Engineering Approaches to

Bio mechanics of Standing

Muscles extending from the neck to the ankle= prevention of buckling

Postural Muscles

Types of skeletal muscle fibers

Mixed composition of muscles

Force proportion of postural to phasic muscles is 3:2

High-level sports or athletic training can change this proportion to even 5: I

The relationship between postural and phasic muscles

Postural muscles tend to spontaneous functional or even anatomical shorteningè higher muscle tonus.

Postural muscles have an inhibiting effect on their phasic partners

With an insufficient variety in muscle useè which leads to inhibition and weakening of phasic muscles (pseudo paresis)è imbalance, postural dysfunction, loss of mobility deformities and increase the joint load.

Biomechanics Of The Pelvis

Largest forces always act in the longitudinal direction of the spine

The position of the Intervertebral discs èPerpendicular to the longitudinal orientation of the spine

Sitting

Evaluation of design criteria on the basis of biomechanical aspects

A good sitting posture characterized minimal muscle effort, which is produced with proper support by armrests, backrest, seat, and footrest.

Ability to change posture regularly.

Weight Transfer

  • Main body weight transferred to a seat. Some transferred to floor, backrest, and armrests.

Advantages

  • Provides stability to tasks involving visual and motor control
  • Less energy consuming than standing
  • Places less stress on lower extremity joints
  • Lessens pressure on lower extremity circulation

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ARMRESTS

The importance of armrests  often underestimated

Armrests unload the shoulder girdle

The weight of the arms is 10 % of body weight

Variety of arm positions

In cars, armrests often absent or too low

A proper armrest may place below the mass centers of an upper and lower arm

In cockpits for pilots= adjustable armrest

Logical error= air passing underneath, the only limited area supported

Back Rest

Provides stability for the vertically erected trunk

In prolonged sitting, the prevention of a lumbar kyphosis èmost important function of the back rest

Click-clack phenomenon

Backrest Dimensions

1.Top height

2.Bottom height

3.Center height

4.Height

5.Width

6.Horizontal radius

7.Vertical radius

8.Seat angle

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Seat

The weight of the trunk, head, and part of the arms almost completely carried through the ischial tuberosities

Horizontal seat always raises friction at the ischial tuberosities, this friction can completely eliminate:

means  of a moderate seat angle and that the angle between seat and back rest optimal between 90 and 95°

Backrest inclination larger in auditoriums, cars, and easy chairs at home

This facilitates a horizontal direction of looking and  the advantage that the back rest contributes in carrying the trunk

Headrests cannot carry the head when they positioned too far forward

Seat Dimensions

1.Height

2.Width

3.Depth (length)

4.Slope

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Chair And Table

A chair important for a good posture

In tasks such as reading and writing involved, the height and inclination of the desk or table play a dominating role

Despite good chairs, postures with the back bent, sagging, or twisted can observed

A various cycle of pain

Leg-crossing

The majority of people cross their legs often when sitting, alternating left over right and right over left

There may be many reasons to do this, but in the literature, there is no scientific proof about the benefit or demerit of leg-crossing

Lying

A bed is a body support surface on which prolonged and complete rest must be found

Combination of a mattress, a mattress carrier, and a bed frame or bedstead

Spread-out mat on the floor

Several kinds of mattress carriers

A good bed should

¤Adapt  to body curvatures

¤ Remain  flat

¤ Have a pleasant spring action

¤Have  good ventilation, and

¤Not  be too warm or too cold

Aspects I to 3 concern body support that evenly spreads the pressure on protruding (bony) parts.

Gives a straight spine when lying on the side, and gives a natural S-shape to the spine in the supine posture

For static equilibrium, muscle action is superfluous

For separate support of the head, a pillow is needed.

The thickness of the pillow is related to the curvature of the spine

Thin vs. thick pillow

Pillows must be pliable to also support the neck.

A bed that is too hard results in the restless change of posture to unload areas with disturbed blood flow.

Normally people change posture 20 times per night

The most unconstrained form of the lumbar spine is found when the angle between the trunk and femur is 135 degree

Sitting In Bed

It is  common, especially in hospitals in which the head portion can be raised upward

Sitting in bed on a horizontal mattress gives rise to shear forces (in the order of magnitude of 100 N) on the skin and underlying tissues of the bottom.

As a consequence, the pelvis moves little by little on the mattress

Taller people have the advantage that they can come to a stop with the feet against the board at the foot end

lumbar support is absent, which causes lumbar kyphosis and the pelvis to rest on the os- coccyx instead of on the ischial tuberosities.

The combination of pressure and shear is a provocation of decubitus skin ulcerations.