Chairless Chair Exoskeleton. Work-physiological-biomechanical Analysis of the Lower Extremities
Phase: NA
Conditions: Healthy
Sponsor: University Hospital Tuebingen

 
 
 
 
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CHAIRLESS CHAIR EXOSKELETON 
WORK-PHYSIOLOGICAL-BIOMECHANICAL ANALYSIS OF THE LOWER 
EXTREMITIES 
 
 
Head of study (UKT) 
 
Dr. Benjamin Steinhilber & Dr. Tessy Luger 
Director 
 
 
Prof. Dr. Monika A. Rieger 
Other researchers (UKT) 
Timothy Cobb (Medicine, Doctoral Student) 
 
 
 
 
Theresa Kreidler (Medicine Technique, Bachelor Student) 
Contact persons 
Dr. Benjamin Steinhilber  
Institut für Arbeitsmedizin, Sozialmedizin und Versorgungsforschung 
Wilhelmstraße 27 
72074 Tübingen 
 07071-29-86805 |  07071-29-4362 
 Benjamin.Steinhilber@med.uni-tuebingen.de 
 
Dr. Tessy Luger 
Institut für Arbeitsmedizin, Sozialmedizin und Versorgungsforschung 
Wilhelmstraße 27 
72074 Tübingen 
 07071-29-84364 |  07071-29-4362 
 Tessy.Luger@med.uni-tuebingen.de 
 
 
 
 

 
 
 
 
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I 
Abstract 
Standing work is associated with increased risks of venous and musculoskeletal disorders; particularly 
low back pain is commonly reported in prolonged standing work. In manufacturing work, work-
stations often do not allow standing aids due to insufficient functional and spatial conditions. In 
2014, the car manufacturer Audi introduced the lower leg exoskeleton developed by Noonee to their 
employees working in the factories. This exoskeleton, the ‘Chairless Chair’, has the advantage that 
standing work can be performed while technically sitting on this device. The exoskeleton offers the 
potential for reduced awkward body postures, but it is unclear which biomechanical loads are 
influenced and how. This study will evaluate the ‘Chairless Chair’ in a laboratory setting, by testing its 
effectiveness in terms of biomechanical parameters, including postural balance, back and neck 
curvature, and muscular activity of selective muscles. For this purpose, different work settings will be 
compared, consisting of a mixture of different working heights and different positions (wearing or 
not wearing the exoskeleton) while performing simulated assembly tasks. The study population will 
consist of 42 healthy men in the age of 18-40 years old, who will perform 7 different experimental 
conditions. 
II 
Keywords 
Exoskeleton, manual materials handling, musculoskeletal disorders, postural balance, standing work 
 
 

 
 
 
 
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Table of Content 
Table of Content ................................................................................................................................ iii 
1 
Abbreviations .............................................................................................................................. 4 
2 
Background .................................................................................................................................. 4 
3 
Objectives .................................................................................................................................... 5 
4 
Timetable ..................................................................................................................................... 5 
5 
Study Design and Study Population .................................................................................... 5 
5.1 
Study Design ........................................................................................................................ 5 
5.2 
Study Population ............................................................................................................... 7 
5.3 
In- and Exclusion Criteria ............................................................................................... 7 
5.4 
Recruitment ......................................................................................................................... 7 
5.5 
Sample Size .......................................................................................................................... 9 
6 
Study Procedure ......................................................................................................................... 9 
6.1 
Experimental Procedure .................................................................................................. 9 
6.2 
Experimental Tasks ......................................................................................................... 10 
7 
Data Collection ......................................................................................................................... 10 
7.1 
Anthropometry ................................................................................................................. 11 
7.2 
Posturography .................................................................................................................. 11 
7.3 
Muscular Stress and Strain .......................................................................................... 12 
7.4 
Reference Measurements ............................................................................................. 12 
7.5 
Work Performance .......................................................................................................... 13 
7.6 
Subjective Assessment ................................................................................................... 14 
7.7 
Participant Evaluation ................................................................................................... 15 
8 
Statistical Analysis .................................................................................................................. 15 
9 
Risk for the Participant ......................................................................................................... 15 
10 Data Protection ........................................................................................................................ 16 
11 Study Finance ............................................................................................................................ 16 
12 References ................................................................................................................................. 17 
 
 

 
 
 
 
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1 
Abbreviations 
ARV 
Average Rectified Value 
COG 
Center Of Gravity 
COM 
Center Of Mass 
COP 
Center Of Pressure 
EMG 
ElectroMyoGraphy 
FSA 
Force Sensing Array 
IED 
Inter-Electrode Distance 
L3 
Lumbar vertebra 3 
MdPF 
Median Power Frequency 
MnPF 
Mean Power Frequency 
PPI 
Pressure Peak Index 
RMS 
Root Mean Square 
RPD 
Rating of Perceived Discomfort 
% RVC 
Percentage of Reference Voluntary Contraction 
SD 
Standard Deviation 
T3 
Thoracic vertebra 3 
UKT 
[Universitäts Klinikum Tübingen] 
2 
Background 
Standing work is associated with an increased risk of venous disorders (Sudol-Szopinska et al. 2011: 
Ziegler et al. 2003) and musculoskeletal disorders (Werner et al. 2010). Particularly low back pain is a 
commonly reported disorder in prolonged standing work (Andersen et al. 2007; Gregory and 
Callaghan 2008). The development of lower limb discomfort is mainly the result of stationary 
standing, and to a lesser extent of dynamic standing (Balasubramanian et al. 2009). Some studies 
suggested using aids during standing work to relieve the load of the back muscles and reduce 
symptoms; however, not every workplace allows the use of standing aids because functional and 
spatial conditions are insufficient. 
One solution to reduce the exposure of employees to associated risks for developing work-related 
musculoskeletal disorders is to use exoskeletons. Using such devices in dynamic environments has 
the advantage over, e.g., robotics because it does not need any programming or teaching of robots. 
Moreover, exoskeletons are worn on the body and do not have to overcome spatial issues. In a 
recent review, 26 different exoskeletons have been described of which only two were designed to 
support the lower body during heavy work (de Looze et al. 2015). For low-intensity work, like 
assembly work in the automobile industry, no study has focused on using exoskeletons to relieve 
employees while performing the work standing. 
Since 2014, Audi has several passive exoskeletons to use in their production lines and factories in 
Southern Germany, where employees perform different types of assembly work while standing (Feigl 
2015; Karius 2015; Stirnemann 2015). The passive exoskeleton, the ‘Chairless Chair’, provides 
support for the lower extremities. The system is developed by the Swiss company Noonee (Saameli 
2014). The ‘Chairless Chair’ offers the potential for a reduction of awkward body postures, even in 
restricted spatial conditions at standing workplaces. It is unclear which physiological and 

 
 
 
 
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biomechanical loads are influenced by the ‘Chairless Chair’ exoskeleton and how. Furthermore, it is 
not clear whether participants of all body heights will experience the same beneficial effects, if any, 
when wearing the system. 
3 
Objectives 
This research proposal aims to provide a sophisticated study design in which participants simulate 
assembly work related to that performed in the Audi factories. With measurements of several 
physiological and biomechanical parameters, work-physiological-biomechanical analyses will be 
performed to investigate the influence of wearing the exoskeleton during assembly tasks. The 
participants included will be grouped according to their body height, enabling us to investigate 
whether body height influences the effectiveness of wearing the exoskeleton. 
4 
Timetable 
The project consists of four phases (Table 1). First, the study design and measurements need to be 
prepared, which will take approximately three months. Participants will be recruited during three 
months and the measurements will take up to three months. The data analysis, evaluation, writing 
and publication will be spread over five months. The four phases will be performed with some 
overlap with a total length of the study of about ten months. 
Table 1. The timetable for the proposed study that will be completed early 2018. 
Month 1 
Month 2 
Month 3 
Month 4 
Month 5 
Month 6 
Month 7 
Month 8 
Month 9 
Month 10 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Preparations 
 
 
 
Recruitment 
 
 
 
Measurements 
 
 
 
Analyses, Evaluation, and Publication 
 
 
5 
Study Design and Study Population 
5.1 
Study Design  
The laboratory study will include several experimental conditions, being a combination of supported 
seating or unsupported standing by the exoskeleton, and three different working heights. These 
seven combinations will be offered to the subjects in a balanced, randomized cross-over design (Latin 
Square Williams Design; Williams 1949). In each of these seven conditions, three manual materials 
handling tasks will be simulated in a fixed order but in three different frontal working distances. 
Because of the three working distances, we will include a multiple of the 14 orders resulting from the 
Williams Design, striving for a population of 42 subjects.  

 
 
 
 
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5.1.1 Independent variables (within-subject factors) 
At first, experimental conditions are designed by a combination of working position and working 
height. We will investigate three different working positions [P]: (1) unsupported standing [US], (2) 
supported high seating [HS], and (3) supported low seating [LS]. We will investigate three working 
heights [WH]: high [H], medium [M], and low [L]. The combination of these two independent 
variables is displayed in Figure 1A. The randomized design for assigning conditions to subjects will be 
based on an orthogonal Latin Square Design (Figure 1B). A third independent variable considered is 
working distance, i.e. the distance of the task set-up from the front of the subject: (1) optimal, (2) far, 
(3) very far; the order of these three working distances will also be varied by assigning them balanced 
to the subjects. For the order of the three working distances, the outcome of a special type of Latin 
Square Design, the Williams Design suggests three different orders (for the complete experimental 
design, see Attachment 8). 
 
Figure 1. A. Incomplete Block Design (Williams Design) based on position (P) and working height (WH), resulting in 7 
different experimental conditions; B. Latin Square Design for 14 subject, which will be preferably elongated to a replicate of 
14 subjects. 
We consider three fixed working positions for each subject, standing, high seating and low seating on 
the Chairless Chair. According to the anatomy of the individual, the optimal working height for each 
working position can be calculated (Equation 1; in accordance with DIN EN ISO 14738:2009-07). 
Optimal working heights in Figure 1 correspond to (A) PSS x WHH, (D) PHS x WHM, and (G) PLS x WHL, 
and are based on an optimal elbow angle of 105° with the upper arm vertical alongside the upper 
body. The non-optimal working heights deviate -10% and +10% from the optimal working height. In a 
similar way, the optimal working distance (WD) with respect to the subject (in accordance with DIN 
EN ISO 14738:2009-07) is calculated (Equation 2) and the non-optimal working distances deviate 
+15% and +30% from the optimal working distance. 
𝑊𝐻𝑜𝑝𝑡𝑖𝑚𝑎𝑙= 𝑙𝑒𝑛𝑔𝑡ℎ𝑒𝑙𝑏𝑜𝑤−𝑓𝑙𝑜𝑜𝑟−(sin(𝛼) ∙𝑙𝑒𝑛𝑔𝑡ℎ𝑒𝑙𝑏𝑜𝑤−𝑔𝑟𝑎𝑠𝑝𝑖𝑛𝑔 ℎ𝑎𝑛𝑑) 
 
 
 
Eq. 1 
𝑊𝐷𝑜𝑝𝑡𝑖𝑚𝑎𝑙= cos(𝛼) ∙𝑙𝑒𝑛𝑔𝑡ℎ𝑒𝑙𝑏𝑜𝑤−𝑔𝑟𝑎𝑠𝑝𝑖𝑛𝑔 ℎ𝑎𝑛𝑑 
 
 
 
 
 
 
Eq. 2 
with α the angle (°) with the horizontal position of the lower arm, which is always 15°. 

 
 
 
 
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5.2 
Study Population  
We will select male participants with a body height corresponding to the average German body 
height of the working population aged 18-40 years old. We will recruit males only, because most 
assembly work at Audi is performed by men. In 2013, German males were on average 180.6 (SD 7.3) 
cm tall (Statistisches Bundesamt 2015). 
5.3 
In- and Exclusion Criteria  
Inclusion criteria (Attachment 2): 
- 
Age between 18 and 40 years; 
- 
Male gender; 
- 
The participant will give his voluntary informed consent after receiving oral and written 
information of the content and goal of the study. 
Exclusion criteria (Attachment 2): 
- 
Aged <18 and >40 years; 
- 
Female gender; 
- 
People under the influence of intoxicants, analgesics, or muscle relaxants; 
- 
Alcohol abuse; 
- 
People with cardiovascular diseases; 
- 
People with a heart pacemaker; 
- 
People with a disability who, due to their restriction at a workplace of this kind, will not be 
able to participate; 
- 
People with Diabetes Mellitus; 
- 
People with severe muscle contractions of the lower extremities, back or arms; 
- 
People with acute ailments or pain; 
- 
People who are unable to complete the examination program due to language or cognitive 
obstacles; 
- 
Depending on the degree of severity, people with diseases of the veins and joints of the 
lower extremities, spine, muscle disorders, symptomatic neurological-psychiatric diseases, 
acute pain syndromes, maladies or other current diseases. 
5.4 
Recruitment 
Volunteering participants will be recruited by means of an announcement e-mail at the University 
Hospital Tübingen, the Eberhard-Karls-University Tübingen, as well as by e-mail to former 
participants who agreed to be informed about future studies (Attachment 7). 
 
 

 
 
 
 
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Recruitment 
Interested in the study 
Participant information 
Participation? 
Check in- and exclusion criteria 
Group allocation according to 
inclusion criteria 
Group 2 
Body height 176 – 186 cm 
N = 26 
Group 1 
Body height < 176 cm 
N = 8 
Group 3 
Body height > 186 cm 
N = 8 
Randomization of the order of the 7 exposure conditions 
Working height (3) | Working position (3) 
Measurement preparation 
Pre-measure I-N 
Subjective rating 
Pre-measure I-N 
Subjective rating 
Reference measurements 
Exposure N 
(20 min) 
Exposure N 
(20 min) 
Post-measure I-N 
Subjective assessment 
Post-measure I-N 
Subjective assessment 
Post-measure II 
Balance 
Cleaning subject 
0 
45 
60 
70 
210 
220 
280 
240 
1× 7 
20× 7 
1 × 7 
Evaluation 
Rest 
(3 min) 
Rest 
(3 min) 
3× 6 
245 
Time (min) 
Figure 2. Study course. During the exposure, posturography, muscle strain and work performance will be continuously 
recorded. 

 
 
 
 
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5.5 
Sample Size 
The planned study serves as a pilot study for a future larger research project and is intended to 
provide, among other things, the basis for a concrete calculation of the number of cases for follow-
up projects. For this reason, and considering that we will include 7 experimental conditions with 3 
working distances, we think that 21 individuals is the minimum for evaluating the exoskeleton, but 
we strive for 42 to have a double balanced randomization controlled cross-over design (double 
Williams Design). The total study population will be divided, based on body height, in three 
subgroups (Figure 2). The groups are based on (1) < 25th percentile, (2) 25th to 75th percentile, and (3) 
> 75th percentile of the average body height of 18-40 German males. As a result, the subgroup 
analyses may statistically be somewhat weaker than the overall statistical assessment of 42 
participants. 
We are mainly interested in recruiting men that fall in between the 25th and 75th percentile of the 
average German body height distribution, corresponding to Group 2 (Figure 2). The percentiles can 
be calculated using the formula: 𝑋= 𝜇+ 𝑍∙𝜎, in which 𝜇 is the population’s mean, 𝑍 the z-value 
belonging to the desired percentile, and 𝜎 the population’s standard deviation. 
6 
Study Procedure 
6.1 
Experimental Procedure 
Before the experiment starts, participants will visit the lab for familiarization with the ‘Chairless 
Chair’ and familiarization with the three manual materials handling tasks. They will perform the 
simulated assembly tasks for about 30 to 45 minutes, of which 15 to 30 minutes while wearing the 
exoskeleton (3.8 kg) in the different positions. During this first visit, the subjects will also complete 
the form with general information (Attachment 1), the form with in-/exclusion criteria (Attachment 
2) and will be explained in a standardized way about how to report and judge subjective feelings of 
discomfort (Attachment 3). After familiarization, participants will visit the laboratory a second time 
during which they will perform the seven experimental conditions (exoskeleton [3], working height 
[3]) in a preset balanced randomized order (using a Williams Design). Before starting with the true 
experiment, participants will first be equipped with the measurement devices. After the 
preparations, participants will perform each experimental condition for ~20 min followed by 3-min 
rest breaks. During the rest break the participant will first walk around and then sit down with 
wearing the exoskeleton. 
After finishing the experimental conditions, subjects will perform 11 trials on the force plate. Two 
trials will be performed while the subject is wearing the exoskeleton and is standing (1) with only his 
feet on the force platform while the feet of the ‘Chairless Chair’ are off the force platform. The trials 
will be recorded while in low seating and in high seating when wearing the exoskeleton and 
performing one work cycle in optimal height. These trials should provide insight into the body weight 
distribution over the own feet and the exoskeleton. Nine other trials will be performed to provide 
insight into the center of pressure’s behavior when performing tasks in different lateral workspaces. 
The nine trials will include three different positions (unsupported standing, supported high seating, 
supported low seating) and three different frontal-lateral workspaces (narrow in a range of 120°, 

 
 
 
 
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intermediate in a range of 150°, and broad in a range of 180°). The order of the three positions and 
the three workspaces will also be randomized using a Williams Design for nine combinations of 
position and workspace (Attachment 8). Finally, the subjects will evaluate the exoskeleton by filling 
out a questionnaire. During the tasks, posturography, muscle strain and work performance will be 
continuously recorded (Figure 2). 
6.2 
Experimental Tasks 
The simulated assembly tasks shall represent the existing assembly tasks at the automotive factories 
of Audi, including screwing, clip fitting, and cable mounting. The three tasks are presented to the 
subject on each side of a triangle (2.7 kg; Figure 3). One full work cycle consists of assembling the 
three subtasks and disassembling the three subtasks: (1) assembling 4 screws with an electrical screw 
driver, (2) fitting two rows of five clips, (3) mounting three cables to one coupling piece, (4) 
disassembling 4 screws with an electrical screw driver in reverse, (5) loosen the clips, and (6) loosen 
the cables to remove the coupling piece. We expressed the task, i.e. the work cycle, in codes of the 
predetermined motion-time system MTM-1 (Maynard et al. 1948). This system is often used in 
industry to assess the work pace, which also applies to Audi factories. Using detailed tables of the 
MTM-1 system, we are able to calculate the MTM-100, the pace corresponding to the work pace that 
a regular worker should be able to reach. Although Audi strives to work at pace MTM-114 at its 
factories, we strive for pace MTM-100, which is equal to ~80 s per work cycle (for an extensive 
determination of the MTM-pace, see Attachment 9). A metronome including both visual and 
auditory feedback will be provided to set this work pace for the simulated assembly tasks, indicating 
how much time the participant has left in a work cycle (the visual feedback is in accordance with Audi 
factories, in which employees are shown how much time is left to finish the work cycle in the green, 
orange or red zone). 
During the balance measurements after the experiment performed at the force platform, a n 
auditory metronome will be set for determining the pace at which subjects should move a 2-kg 
weight from the one side to the other. 
 
 
 
Figure 3. Three manual materials handling tasks. One work cycle consists of assembling the screws, clips, and cables and 
also disassembling them. 
7 
Data Collection 
Objective data collection includes that of posturography, muscle strain, and work performance; 
subjective data collection includes feelings of discomfort and an evaluation questionnaire about 
working with the exoskeleton. All measurement devices are CE certified and data will be collected by 
researchers familiar with the measurements devices, data extraction, and pseudonymization. 

 
 
 
 
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7.1 
Anthropometry 
At their first visit to the laboratory, participants will be asked to provide us with their anthropometric 
data including their age, body height, and body weight. This information will be documented 
together with the length measurements of several body parts (Attachment 1). 
7.2 
Posturography 
The study of Lafond et al. (2009) showed that low back discomfort is more prevalent when people 
make fewer postural adjustments during stationary standing. This means that posturographical 
analyses may provide information about postural control mechanisms related to low back 
discomfort. For this purpose, we should include both the lower limbs and trunk because they 
cooperate with each other when stabilizing a working posture (Leyman et al. 2001). This became 
particularly clear by a recent study, in which participants had to maintain their postural balance while 
standing on a narrow unfixed base of support (Mani et al. 2016). With practice, the distance between 
center of pressure (COP) and the projection of the center of mass (center of gravity, COG) decreased, 
while the motion of the lower extremity did not change. This result shows that our body is highly 
redundant and indeed controls postural balance with strong intersegmental cooperation. 
When registering and reporting on posturography, both limb motions as well as balance control are 
two important and interrelated concepts. Limb motions define the control of the postural balance, 
including concepts as center of pressure (COP), center of gravity (COG; Figure 4). The COP is the point 
where the resultant of all ground reaction forces act. The center of mass (COM) is the point where all 
the mass of a body is concentrated. The COG is the vertical projection of the center of mass to the 
ground, when one is on earth within the gravitational force field. 
 
Figure 4. The figure illustrates the most important concepts related to postural balance in sagittal view, including the center 
of mass (COM), the center of gravity (COG), and the center of pressure (COP). Source: 
http://www.pt.ntu.edu.tw/hmchai/BM03/BMClinic/Stance.htm. 
Dynamic stability, representing postural control, can be tracked by having participants stand on a 
force plate (Type 9287CA, Kistler, Toronto, Canada) while working. When assuming that the 
placement of the feet is known, the force plate can record the pressure distribution in the known 
area using a transformer (DAQ-System, Type 5691A1, Kistler, Toronto, Canada) which allows us to 
track the COP in the anterior-posterior and medial-lateral directions (BioWave, Kistler, Toronto, 
Canada). 

 
 
 
 
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Joint motion will be recorded using gravimetric position sensors (THUMEDI, Thum-Jahnsbach, 
Germany) which have a resolution of 0.1° and 125 ms in time, and a maximum static error of 0.5°. 
The sensors will be attached to the lumbar spine at the third lumbar vertebra (L3), the thoracic spine 
at the third thoracic vertebra (T3), and at the forehead. After calibration, each sensor measures its 
inclination angle with respect to the absolute perpendicular (gravitational axis) in the anterior-
posterior (flexion) and medio-lateral (lateral flexion) directions. The difference between the flexion 
angles of the sensors at L3 and T3 results in the curvature of the spinal column; the difference 
between the flexion angles of the sensors at T3 and the forehead results in the neck 
flexion/extension angle. 
7.3 
Muscular Stress and Strain 
During standing work, evaluation of the exoskeleton also includes assessing the level and 
development of muscle activity in selected muscles. Furthermore, it might also be interesting to 
investigate whether wearing or not wearing the exoskeleton is associated with any manifestations of 
muscle fatigue. Activity of selective muscles at the legs, back, and shoulders will be recorded. In the 
lower leg, the Gastrocnemius Medialis is involved in knee bending. In the upper leg, the Vastus 
Lateralis contributes to standing by supporting the knee and hip. At the lower back, the Erector 
Spinae Lumbalis helps holding up the spine. At the shoulder region, the Trapezius Descendens is one 
of the prone muscles during manual materials handling and is responsible for scapular movements 
and supports arm movements. 
Activity of the muscles will be tracked continuously during the experiment using surface 
electromyography (EMG). This method uses recording electrodes (42×24 mm, KendallTM H93SG ECG 
Electrodes, Covidien, Zaltbommel, the Netherlands) that are attached to the skin’s surface overlaying 
the muscle of interest with an inter electrode distance (IED) of 25 mm. The differential signal (bipolar 
derivation) of two electrodes placed over one muscle belly, recorded by an amplifier (PS12, 
THUMEDI® GmbH & Co. KG, Thum-Jahnaback, Germany), gives a firm estimation of that muscle’s 
electrical activity in microvolts (µV). This electrical activity pattern enables us to estimate the 
magnitude of the muscle activity (amplitude) and the overall firing frequency of the muscle (power 
frequency). The amplitude can be calculated by the overall root-mean-square (RMS) of the power 
spectrum of the EMG signal. The median power frequency (MPF) can be calculated from the power 
spectral density of the EMG signal by calculating the median. 
7.4 
Reference Measurements 
Before starting with the experiment and measurements, we need reference recordings for 
posturography and muscle strain. For the postural reference, the participant will stand upright with 
his back straight, his arms alongside the body, and hand palms facing forward. While the participant 
is standing in this position against a vertically levelled wall, a recording will be saved. The joint angles 
from this reference recording represent all the zero angles, which can be used for referencing. 
Muscle strain, as measured by EMG, will be normalized to a submaximal reference contraction. Each 
muscle that will be recorded has its own reference contraction and submaximal force level (Table 2; 
Coorevits et al. 2008). The posture during the reference contractions is continuously judged by one 
of the experimenters. After the reference contractions, the participant is provided with a short 

 
 
 
 
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period of rest to recover from the exertions. The reason for this submaximal reference contraction is 
to normalize the EMG signals and decrease the variance between subjects. The measured muscle 
activity levels are, therefore, expressed as a percentage of the submaximal reference voluntary 
contraction (% RVC). 
Table 2. Body segments with muscles of interest for investigating muscular strain while assembling. The reference 
contractions are performed for 30 s and the reported angles represent flexion angles (see also Figure 5). 
Segment 
Muscle(s) 
Reference voluntary contraction 
Force level 
Lower legs 
Gastrocnemius 
Medialis 
Lay prone, 90° hip, 90° knee, foot neutral (90°), hold position (with strap 
with weight around toes) 
10 kg 
Upper legs 
Vastus Lateralis 
Lay supine, 90° hip, 90° knee, hold position (with weight hanging just 
ventrally over ankle) 
5.0 kg 
Lower back 
Erector Spinae 
Lumbalis 
Lay prone, 0° hip, 0° knee, horizontal back extension with arms crossed 
on the chest, upper body off the bench and the legs fixed with straps 
Weight 
upper body 
Shoulders 
Trapezius 
Descendens 
Standing upright, arms 90° flexed upward, 0° elbows, holding weight in 
both hands 
1 kg 
 
 
 
 
Figure 5. Reference contractions for EMG normalization for the four muscles: (left up) vastus lateralis; (right up) 
gastrocnemius medialis; (left down) trapezius descendens; (right down) erector spinae lumbalis (see also Table 2). 
7.5 
Work Performance 
Useful information for industry is work performance with respect to wearing the ‘Chairless Chair’ 
exoskeleton. We will continuously track the error rate, which represents a mistake made during the 
assembly process. Also, the timing error will be registered, which is defined as the wrong timing with 
respect to the metronome (Bosch et al. 2012). When the participant is faster than the metronome, 

 
 
 
 
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this is a positive timing error; when the participant is slower than the metronome, this is a negative 
timing error. 
Bitte geben Sie die Intensität des aktuellen Beschwerden auf einer Skala von 0 (kein Beschwerden) 
bis 10 (schlimmste Beschwerden vorstellbar) an. 
 
 
Figure 6. The 11-point numerical rating scale (McCaffery & Beebe 1989) and body map (Kuorinka et al. 1987) as used in this 
study. 
7.6 
Subjective Assessment 
Participants will rate their overall subjective feeling of discomfort (RPD) on a 11-point numerical 
rating scale (McCaffery & Beebe 1989; Figure 6; Attachment 3). When participants experience a 
feeling of discomfort > 0, we ask them to define the body region (Kuorinka et al. 1987) which is 
causing this sensation. 
1
9
8
7
6
5
4
3
2
1
0
kein
leich
bescheide
stark

 
 
 
 
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7.7 
Participant Evaluation 
When the participants have finished the experiment, we will provide them with a short questionnaire 
to report their opinion about using the exoskeleton for the various experimental conditions. This will 
provide us information about the acceptance of the worker regarding the use of the ‘Chairless Chair’. 
The questionnaire will include items on feasibility, comfort, and usability (Attachment 6). 
8 
Statistical Analysis 
When all data are collected, we will have a number of dependent variables as listed in Table 3. Each 
of these parameters will provide a(n) (average) value per condition, resulting in four values. For the 
continuous measurements, i.e. posturography, muscle strain, and work performance, we will average 
the values over each of the seven conditions. For the discontinuous measurements, i.e. subjective 
discomfort ratings, we will calculate the difference value of the pre- and post-condition 
measurements. Participant evaluations will not be tested but only discussed narrative. 
Table 3. Summary of the model components that will be included in the statistical analyses. 
Model component 
Method / Name 
Parameter 
Dependent variable 
Posturography 
Center of Pressure (COP) 
Joint angles 
Muscle strain 
Amplitude (ARV) 
Median power frequency (MdPF) 
Work performance 
Error rate 
Timing error 
Subjective assessment 
Rating of perceived discomfort (Borg RPE) 
Within-subject factor 
Working postures 
Without, with (high), with (low) exoskeleton 
Working height 
High, medium, low 
Working distance 
Optimal, too far I, too far II 
Between-subject factor 
Group 
Body height (S/M/L) 
 
The current study has a repeated measures design including three within-subject factors 
(exoskeleton, working height, and working distance). If we have enough participants, we could also 
include a between-subject factor (group, based on body height), or another post-hoc identified 
discriminator. We will use a linear mixed effects model to perform the statistical analyses of all 
dependent variables (Table 3) using SPSS (IBM SPSS Statistics 23.0). This model allows observations 
to be not independent, because it adds a random effect for subject, i.e. modeled by assuming 
different random intercepts for each subject. This model also allows including both repeated 
measures, i.e. different conditions (within-subject factor: working height, working posture, working 
distance), and comparing independent groups of participants (between-subject factor: group). 
9 
Risk for the Participant 
The experiment has no threatening risks for the participant. However, there can be some side-effects 
of study-related procedures about which we will inform the participant, including: 

 
 
 
 
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- 
Skin irritation or reddening due to skin preparation and of the attachment of the adhesive 
electrodes; in rare cases, participants may experience allergic reactions; 
- 
Tingling or tension sensations in the area of the adhesive electrodes due to cable pull; 
- 
Sensations of tension or muscle fatigue along the duration of the experiment; 
- 
Possible sensations of dizziness, loss of blood pressure, and a very low risk of impotence due 
to the semi-standing for a total of 70 minutes. 
10 
Data Protection 
The participants as well as their personal and recorded data will be numerically pseudonymized by 
assigning a randomly generated, five-digit identification number to the examined participant. 
Relating analyzed outcome data to the natural person is only possible by inspection of the decoding 
list. This list, together with the informed consents of all volunteering participants, will be stored 
separately from the measured data in the storage of the Institute of Occupational and Social 
Medicine and Health Services Research. All these participant data will be stored for a period of ten 
years after publication of the results. The destruction of the data in paper form is carried out by 
means of the disposal boxes for data protection at the University Hospital, and the destruction of the 
data in electronic form is carried out by means of transferring the data off the memory cards of the 
devices. 
The measurements as part of this proposal are all recorded on portable computers that are 
accessible by means of a password only and cannot be accessed by any network. The pseudomynized 
data are processed within the University Hospital (UKT) network, of which the time of the 
measurement collection is removed from the operating software whenever possible. The measuring 
devices are configured identically for all measurements to exclude a possible association between 
the measurement configuration and the participants. After completing the data analysis, the data of 
the participants are stored together with the photo documentation on Blu-ray discs. 
Only employees directly involved in the study have access to the data. In addition, all employees of 
the Institute for Occupational and Social Medicine and Health Services Research who are involved in 
the study are subject to medical confidentiality. 
At the start of the study, participants are asked whether they wish to be informed about the primary 
results of the study. In case of several interested parties, an information event will be organized for 
the participants after completion of the data evaluation. If there is only little interest, a brief written 
presentation of the primary results will be spread and sent to interested parties by e-mail. 
11 
Study Finance 
The financing of the study, including personnel costs and costs for technical equipment and 
assistance, are funded by Audi AG. The volunteering participants will receive a compensation of 
about € 10 per hour. The two days of testing together take approximately five hours, which results in 
a total amount of € 50. 

 
 
 
 
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12 
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