ANALYSIS OF QUASISTATIC POSTURES day 2 PHOLG017 SURGG005

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ANALYSIS OF QUASI-STATIC POSTURES(day
2)PHOLG017 / SURGG005
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PEOPLE DIFFER IN SIZE AND SHAPE. HOW MAY THEIR
POSTURES BE ASSESSED BIOMECHANICALLY?
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SOME M.Sc. STUDENTS AT UCL
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Linear dimensions are normally distributed, body
mass is positively skewed
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THE JOINTED SKELETON LARGELYDEFINES THE LIMITS
TO THE BODY SEGMENTS
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AMYRIADPOSTURESARE POSSIBLE.EACH CREATES
UNIQUEMECHANICAL DEMANDS
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THE DEMANDS IMPOSED BY A POSTURE MAINLY ARISE
BECAUSE THE BODY AND ITS SEGMENTS HAVE WEIGHT

• DO NOT CONFUSE MASS AND WEIGHT

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OUR BODY WEIGHT IS THE ATTRACTION WE HAVE TO THE
CENTRE OF THE EARTH
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TO HOLD A POSTURENEEDSMUSCLES (SOMETIMES
LIGAMENTS)DEMANDS MAY BE MODEST OR EXCESSIVE
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ANEXPERIENCEDEYE CANRECOGNISE POSTURAL
DEMANDSBUT NOT EASILYRATE OR RANK THEM
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SEGMENTAL ANTHROPOMETRYTHE SIZE, WEIGHT AND
CENTROID LOCATION OF EACH SEGMENT
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DEMPSTER (1955) DISMEMBERED FROZEN CORPSES INTO
ANATOMICALLY DEFINED BODY SEGMENTS. HIS BASIC
TOOLS WERE

• 1. SAW AND SCALPEL
• 2. RULER
• 3. WEIGHING SCALES
• 4. BALANCE BOARD
• 5, (STOP WATCH re. inertias)
• Ref Space requirements of the seated operator.
  WADC Tech. Note 53-159. Wright Patterson Air
  Force Base, OH

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WHAT DEMPSTER DID
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WHAT DEMPSTER DID
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WHAT DEMPSTER DID
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WHAT DEMPSTER DID
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DEMPSTER ALSO DETERMINED THE MOMENTS OF INERTIA
OF SEGMENTS WHICH ARE ONLY RELEVANT TO
ACCELERATIVE MOVEMENT
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LENGTH, WEIGHT, CENTROID LOCATION (AND MOMENT OF
INERTIA) ARE REFERRED TO AS BODY SEGMENT
PARAMETERS. THEY ARE ASSUMED TO BE CONSTANT FOR
THE INDIVIDUAL FROM DAY TO DAY.WHAT ARE THE
FLAWS IN THE ASSUMPTION AND DO THEY MATTER WHEN
CONSIDERING POSTURE?
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OTHER ESTIMATES OF SEGMENT PARAMETERSHAVE BEEN
OBTAINED WITH LIVING SUBJECTS
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Zatsiorsky Seluyanov(1993)determined segment
parameters by gamma-ray scanning.Gamma rays are
absorbed in proportion to the amount of matter
they pass through
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Water displacement method (live
subjects).Another approach to segment parameters
is to measure the volume distribution and assume
density distribution
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Values for an individual are calculated by
scaling.Tables exist which quote the parameters
in units of stature and body mass.

• see BIOMECHANICS OF THE MUSC-SKELETAL SYSTEM
  (1999) 2nd Ed. by NIGG HERZOG Wiley for
  detailed discussion off segment anthropometry .

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DIMENSIONSANDMASSESOF SEGMENTS(based on
Dempster)
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Zoom in toCHART DATARELATING TO THEUPPERLIMB
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Assuming that your upper limb was truly
horizontal, you should be able to work out the
demand on your shoulder, using the data given
and scaling to your height and weight.Which
muscles would be involved?
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Assume body mass 75kgstature 1.75mShow that
the turning momentabout the shoulder joint due
to gravityis 11.11 Nm
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If a 50N weight can just besupported (briefly),
the mvc of shoulder abductors is11.11 50 x
0.652 43.7Nm.The posture alone, without the
weight, requires 11.1/43.7 i.e. 25 of mvc.How
long could the unloaded posture be maintained?
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From the above exercise, note that the demand
imposed by gravity upon a joint in a posture was
(a) quantified, and (b) rated against available
strength.The implications for possible fatigue
in that posture are considered later.
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The next example is an extension of the above to
analysis of symmetrical whole body postures
(which featured in the workshop)
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Postures described by segment angles
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POSTURAL ANALYSIS 1.
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POSTURAL ANALYSIS 2.
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SINCE ROUGHLY HALF OF BODY WEIGHT RESIDES IN THE
TRUNK,SMALL VARIATIONS OF TRUNK POSTURE HAVE
LARGE INFLUENCE UPON GRAVITATIONAL DEMANDS ON THE
HIPS LOW BACK.
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BONES ARE LIVING TISSUES.HABITUAL POSTURES CAN
LEAD TO DEFORMITIES
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THERE ARE SEVERAL METHODS OF POSTURAL ASSESSMENT
WHICH MAY BE USED IN THE FIELD (OWAS, RULA AND
BODY TARGETTING)

• See Evaluation of Human Work (1995) Ed. Wilson
  Corlett (Taylor Francis) 2nd Ed

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SOME POSTURES ARE DIFFICULT TO MAINTAIN. Why?

• FOOT BASE IS INADEQUATE
• POSTURE IS FATIGUING

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THE BODY TOPPLES IF THE LINE OF WEIGHT MOVES
OUTSIDE THE FOOT BASE!
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POSTURES THAT INVOLVE PROLONGED STATIC USE OF
MUSCLE MAY CAUSE FATIGUE AND DISCOMFORT
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SIMILARFORMANYOTHER MUSCLEGROUPSWHAT IS
HAPPENING?
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COMPLEX WORKING POSTURES MAY BE ASSESSED
PSYCHOPHYSICALLY AND BY BODY MAPPING OF DISCOMFORT
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e.g. a mixed manual / visual task
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BODY MAPPING OF DISCOMFORT
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Electro-myographyas a tool forposturalstudies
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FREQUENCY CONTENT OF EMG SHIFTS IN FATIGUE
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LOW BACK PAIN IS ASSOCIATED WITH BOTH STANDING
AND SEATED POSTURES. TO END, CONSIDER STRESS ON
THE SPINE AND ONE TECHNIQUE FOR ASSESSING IT.

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SPINAL STRESSES
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INTRADISCAL PRESSURE INDICATOR OF SPINAL STRESS
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THATS ALL FOR DAY2