Work-related neck and shoulder pains are highly prevalent in jobs with low physical exposure. Myalgia of the trapezius muscle is one of the most prevalent work-related neck-shoulder disorders and muscle fatigue is widely considered a precursor of such disorders. There is evidence that long-lasting low-level activity of the trapezius muscle appears as a crucial link in the pathway from workplace physiological and psychological demands to the development of work related neck pain. A possible approach to reduce the risks associated with muscle fatigue is to disrupt the monotonous muscle activity by adding frequent, active breaks during the working task. In the first phase of our investigation the long lasting component of trapezius muscle fatigue resulting from low level, sustained working task and spatio-temporal distribution of EMG activity are investigated in two conditions including passive break or active disruption of muscle contraction. Muscle fatigue develops and persists after the end of the workday. It appears that the alteration of force control may be associated with the corresponding fatigue. However, these phenomena seem to be counteracted by disruption of muscle contraction monotony by active interventions during the workday. Indeed, the presence of active disruptions also induces changes in the timing and degree of EMG activity as well as features of trapezius active areas. The extent of these adaptations appears to be subject and work task dependent but seem to be beneficial for the reduction of muscle fatigue.

1. Pain in the neck: A likely
neuromuscular control issue:
I, the fatigue approach
DG Kim, S.D.H. Soedirdjo
C. Nicoletti,
A. Botter,
P. Wild,
T.Laübli,
B. Martin
LISiN
Laboratory for
Engineering of the
Neuromuscular
System
Politecnico di Torino

3. • Background:
Who is complaining?
Fatigue is considered as a precursor to MSDs
for multiple reasons:
Physiological mechanisms (ECC, +++)
Control mechanisms (Cinderella)
Trapezius myalgia very common

5. Hypothesis
monotonous muscle activity may also be centrally driven
hence focalization of activity leading to fatigue and myalgia
Aims
1)Quantify trapezius muscle fatigue
(focus on long lasting component)
2) Analyze Pattern of muscle activity (2D EMG)
3) Intervention
In predominantly static job

6. Methods
Can you find 7 errors ?
Typing + other tasks

7. Experimental Protocol

8. Work activities
• Typing text desk adjusted to anthropometry
• Typing text high desk
• Solitaire
• Stroop test
• Puzzle assembly

9. Intervention activities
Laughing (if joke was good)
engaging in conversation
Resting feet on desk
Shrugging the shoulders
…various ways of disrupting static posture
without added stress

10. ELECTRODES and IMUs set up
Isometric force ramp control

11. Muscle Twitch (Stimulation Posture)

12. Methodology for measuring Muscle twitch 3D acceleration
Electrical Stimulation:
1 ms, 1 Hz, 20-30 mAmp
Control: 2 series of 15 stimulations separated by repositioning
then 30 stimulations for each measure

13. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
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Day 1 BaselineS02
Mag=0.10
AZ=165.55
EL=56.36
̅𝑥𝑥2 + �𝑦𝑦2 + ̅𝑧𝑧2
Example of acceleration along X, Y, Z directions for one subject
Mean, +SD
( synchronized and ensemble average)

14. Some Results
A real pain in the neck

15. Slope of Force ramp control

16. Peak of Acceleration (X,Y,Z directions)

17. First Conclusions
Alteration of acceleration pattern
stemming from changes in muscle contraction pattern
Alteration of force control
Very low level of sustained muscle activity (≈ 2-5 %MVC)
Disruption of monotony appears to be effective in reducing fatigue effects

18. II. neuromuscular control issue; trapezius contraction patterns
LISiN
Laboratory for
Engineering of the
Neuromuscular
System
Politecnico di Torino

20. BACKGROUND
Myalgia of the trapezius muscle is one of the most
prevalent work-related neck-shoulder disorders
(Hägg, 1991, Buckle and Devreux, 1999)
Long-lasting low-level activity of the trapezius muscle is a crucial link in the pathway from
workplace physiological and psychological demands to the development of work-related
neck pain (Hägg, 1991)
Selective activation of neuromuscular
compartments within the human
trapezius muscle
Holterman et al JEK (2009) 19:896–902
Functional Subdivision of the Upper Trapezius
Muscle during Maximal Isometric Contractions
Jensen and Westgaard JEK (1995) 5:227-237
Functional Subdivision of the Upper Trapezius
Muscle during low-level activation
Jensen and Westgaard JEK (1997) 76:335-339
Trapezius muscle is anatomically and functionally compartmentalized
The activity in the three regions of the trapezius under
controlled loading conditions—an experimental and
modelling study
Johnson and Pandyan (2005) 20:155-161

21. Experimental muscle pain changes the spatial distribution of upper
trapezius muscle activity during sustained contraction
Madeleine at al (2006) Clin Neuropysiol 117:2436-2445
Changes in spatial distribution of EMG amplitude over time during a sustained
contraction may reflect a mechanism to counteract fatigue during prolonged muscle
activity.
Falla and Farina Exp Brain Res (2007) 182:99–107
Farina et al JEK (2008) 18:16–25
The change in spatial distribution of upper
trapezius muscle activity is correlated to
contraction duration
Active pauses induce more variable EMG pattern of
the trapezius muscle activity during computer work
Samani et al JEK (2009) 19:e430–7
Surface EMG mapping of the human trapezius muscle
Kleine et al (2000) Clin Neuropysiol 111:686-693
Changes in the spatial distribution sEMG activity over the trapezius muscle were shown
to be associated with fatigue, pain, and working conditions.
BACKGROUND

22. AIM
To investigate the association
between spatio-temporal
patterns of trapezius activity
and working condition during
a simulated, working day of
computer work.
BACKGROUND
By detecting the sEMG activity from different location over the muscle, high density surface
EMG (HD-sEMG) allows the analysis of temporal and spatial changes in muscle activation
and their association with the performed task, fatigue or pathological conditions (Merletti et
al 2010 CRBE)

23. Act 1 Act 2 Act 3 Act 4 Act 5
Preparation
TBMO
TBMI1
TBMI2
TBA1
TBA2
20
70
80
140
190
200
130
215
245
260
310
320
380
370
400
450
430
0
time (min)
Lunch
Control day
Act 1 Act 2 Act 3 Act 4 Act 5
Preparation
TBMO
TBMI1
TBMI2
TBA1
TBA2
20
70
80
140
190
200
130
215
245
260
310
320
380
370
400
450
430
0
time (min)
Intervention day
= interuptions during the activity Lunch
EXPERIMENTAL PROTOCOL

24. 1. Writing Low: writing a text with the table height
set according to ergonomic standards [1]
2. Writing High: writing a text with the table 10cm
higher than Writing Low
3. Stroop: performing STROOP TEST [REF2]
4. DD_Puzzle: playing Drag and Drop Puzzle
5. DD_Sol: playing Drag and drop Solitaire
Act 1 Act 2 Act 3 Act 4 Act 5
Preparation
TBMO
TBMI1
TBMI2
TBA1
TBA2
20
70
80
140
190
200
130
215
245
260
310
320
380
370
400
450
430
0 time (min)
Intervention day
interruptions
during the
activity
Lunch
Working activities
Working activities and interruptions
were performed in randomized order
EXPERIMENTAL PROTOCOL
Interruptions
1. Move shoulder and upper back
2. Swing arms near the body
3. 3x5 Jakobson Test: slow but forceful shoulder elevation
4. Stand up and stretch: stretching exercise of the upper
body/shoulder/neck area
5. Turn head slowly in all possible directions
6. Tell a joke: make a couple of jokes and engaged the participant
into a conversation
7. Stand up and have a drink: have a little walk with the subject to
get some water and drink it in a standing position
8. Concentrate on relaxing the trapezius
9. Comments on actual posture and feelings of comfort and
discomfort
10. Laying on the couch: walk to the couch, lay down and rest
11. No interruptions (on control day)
[1] Chaffin DB, et al. Occupational Biomechanics. New York: Wiley; 2006.
[REF2]
[REF3]
ACTIVEInterruptionsPASSIVEInterruptions

25. EXPERIMENTAL SETUP
C7
Acromion
Grid of sEMG
electrodes
IMU
Bipolar EMG
electrodes
Stimulation
electrodes
IMU
IMU
(Forehead)
Grid of sEMG electrodes (4x16), 10mm inter-
electrode distance in both directions

26. DATA ANALYSIS
0
50 µV
• The monopolar sEMG signal from each electrode was
divided in 30ms epochs
• The RMS amplitude was computed for each epoch and for
each channel
• A RMS amplitude map was computed for each epoch
Example of RMS
amplitude map for
a 30ms epoch

27. DATA ANALYSIS
0
50 µV
• For each channel a threshold based on the baseline sEMG
signals was defined (3std of the REST RMS distribution)
• The threshold was used to identify the active regions
(groups of at least three adjacent active electrodes ) and the
associated active epochs (time intervals where there was at
least one active regions)
EMG amplitude map (RMS)
for the selected epoch
Prox
Med
Not Active
Active
Active
Spatial localization of the active
regions for the selected epoch
Thresholding

28. DATA ANALYSIS From the map of active regions we identified:
1. The Centroids of each active region (Red dots)
2. The size of each active region (i.e. number of
channels included in each active region)
3. The 90p amplitude (RMS) of sEMG in the active area
Prox
Med
NA
A
A
Spatial distribution of the centroids
(Red dots) of active regions for all
the active epochs identified during
the activity DD_Puzzle_Act
Feature extraction
for a single epoch
of 30ms
RMS map
Active region
Number and Sizes
Centroid
positions

29. RESULTS – Comparison between working activities
0.2.4.6.8
exp(predict(xb))/(1+exp(predict(xb)))
DD_Puzzle_Act DD_Sol Stroop Writing_Tab1 Writing_Tab2
Task
Control Interv
p task= 0.0000 p session=0.5726 p interaction= 0.9412
proportion of proximal cluster
all epochs
1.41.51.61.71.8
LinearPrediction,FixedPortion
DD_Puzzle_Act DD_Sol Stroop Writing_Tab1 Writing_Tab2
Task
Control Interv
p task= 0.0000 p session=0.6827 p interaction= 0.8762
median log10(P90) of first cluster
all active epochs
152025
LinearPrediction,FixedPortion
DD_Puzzle_Act DD_Sol Stroop Writing_Tab1 Writing_Tab2
Task
Control Interv
p task= 0.0021 p session=0.4402 p interaction= 0.7202
area of first cluster
all active epochs
Activation duration:
proportion of active epochs
Area of active regions
sEMG Amplitude: median log10(P90) of the
active regions
Note that:
Writing_Tab 1 is Writing Low: Typing text with the table height
set according to ergonomic standards [1]
Writing_Tab 2 is Writing High: Typing text with the table 10cm
higher than Writing Low
*
*
*
The condition showing statistically significant differences
with respect to the other conditions in all sEMG variable is
Writing High (Writing_Tab2 in the graphs)
During a high cognitive load (Stroop test) sustained trapezius
muscle activity was not observed.

30. RESULTS – Comparison between working activities
Effect of table height
Paired Wilcoxon rank test
12345678910111213141516
TableHigh
When compared to “low” condition,
“high” condition showed:
1. higher and more prolonged EMG
activity during computer typing
2. More localized spatial
distribution of sEMG (smaller
area of active regions), indicating
the continuous activation of
specific subgroup of MUs
Risk factors for the development of
work-related neck pain.
Note that Activation duration is defined as number of active
epoch w.r.t. the total number of epochs (% values)
Low condition High condition
10
µV
25
sEMG distribution and Active Areas

31. RESULTS – Comparison between intervention approaches
1.451.51.551.61.65
LinearPrediction,FixedPortion
no
interm
ove
shoulder
tella
joke
swing
armstairs
up-drink
relaxing
trapezius
com
m
ents3x5s
Jakobson
stairs
up-laystand
up
stretch
head
m
ovem
ents
Int
Control Interv
p type intervention 0.4999
median log10(P90) of first cluster
all active epochs
161820222426
LinearPrediction,FixedPortion
no
interm
ove
shoulder
tella
joke
swing
armstairs
up-drink
relaxing
trapezius
com
m
ents3x5s
Jakobson
stairs
up-laystand
up
stretch
head
m
ovem
ents
Int
Control Interv
p type intervention 0.0069
area of first cluster
all active epochs
.1.2.3.4.5
exp(predict(xb))/(1+exp(predict(xb)))
no
interm
ove
shoulder
tella
joke
swing
armstairs
up-drink
relaxing
trapezius
com
m
ents3x5s
Jakobson
stairs
up-laystand
up
stretch
head
m
ovem
ents
Int
Control Interv
p type intervention 0.0973
proportion of proximal cluster
all epochs
Activation duration:
proportion of active epochs
sEMG Amplitude: median log10(P90) of the
active regions
Area of active regions
Jakobson test leads to a reduction of the
activation duration of upper trapezius and to
less localized active regions, without an
increase of EMG amplitude.

32. CONCLUSIONS
Other strategies aimed at disrupting a continuous muscle activation do not seem to
induce significant effects on the spatio-temporal patterns of EMG activity over the
upper trapezius
General, methodological considerations
Comparison between working tasks
Comparison between working-interruption strategies
Among the tested strategies, Jakobson test seems the most effective in inducing the
wished changes in upper trapezius activity
Major differences were observed in the comparison between two versions of writing
task (high vs low table height)
During a high psycho-mental load (Stroop test) sustained trapezius
muscle activity was not observed.
By providing spatio-temporal information of muscle activation, High Density sEMG has
the potential to allow an in-depth analysis of the interactions between work context and
individual physiological behavior

33. Thank you for your attention