Friday, January 13, 2012

Sitting ergonomics: Different sitting postures & analysis of chair sitting muscle work

Different varieties sitting postures:

A. Common sitting postures:

1.    Chair sitting
2.    Crossed sitting
3.    Crossed sitting with arms wrapped around both knees & locked in front
4.    Half crossed sitting
5.    Crook sitting
6.    Inclined sitting (to back)
7.    Inclined sitting (to sides)
8.    Inclined long sitting
9.    Side sitting
10.    Stoop sitting
11.    Fall out sitting
12.    Ride sitting
13.    Kneel sitting
14.    Crouch sitting

B. Activities in sitting:

1.    Twisting in sitting
2.    Bending & reaching in sitting (sidewise- office works & in front- driving)
3.    Hitching & Hiking (to relieve pressure on buttocks in prolonged sitting)

C. Co-existing unavoidable stress factors in sitting:

1.    Whole body vibration (driving)
2.    Noise stress
3.    Visual stress
4.    Psychological stress

Analysis of muscle work in sitting posture:

Ideal sitting posture: The following discussion is in the context of a quiet ideal chair sitting posture without any upper limb activity. The position is taken on a flat base chair or stool, the height & width of the sitting area allow the thighs to supported & hips and knees is flexed to 900 . In ideal sitting femora are parallel to each other & feet rest on the floor with ankle at 900 where as hells are vertically below the knees.

Muscle work in ideal sitting posture:

a.    Joints of lower extremity have no muscle work except at hip. Flexors of hip work in reverse origin insertion fashion to prevent slumping of the lumbar spine.

b.    Joints of spine:
i. Global extensor muscles of the spine (Ex-Multifidus): these postural muscles keep the trunk upright. Action of these muscles may be counterproductive at lumbar & cervical spine where it’s action produces a bow string effect & increases the lordotic curvature leading to reduction in the over all height of the spine at these places. Therefore at the lumbar & cervical spine this action must be counteracted by the local flexors (lumbar & cervical spine flexors) to ensure local spine lengthening and maintain the correct & ideal local spine posture.  
ii. Flexors of the lumbar spine (Abdominals): In sitting they must work to prevent the bow string effect produced by the global extensor muscle. Scientific literature indicates they contract in an in to out fashion. Hence transverse abdominis is of prime importance in maintaining the core stability & correct spinal alignment. Where as the straight abdominals (rectus abdominis) maintain the correct pelvic tilt matching the spine alignment so that correct contact points are maintained at the chair base- body interface.
iii. Flexors of the cervical spine (pre-vertebral neck muscles) act to prevent the bow string effect produced by the global extensor muscle.
iv. Posture of the head on the cervical spine is finely controlled at the CVJ & at atlanto-axial joint by own set of flexors-extensors to maintain this sagittal posture.
c.    Other joints in sitting:
i. TMJ: elevators of the mandible close the mouth against the pull of the gravity.
ii. Thoraco-scapular junction: Thoraco scapular muscle (rhomboids) retracts the scapula so that the glenoid cavity faces laterally. Cervico scapular muscles (levator scapulae) work to elevate a depress scapula due to the pull of the gravity.

d.    Joints of upper extremity: No muscle work is required for quite sitting but sitting with occupational arm demands may leads to more activation of the external rotators of the arm, abductors, elbow flexors, forearm pronators, wrist extensors & finger flexors.