The Pelvis

Good posture in terms of seating, and therefore a seating assessment, begins with the pelvis.

During an assessment, the Anterior Superior Iliac Spine (ASIS) and the Posterior Superior Iliac Spine (PSIS) should be your indicators for the position of the pelvis.

The position of these points in space and in relation to each other is the basis from which you start your seating assessment. It is important to locate these points on the pelvis and record their position at the various stages of the assessment.

Normal and Abnormal Posture

In order to maintain a good seated position, the ASIS and the PSIS must be level, with the pelvis in minimal anterior pelvic tilt. This enables weight to be taken evenly through both Ischial Tuberosities (IT’s) with the head and spine balanced and aligned above the hips. This is considered normal posture.

This posture however, is difficult to maintain due to the force of gravity. Postural control requires an effectively functioning neuromuscular and musculoskeletal system; i.e.

  • Intact nervous system
  • Healthy muscles
  • Flexible joints

Some physical and medical conditions can lead to a deterioration of this posture to an abnormal posture.

Fixed or Flexible?

An abnormal posture can be either fixed or flexible and it is vital that you establish this during an assessment. If the posture is fixed, you must accommodate this posture using the functions available to you in the chair.  This will help to slow down deterioration of this abnormal posture.

If the posture is flexible or partially correctable, you can attempt to correct this posture using the chair. This will bring the person into a better seating position and improve their functional ability while preventing or slowing down deterioration of this posture.


Normal Sitting Posture

Normal Posture

Abnormal Posture

Pelvic Presentations

Commonly referred to as “sacral sitting”, this is identified by the PSIS becoming lower than the ASIS.
  • May cause difficulty in swallowing, communicating and breathing.
  • Can cause development of a kyphotic posture and sliding from the chair.
  • Increased loading on the sacrum and less pressure going through the ITs can often lead to sacral pressure ulcers.
  • Ulcers can occur on apex of the spine due to kyphosis and on the heels as a result of the person ‘anchoring’ themselves to reduce sliding.

Clinical Causes

  • Sliding forward in seat
  • Limited hip flexion
  • Abnormal tone
  • Trunk muscles unable to hold spine upright against gravity
  • Obesity
  • Tight hamstrings

Technical Causes

  • Seat depth too long
  • Inadequate foot loading caused by seat to floor height too high
  • Footplates too low
  • Back too vertical
  • Arm rest too low
  • Seat depth too short
  • Inadequate femoral thigh loading
A pelvic obliquity is characterized by one side of the pelvis being higher than the other.
  • Weight is taken unevenly through the ITs and can cause associated pressure issues on the lower IT.
  • Postural issues resulting from this can be scoliosis and issues with the ribcage and organs on the affected side.

Clinical Causes

  • Scoliosis
  • Hip dislocation or subluxation
Asymmetry in;
  • Muscle strength
  • Bone structures
  • Hip flexion
  • Muscle tone

Technical Causes

  • No solid base of support i.e. unstable cushion or no baseboard
  • The person leans to one side to gain contact with the chair
Their body has not been loaded/supported by the chair, ie;
  • Chair too wide
  • Arm supports too low or too high
  • Femurs and feet unsupported
A pelvic rotation is characterized by one ASIS being in front of the other.
The ASIS is named from the patient’s perspective (see diagram) and a pelvic rotation is named according to the direction of rotation. Think of it as the direction of movement of a car in relation to the steering wheel – if the right ASIS is forward, it is a left pelvic rotation and vice versa.
  • It is often present in those with a pelvic obliquity
  • Can lead to a windswept deformity

Clinical Causes

  • Leg length discrepancy
  • Hip dislocation or subluxation
  • Asymmetrical hip flexion
  • Person is a unilateral foot propeller
  • Asymmetrical hip adduction

Technical Causes

  • Trunk not supported
  • Back rest does not support the posterior pelvis
  • Seat too wide
A windswept deformity is identified by the abduction and external rotation of one hip with the adduction and internal rotation of the other. It is named according to which side of the body the legs are windsweeping. May occur in association with; hip dislocation, scoliosis, pelvic rotation.

Spinal Presentations

A lordosis is identified by an increased lumbar curve. It can be associated with;
  • Anterior pelvic tilt
  • Increased tone in hip flexors
  • Weakened abdominals relative to extensors
A scoliosis is identified by the “C” or “S” shaped spine and can be associated with;
  • Pelvic obliquity
  • Osteoporosis
  • Pelvic Rotation
With a severe scoliosis there can be issues with the ribcage and with compression of internal organs.
A kyphosis is identified by a marked curvature in the thoracic or cervical spine and can greatly reduce physiological functioning. It is associated with;
  • Posterior pelvic tilt
  • Osteoporosis
Pressure ulcers can occur on apex of the spine and on the heels if the person ‘anchors’ themselves to reduce sliding out of their chair.