forward onto a support rest with knees below the
hip joints (TF Kbelow ), trunk leaning forward onto a support rest with knees above the hip joints (TF Kabove ) and
_ 2 similar to that under racing conditions,
To maintain a VO
the subject was provided with a visual display (Dansprint
Aps, Hvidovre, Denmark) of his power output (W). Data
Fig. 1 Subject in the three different sitting positions wearing a fullbody set of reflective markers. a Trunk lean forward onto a support
with the knees positioned below the hip joint (TF Kbelow ). b Trunk
leaning forward onto a support with the knees positioned above the
hip joint (TF Kabove ). c Trunk leaning backward, with the knees level
with the hip joint (TB KinLevel )
2.3 Experimental setup
The influence of sitting posture on mechanics and metabolic energy requirements during sit…
were recorded with the ThoraxTrainer Analyser Software.
Strain-gauge force sensors equipped with amplifiers (Biovision, Wehrheim, Germany) were calibrated and used to
measure resultant pole forces at a sampling frequency of
250 Hz. The load cells were mounted between the grip and
pole, and the signals transmitted wirelessly (TeleMyo
2400T G2) to a receiver (TeleMyo 2400R G2, Noraxon
USA Inc., Scottsdale, USA) and synchronised with the
kinematic data.
Three-dimensional kinematic data were recorded with
eight Oqus3? (Qualisys AB, Gothenburg, Sweden) cameras and the QualysisTrackManager software at a sampling
frequency of 250 Hz (standard deviation of calibration
wand length during calibration was 0.57 mm, as computed
by the software). A full-body marker set, including four
markers on the poles, was employed. The retroflective
markers were placed on the skin and covered with tightfitting perforated Lycra wear. Respiratory variables,
ambient conditions (Vaisala PTU200, Vaisala Oy, Helsinki, Finland), and heart rate (RS400, Polar Electro Oy,
Kempele, Finland) were continuously monitored during all
trials. Respiratory variables were monitored with the mixed
expired procedure, employing an ergo-spirometry system
(AMIS 2001 model C, Innovision A/S, Odense, Denmark)
equipped with pneumotachograph to measure inspiratory
flow. This system was calibrated with the standardized
procedure described by Björklund et al. [10]. The subject
wore a mouthpiece and nose clip during the tests.
2.4 Data analysis
Respiratory variables were analysed in Excel (Office Excel
2010, Microsoft Corporation, Redmond, USA). The oxy_ 2 (l/min) was computed as mean
gen consumption rate VO
of the fifth minute, where steady-state were achieved, at the
pre-defined constant power output in the submaximal tests
_ 2 was compared between the three different
(80 %). VO
sitting postures. The Visual3D (v5 professional, C-Motion
Inc., Germantown, USA) software was used to process
kinematic data from the right side of the body. All signals
were smoothed by a low-pass Butterworth filter with a cutoff frequency of 6 Hz.
One poling cycle was defined between consecutive
occasions of the right pole tip reaching its most anterior
position. The poling cycle was divided into a pushing phase
and a return phase: the pushing phase started when right
pole tip reached the most anterior position and ended in the
most posterior position, and the return phase was defined as
the opposite. Cycle length was obtained by multiplying
cycle time by the speed provided by the ThoraxTrainer
Analyser Software. Resultant pole forces were used to
calculate peak pole forces and horizontal impulses.
215
The kinematic model was applied to extract elbow,
shoulder, knee, and hip joint angles from the marker trajectories. The joint angles () were defined in the elbow
(between the upper arm and forearm, fully extended 0),
shoulder (flexion (?) and extension (-), hip (between the
femur and pelvis, flexion (?) and extension (-), standing
0), knee [flexion (?)] and thorax [forward lean (?)], and
pole in the sagittal plane [vertical (0) and forward lean
(?)].
Kinetic and kinematic measures were analysed and
averaged for ten cycles. Descriptive statistics were
employed and results are presented as mean ± standard
deviation (SD) taken during the fifth minute of each submaximal trial to ensure that the subject was in a steadystate condition.
3 Findings and outcomes
_ 2 max test, the athlete attained a RER of
During the VO
1.35 ± 0.03 and RPE of 18, thereby fulfilling the criteria
for maximal effort. During all submaximal trials, the subject was able to maintain the designated level of power,
performing DP at a constant power output.
3.1 Physiological responses
In Table 1, results are shown for power output, cardiorespiratory responses, cycle characteristics, and kinetic
parameters during the maximal test and the three 7-min
_ 2 max) in three different trunksubmaximal tests (80 % VO
stabilising postures. All values are expressed as
mean ± SD (n = 10 cycles).
_ 2 , minute ventilation (VE) (l/min), and tidal
The VO
volume (VT) (l/breath) were all higher during TF Kabove
than the other conditions, TB KinLevel and TF Kbelow
indicating a higher demand in the posture TF Kabove .
Breathing frequency and heart rate (HR) were lower during
TF Kbelow than TF Kabove and TB KinLevel (Table 1).
3.2 Cycle characteristics
The athlete demonstrated the longest cycle length during
TF Kbelow , followed by TF Kabove and then TB KinLevel ,
with the same order for absolute pushing, return, and cycle
times (Table 1). Cycle frequency was lowest for
TF Kbelow , followed by TF Kabove and TB KinLevel . The
locomotor-respiratory coupling (ratio of cycle frequency to
respiratory frequency) was 1:1.44, 1:1.42, and 1:1.22 for
TF Kbelow , TF Kabove , and TB KinLevel , respectively
(Table 1). The shortest relative pushing time was observed
216
Table 1 Power output, cardiorespiratory responses, cycle
characteristics, and kinetic
parameters of an XCSS athlete
during the maximal test and the
three 7-min submaximal tests
_ 2 max) in three
(80 % VO
different trunk-stabilising
postures
K. B. Hofmann et al.
_ 2 max
VO
TF Kbelow
TF Kabove
TB KinLevel
Power (W)
53 ± 19
37 ± 5
37 ± 5
37 ± 6
Speed (m/s)
2.85 ± 0.48
2.38 ± 0.12
2.37 ± 0.09
2.37 ± 0.13
Physiological responses
_ 2 (l/min)
VO
1.98 ± 0.02
1.61 ± 0.05
1.80 ± 0.15
1.61 ± 0.03
VE (l/min)
128 ± 2.46
50.8 ± 1.79
60.2 ± 5.44
53.0 ± 2.01
VT (l/breath)
2.79 ± 0.03
2.00 ± 0.17
2.15 ± 0.26
1.46 ± 0.13
Breathing frequency (bpm)
45.9 ± 0.42
25.6 ± 2.17
28.2 ± 2.80
36.5 ± 2.86
HR (bpm)
171 ± 1
142 ± 1
152 ± 1
149 ± 1
Cycle characteristics
Cycle time (s)
1.62 ± 0.03
1.49 ± 0.03
1.35 ± 0.01
Cycle frequency (cycles/min)
37.0
40.3
44.4
Pushing time (s)
0.80 ± 0.03
0.75 ± 0.02
0.68 ± 0.02
Return time (s)
0.82 ± 0.02
0.74 ± 0.02
0.68 ± 0.02
Cycle length (m)
3.86 ± 0.07
3.53 ± 0.07
3.20 ± 0.02
109 ± 5.76
47.4 ± 3.47
112 ± 3.84
49.4 ± 4.63
100 ± 8.41
37.4 ± 3.07
Kinetic parameters
Peak pole force (N)
Impulse (Ns)
All values are expressed as mean ± SD (n = 10 cycles)
TF Kbelow trunk leaning forward with knees below hip joints
TF Kabove trunk leaning forward with knees above hip joints
TB KinLevel trunk leaning backward with knees level with hip joints
of the pushing phase that ended approximately after the
first quarter of the return period (Fig. 2). Thereafter, pole
forces showed a slight elevation prior to the next pushing
phase. The resultant peak pole force and amplitude of the
signal during TF Kbelow and TF Kabove were 109 ± 5.76
and 112 ± 3.84, respectively, whereas peak pole force
during TB KinLevel was lower, 100 ± 8.41, which was also
the case for the impulse outcomes (Table 1).
3.4 Angular movement
Fig. 2 Pole force for ten representative cycles by an XCSS athlete
performing three 7-min submaximal tests (80 % VO2 maxÞ in three
different trunk-stabilising positions (TF Kbelow , TF Kabove ,
TB KinLevel ). The vertical lines indicate the change from pushing to
return phase
during TF Kbelow and closely followed by TB KinLevel and
TF Kabove (Fig. 2).
3.3 Kinetics
For all trials, the time-course of the resultant pole force
exhibited a pre-peak plateau, followed by a rapid rise to
peak force and then a moderate reduction in the latter half
In Table 2, the results of the kinematics of the submaximal
_ 2 max) are shown. The values presented are
tests (80 % VO
mean ± SD (n = 10 cycles).
The range of motion (ROM), as well as maximum
(Max) and minimum (Min) angles for the elbow and
shoulder were similar for TF Kbelow and TF Kabove ,
whereas TB KinLevel was characterized by lower values for
these parameters. During all trials, the Min pole angle
occurred at the beginning of the pushing phase and the Max
near the start of the return phase. In contrast to TF Kabove
and TB KinLevel variables, TF Kbelow was associated with
the smallest overall angular displacement of the extracted
hip, knee, and thorax (Table 2). However, these displacements were expected to be low in all cases to maintain a
stable sitting posture.
The influence of sitting posture on mechanics and metabolic energy requirements during sit…
Table 2 Angles () at the
elbow (between the upper arm
and forearm), shoulder [flexion
(?) and extension (-), hip
(between the femur and pelvis),
flexion (?) and extension (-),
standing 0], knee [flexion (?)]
and thorax [forward lean (?)],
and pole in the sagittal plane
[vertical (0) and forward lean
(?)] for an XCSS athlete during
the fifth minute of 7-min
submaximal tests (80 %
_ 2 max) of double-poling in
VO
three different sitting postures
217
Angle ()
Elbow
Shoulder
Hip
Knee
Thorax
Pole
TF Kbelow
ROM
59.9 ± 4.6
84.5 ± 2.7
3.4 ± 0.5
1.4 ± 0.3
3.8 ± 0.6
60.2 ± 2.0
Max
81.7 ± 5.3
69.5 ± 1.4
78.8 ± 0.5
158 ± 0.2
10.4 ± 0.4
80.8 ± 0.5
Min
21.8 ± 3.0
-14.9 ± 2.0
75.4 ± 0.4
157 ± 0.2
6.6 ± 0.3
20.6 ± 1.8
TF Kabove
ROM
60.9 ± 5.0
85.2 ± 3.0
14.7 ± 3.1
15.0 ± 3.1
4.1 ± 1.0
56.2 ± 2.3
Max
87.2 ± 6.7
71.8 ± 1.6
120 ± 0.7
60.2 ± 1.0
15.3 ± 1.2
81.7 ± 0.6
Min
26.2 ± 2.7
-13.4 ± 2.1
105 ± 2.9
45.2 ± 2.9
11.2 ± 0.4
25.5 ± 2.5
ROM
48.7 ± 2.3
71.1 ± 1.6
9.8 ± 1.6
4.8 ± 0.5
9.1 ± 1.7
58.1 ± 1.1
Max
Min
78.3 ± 2.5
29.6 ± 2.9
63.4 ± 1.1
-7.6 ± 2.0
90.4 ± 1.7
80.6 ± 0.7
61.3 ± 0.5
56.5 ± 0.4
0.9 ± 1.9
-8.2 ± 0.9
81.0 ± 0.4
23.0 ± 0.9
TB KinLevel
The values presented are mean ± SD (n = 10 cycles)
TF Kbelow trunk leaning forward with knees below hip joints
TF Kabove trunk leaning forward with knees above hip joints
TB KinLevel trunk leaning backward with knees level with hip joints
4 Discussion
The purpose of this study was to investigate how sitting
posture affects double-poling submaximal workloads for a
Paralympic sit-skier. The main finding was that the posture
TF Kabove had 12 % higher oxygen consumption rate than
TF Kbelow and TB KinLevel at the same power output. In
posture TF Kbelow , a longer cycle time and a greater cycle
length were observed indicating that this posture might be
the optimal out of these three postures for this subject.
Only limited knowledge concerning the influence of
different XCSS body postures on biomechanical and
physiological responses is presently available [11]. Previous studies have considered the potential impact of seating
position on the performance of elite athletes during a DP
XCSS race [12], and the significance of seating position for
efficient movement during sit-skiing has also been pointed
out [7].
VT was smallest in TB KinLevel , while breathing frequency was largest and the product of these two resulted in
the largest VE. Breathing frequency was coupled to poling
frequency, but the locomotor-respiratory ratios were higher
than 1:1, as observed during double poling while standing
by able-bodied athletes [13].
Even though oxygen consumption rates in this study
were similar for the postures TF Kbelow and TB KinLevel
(1.61 l/min), we interpret the TF Kbelow as the most optimal posture for our case subject. This interpretation was
supported by an integrated biomechanical and physiological perspective, with the TB KinLevel results showing lower
elbow and shoulder ROM with higher cycle frequency.
Higher cycle frequency was connected to higher breathing
frequency, and with higher breathing frequency, smaller
VT was observed. The interpretation that the posture
TF Kbelow was the most optimal was also supported by the
lowest HR being recorded for this position.
As demonstrated previously, faster able-bodied skiers
exhibit longer absolute pushing and return phases, resulting
in an overall longer cycle length [14, 15], and similar
observations have been made on sit-skiers [10]. In the
present case, to maintain speed with the shorter cycle
length and cycle time in TB KinLevel , the athlete increased
the cycle frequency, accompanied by higher breathing and
smaller elbow and shoulder ROM (Table 2). An increased
cycle frequency requires faster muscle contractions which
reduce active muscle force generation according to the
force–velocity relationship, therefore, leading to less
effective poling. It may also be the case that a forward
orientation of the trunk allows for a more effective use of
the shoulder extensors, as it has been indicated that an
increased shoulder flexion results in a larger ROM with an
increased extension force [16]. Based on the present
results, it cannot be predicted how these two mechanisms
interact in different athletes, warranting further research.
5 Limitations
As this study is a case study for one individual with a
specific training status and level of handicap results can,
obviously, not be transferred to any sit-skier. The order of
testing might have influenced our results. However, since
218
TB KinLevel , the last position to be tested, resulted in a
lower oxygen consumption value than TF Kabove , it is
unlikely that the subject was fatigued. Another limitation
of this study is the application of the strain-gauge sensors.
Pole forces should be interpreted with caution because of
the potential proportion of bending stress during the transition between pushing-to-return phase. Moreover, the trials were carried out on an ergometer that did not allow for
a minimal pole angle at the beginning of the pushing phase
and the ergometer required considerable pulling forces
during the return phase, which may alter the technique and
timing of joint actions compared to on-snow sit-skiing.
Even though the athlete in this study was not reliant on
trunk support, the present results demonstrate how sledge
seat construction potentially influences the poling performance, which might be of particular importance to athletes
lacking sufficient trunk stability.
6 Conclusion
With a forward leaning trunk and the knees below the hips
while sitting, TF Kbelow , poling mean oxygen consumption
rate was similar to leaning backward, TB KinLevel , whereas
a forward lean with the knees above the hip, TF Kabove ,
showed a 12 % higher oxygen consumption rate. The
posture TF Kbelow showed longer cycle time, longer
pushing length, larger VT, and lower HR and, therefore,
this posture was interpreted as the most optimal out of the
three for this subject.
This also indicates that athletes with insufficient trunk
control could likewise benefit from such posture. Future
investigations should examine the effects of various hip
and knee angles, as well as pole angle at the time of pole
plant on cycle characteristics during XCSS.
Compliance with ethical standards
Conflict of interest The authors have no conflict of interest to
declare.
K. B. Hofmann et al.
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