TI 2111 Work System Design and Ergonomics 11. Occupational Biomechanics & Physiology

TI 2111 Work System Design and Ergonomics

11. Occupational Biomechanics & Physiology


Biomechanics

Biomechanics uses the laws of physics and engineering mechanics to describe the motions of various body segments (kinematics) and understand the effects of forces and moments acting on the body (kinetics).

Application:

Ergonomics

Orthopedics

Sports science


Occupational Biomechanics

Occupational Biomechanics is a sub-discipline within the general field of biomechanics which studies the physical interaction of workers with their tools, machines and materials so as to enhance the workers performance while minimizing the risk of musculoskeletal injury.

Motivation: About 1/3 of U.S. workers perform tasks that require high strength

demands Costs due to overexertion injuries - LIFTING Large variations in population strength Basis for understanding and preventing overexertion injuries


Problems (example)


Free-Body Diagrams

Free-body diagrams are schematic representations of a system identifying all forces and all moments acting on the components of the system.


2-D Model of the Elbow:

From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.2

17.0 cm

35.0 cm

180 N

10 N

Unknown Elbow force and moment


2-D Model of the Elbow

From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.7


Biomechanics Example

10 N180 N

FB?

35.0 cm17.0 cm

5 cm

Free-body Diagram: Unknown values: Biceps and external elbow force (FB and FE), and any joint contact force

between upper and lower arms (FJT)

External elbow moment (ME)

Lower arm selected as free body

HANDCOMELBOW


General Approach

1. Establish coordinate system (sign convention)

2. Draw Free Body Diagram, including known and unknown forces/moments

3. Solve for external moment(s) at joint

4. Determine net internal moment(s), and solve for unknown internal force(s)

5. Solve for external force(s) at joint [can also be done earlier]

6. Determine net internal force(s), and solve for remaining unknown internal force(s)


Example : Solution+Y

+X

+Z

FBD:

E HW

LA=m

LAg

=10NF

H=m

Hg=

180N

FB=??

FJT

=??

ME=??

• ME = 0 M

E + M

E M

E = -M

E

• ME = M

LA + M

H = (W

LA x ma

LA) + (F

H x ma

H) =

• (-10 x 0.17) + (-180 x 0.35) =

• -1.7 - 63 = -64.7 Nm, or 64.7Nm (CW)

• ME = -M

E 64.7 = F

B x ma

B = F

B x 0.05

• FB = 1294N ( )

ME = 0 = ME + ME -> ME = -ME

ME = MLA + MH = (WLA x maLA) + (FH x maH)

ME = (-10 x 0.17) + (-180 x 0.35) = -1.7 - 63

ME = -64.7 Nm (or 64.4 Nm CW)

ME = -ME -> ME = 64.7

ME = (FJT x maJT) + (FB x maB) = FB x 0.05

FB = 1294 N (up)

_ _

_ _

External moment is due to external forces

Internal moment is due to internal forces

_


Example 1: Solution

FE = 0 = FE + FE -> FE = -FE

FE = WLA + FH = -10 + (-180)

FE = -190 N (or 190 N down)

FE = - FE -> FE = 190

FE = FJT + FB

FJT = 190 - 1294 = -1104 N (down)

_ _

_ _

_

Thus, an 18 kg mass (~40#) requires 1300N (~290#) of muscle force and causes 1100N (250#) of joint contact force.


Assumptions Made in 2-D Static Analysis

Joints are frictionless No motion No out-of-plane forces (Flatland) Known anthropometry (segment sizes and weights) Known forces and directions Known postures 1 muscle Known muscle geometry No muscle antagonism (e.g. triceps) Others


3-D Biomechanical Models

These models are difficult to build due to the increased complexity of calculations and difficulties posed by muscle geometry and indeterminacy.

Additional problems introduced by indeterminacy; there are fewer equations (of equilibrium) than unknowns (muscle forces)

While 3-D models are difficult to construct and validate, 3-D components of lifting, especially lateral bending, appear to significantly increase risk of injury.


From Biomechanics to Task Evaluation

Biomechanical analysis yields external moments at selected joints

Compare external moments with joint strength (maximum internal moment) Typically use static data, since dynamic strength data are

limited Use appropriate strength data (i.e. same posture)

Two Options: Compare moments with an individuals joint strength Compare moments with population distributions to obtain

percentiles (more common)


Example use of z-score

If ME = 15.4 Nm, what % of the population has sufficient strength to perform the task (at least for a short time)?

= 40 Nm; = 15 Nm (from strength table)

z = (15.4 - 40)/15 = -1.64 (std dev below the mean)

From table, the area A corresponding to z = -1.64 is 0.95

Thus, 95% of the population has strength ≥ 15.4 Nm


Task Evaluation and Ergonomic Controls Demand (moments) < Capacity (strength)

Are the demands excessive? Is the percentage capable too small? What is an appropriate percentage? [95% or 99% capable

commonly used] Strategies to Improve the Task:

Decrease D Forces: masses, accelerations (increase or decrease, depending on

the specific task) Moment arms: distances, postures, work layout

Increase C Design task to avoid loading of relatively weak joints Maximize joint strength (typically in middle of ROM) Use only strong workers


UM 2-D Static Strength Model


Work Physiology

Aerobic Metabolism

Anaerobic Metabolism

Oxygen Food

Lactic Acid WORKHEAT Carbon Dioxide


Aerobic vs. Anaerobic Metabolism

Aerobic Use of O2, efficient, high capacity

Anaerobic No O2, inefficient, low capacity

Aerobic used during normal work (exercise) levels, anaerobic added during extreme demands

Anaerobic metabolism -> lactic acid (pain, cramps, tremors)

D < C (energy demands < energy generation capacity)


Oxygen Consumption and Exercise

Oxygen Uptake

or Heart Rate

Max. Aerobic Capacity

Time

Start Work End Work

Oxygen Debt Recovery

Job Demands

Oxygen Deficit

Basal Rate

steady state


Oxygen Uptake and Energy Production

RespiratorySystem

CirculatorySystem

Muscle

Oxygen Available

Tidal Volume

RespiratoryRate

Blood

StrokeVolume

Heart Rate

CapillarySystem

Atmosphere

Oxygen Uptake (VO2)

Energy Production (E)


Changes with Endurance Training Low force, high repetition training increased SVmax => increased COmax incr. efficiency of gas exchange in lungs

(more O2)

incr. in O2 carrying molecule (hemoglobin) increase in #capillaries in muscle


Problems with Excessive Work Load Elevated HR

cannot maintain energy equilibrium insufficient blood supply to heart may increase risk of heart

attack in at-risk individuals Elevated Respiratory Rate

chest pain in at-risk individuals loss of fine control

General and Localized Muscle Fatigue insufficient oxygen -> anaerobic metabolism -> lactic acid ->

pain, cramping A fatigued worker is less satisfied, less productive, less

efficient, and more prone to errors


Evaluating Task Demands: Task demands can be evaluated the same

way that maximum aerobic capacity is evaluated – by direct measurement of the oxygen uptake of a person performing the task.

Indirect methods for estimating task demands: Tabular Values Subjective Evaluation Estimate from HR Job Task Analysis

More ComplexMore Accurate

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TI 2111 Work System Design and Ergonomics 11. Occupational Biomechanics & Physiology