Muscular work, physical load, energy
expenditure, work-rest cycles
Oğuzhan Erdinç
Associate Professor
Istanbul, 2020
1
Muscular work
• The motor nerves (efferent
nerves) trigger contraction in
the muscle fibers.
• The motor unit varies in size
based on the number of
muscle fibers
(Efferent nerve)
– Small: precise control (ex: eyes)
– Large: coarse action (ex: biceps)
• Sensory nerves (affarent
nerves) reports the tension in
the tendons to the spinal cord
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc
2
Muscular work – sliding filament model
Actin
Myosin
• Actin and myosin
filaments contract to
generate force
http://philschatz.com/
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc
3
Muscular work – types of muscle contraction
Dynamic
activity
Static
activity
http://philschatz.com/
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc
4
Energy transformation in muscular work
• Oxygen is the key to healthy
muscular work!
• Means of transporting oxygen
into the muscles:
– Respiration - blood circulation
– Dynamic contraction in the
muscles
– Blood pressure
• Static activity impedes oxygen
transportation
• Lactic acid causes muscle fatique
– Lactate accumulation should be
prevented
http://www.medbio.info/
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc
5
Energy expenditure – classification of tasks
• Kcal: the amount of energy necessary to increase the temperature
of 1 kg of water by 1o C. (1 kcal/hr = 1.16 watt)
• Literature offers various task classification/intensity schemes,
based on:
• Energy expenditure (see Ergonomics textbook page 245.)
• Heart Rate
• Oxygen consumption
• Pulat model for work-rest cycles addresses;
–
–
–
–
Task intensity using energy requirement (kcal/min)
Task duration
Basal metabolism
Personal factors (gender, age)
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc
6
Measuring muscular activity – Electromyography (EMG)
•
•
•
•
Records myoelectric signals when a muscle is in use
Shows muscle loading during a physical activity
Shows muscle fatique
Used to compare tasks / postures / worker groups
http://www.dr-lebmeier.de/
Increased variation in the activity
shows fatique
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc
7
Strength measurement
• Measures the force that the person can apply (in kilograms)
– using various muscle groups (ex: grip strength), carrying out different tasks
(ex: lifting), in different postures (ex: sitting)
• Measure of the maximum force: Maximum Voluntary Contraction
(MVC)
• Provides muscular strength data for product and task design
http://www.topendsports.com/
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc
8
EMG – Strength measurement Video
https://www.youtube.com/watch?v=7Xdollkmg-Q
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc
9
Measures of energy expenditure – Heart Rate (HR)
• HR is an estimator of energy expenditure and workload
– HR during work, HR max.(≤ 140 b/min),
– %HR change during activity (≤ 35 b/min),
– Recovery pulse
Graph: Fitting The Task To The Human, Fifth Edition, E. Grandjean, Karl H.E. Kroemer
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 10
Measures of energy expenditure – Heart rate
• Factors that affect HR disproportionately:
– Static muscular activity, climate (heat, humidity), psychologic stress
HR during static effort
HR under various conditions
Graph: Fitting The Task To The Human, Fifth Edition, E. Grandjean, Karl H.E. Kroemer
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 11
Measures of energy expenditure – Oxygen consumption
• 1 liter oxygen intake = 4.8 kcal energy
• VO2 : The amount of O2 that can be used in a minute (liter/minute)
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 12
Measures of energy expenditure – Oxygen consumption
• VO2 max: The maximum amount of O2 that body can use (per min.)
– Indicator of human capacity for aerobic work
– Work in an 8 hour shift should require ≤ 33% VO2 max
– If physical workload > 50% VO2 max, worker’s health can be adversely
affected
http://www.srm.de/
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 13
VO2 Measurement Video -1
https://www.youtube.com/watch?v=i7kn3mkO7Ec
VO2 Measurement Video -2
https://www.youtube.com/watch?v=fn3Yr-LS_l0
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 14
Measures of energy expenditure – Oxygen consumption
• Douglas bag method:
– Composition of the exhaled air is analyzed
– The difference between inhaled-exhaled amounts of O2 gives a measure of
the oxygen consumption
www.aber.ac.uk
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 15
Douglas Bag Video -1
https://www.youtube.com/watch?v=SSOTcvAe3FA
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 16
Key points for ergonomic task design
• Avoid static work
Static work
Dynamic work
Sustained muscle contraction
Contraction – relaxation cycle
Reduced blood flow
Increased blood flow
Hinders flow of oxygen and nutrients
Allows flow of oxygen and nutrients
Causes lactate formation
Does not allow lactate formation
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 17
Key points for ergonomic task design
• Avoid intensive increase in the HR during physical work
• Prefer “Short - frequent breaks” to reduce lactate accumulation
Heart rate
Lactic
Acid
(mg/ml)
Lactic acid
Heart rate
(beat/min)
Physical work(min)
Rest(min)
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 18
Key points for ergonomic task design
• Allow workers to use different
muscle groups (ex: cellular
production system)
http://leansixsigmadefinition.com/
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 19
Cell Manufacturing Example Video - Roland
https://www.youtube.com/watch?v=Dz4xCCPkni0
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 20
Work rest cycles – Pulat model
•
•
•
•
•
Rt : Allowed rest time (min.)
K = Energy required for the task (kcal/min.)
S = Acceptable energy requirement standard (Male =5 kcal/min. Female =4 kcal/min.)
T = Total expected duration of task (min.)
BM =Energy required for the basal metabolism (Male = 1.7 kcal/min , Female = 1.4
kcal/min.)
RT = 0
for K < S
T (K − S)
K

−
1

100
+


S
K − BM


RT =
2
for S ≤ K < 2S
RT =
T (K − S)
1.11
K − BM
Age
≥20
≥40
≥50
≥60
≥65
Multiplier
1.0
1.04
1.1
1.2
1.25
for K ≥ 2S
Age adjusted Rt = Rt x Age multiplier
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 21
Work rest cycles –Pulat model example
The male worker in the picture is carrying the
waste boxes.
He is 36 years old and estimated total task
duration is 100 min.
The energy requirement for this task is 7.3
kcal/min.
Calculate the total rest time the worker would
need for this task. If you were his manager,
how would you arrange work-rest cycles?
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 22
Work rest cycles –Pulat model example
The male worker in the picture is carrying the waste
boxes. He is 36 years old and estimated total task
duration is 100 min. The energy requirement for
this task is 7.3 kcal/min. Calculate the rest time that
the worker would need. If you were his manager,
how would you arrange work-rest cycles?
RT = 0
for K < S
T (K − S)
K

−
1

100
+


S
K − BM


RT =
2
for S ≤ K < 2S
RT =
T (K − S)
1.11
K − BM
5 < 7.3 < 10
for K ≥ 2S
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 23
Work rest cycles –Pulat model example
The male worker in the picture is carrying the waste
boxes. He is 36 years old and estimated total task
duration is 100 min. The energy requirement for this
task is 7.3 kcal/min. Calculate the rest time that the
worker would need. If you were his manager, how
would you arrange work-rest cycles?
S (male) = 5 kcal / min.
BM (male) = 1.7 kcal /min.
K =7.3 kcal/min
T=100 min.
k = 7.3 kcal/min
100 ( 7.3 − 5 )
 7.3 
− 1100 +

5
7.3 − 1.7

Rt = 
 43.5 min .
2
Age adjusted R t = 43.5*1 = 43.5 min .
Age
≥20
≥40
≥50
≥60
≥65
Multiplier
1.0
1.04
1.1
1.2
1.25
Textbook: Pamela McCauley Bush, Ergonomics: Foundational Principles, Applications, and Technologies, CRC Press, (2012)
O.Erdinc 24