Engineering Approaches to
Bio mechanics of Standing
Muscles extending from the neck to the ankle= prevention of buckling
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
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.
- Main body weight transferred to a seat. Some transferred to floor, backrest, and armrests.
- 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
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
Provides stability for the vertically erected trunk
In prolonged sitting, the prevention of a lumbar kyphosis èmost important function of the back rest
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
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
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
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.