Posterior Pelvic Tilt (PPT) is the most commonly seen detrimental posture not only in the users of mobility equipment, but people in general. When the human body is forced to hold itself up against gravity, it naturally seeks out the most stable, least fatiguing position. While sitting, the position of stability is often a posterior pelvic tilt because it allows the person to lower their center of gravity (COG), shift its posterior in relation to their base of support (BOS) and allow the trunk to either “hang” on the ligaments of the spine and/or rest against a back support.
Ideally, the pelvis is positioned in a neutral to a slightly anterior tilted position to allow the remainder of the body to stay in the best biomechanical position. In this position, the anterior superior iliac spine (ASIS) and posterior superior iliac spine (PSIS), is level and parallel to the seating surface (see figure 1).
The ischial tuberosities will be oriented vertically and there should be little to no contact of the sacrum with the seating surface. PPT occurs when a person’s pelvis shifts from a neutral position where the anterior (ASIS) and posterior superior iliac spines (PSIS) are aligned parallel to the seating surface, to a position where the anterior spine is pointed up and the PSIS pointed in a more downward direction (see figure 2).
If PPT is a more stable position, then why are we concerned when a wheelchair end user is seated that way? PPT is a concern as it can lead to serious problems for someone that utilizes a wheelchair for prolonged periods throughout the day and cannot move out of the position on their own. This position places an individual at higher risk for pressure/shear injuries on the seating surface and sliding forward in (and potentially out of) the wheelchair. It also contributes to increased thoracic kyphosis, flexed cervical spine with resultant hyperextension of the upper cervical spine, decreased visual field, difficulty swallowing, and compromise respiratory function, to name a few. In addition, over time this can lead to shortening of the hip flexor and abdominal muscles, changes in the shape of the bones in the spine, pelvis and lower extremities and result in non-reducible deformities.
In order to effectively correct and support the person’s pelvis and prevent PPT, it is important to identify the specific cause for the client. This may include proximal/core weakness, insufficient proximal support at the pelvic/lower extremities, extension hypertonicity/spasticity, tight hamstring muscles, restricted hip flexion range of motion, and improper mobility base/seating system configuration.When the person has impaired postural control or core muscular weakness, they are even more susceptible to PPT because the body is going to seek a position of stability.
Increased extensor tone or spasticity in the trunk and/or lower extremities often causes the posterior lower extremity muscular to pull the pelvis posteriorly as the legs push down into the seating surface.
The hamstring muscles originate on the ischial tuberosities, cross the hip and knee joints and insert on the tibia/fibular head. When the hamstrings are tight and stretched, it can pull the pelvis into a posterior position, especially if there is additional stretch caused by movement of the knee joint towards extension.
If the person has limited hip flexion range of motion, then they tend to move into a posterior pelvic tilt because it takes at least 90 degrees of hip flexion to sit with the pelvis in neutral. If the person has 90 degrees of flexion but is transferring to the chair themselves, then a minimum of 120 degrees of flexion is required as the trunk comes forward to counterbalance the body during the transfer.
Lastly, a posterior pelvic tilt can be caused by an inaccurate upper leg length measurement that is then translated into a wheelchair seat depth that is too long. As the person sits, they are unable to get their hips to the back of the chair. As a result, the pelvis will rotate posteriorly as it seeks the stability of the back support.
We will look deeper into the proper set up of the wheelchair and other interventions that can be made to address PPT in the next segment of this blog.
About Wade Lucas: Wade is a physical therapist and the clinical education manager for Quantum Rehab in the western U.S. In his spare time, Wade enjoys watching his kids compete in their activities, as well as playing golf and watching football.