Postural stability and the effects of wheelchair cushions


Background: Postural stability refers to the continuous process of postural changes during sitting.

The capacity to maintain postural stability in sitting position is a prerequisite to perform activities of daily living (ADLs), reduce risk of shoulder injury, pressure injuries and falling, but also social functioning, satisfaction with equipment, and thus quality of life. Deficits in postural stability can severely limit performance in these aspects of life.

There are different ways to measure postural stability, namely by center of pressure, reach test, wheelchair skills test, among others. And 2 ways to measure stability of a cushion itself, namely horizontal stiffness test and the lateral stability tilt test.

Aim: Determine what factors influence postural stability, more specific of the wheelchair cushion,

and how this data is relevant for Vicair wheelchair cushions.

Method: Literature review, search in PubMed, Google Scholar and references in May 2023.

Results: 25 studies are included.

There are several contributing factors to obtaining and retaining postural stability in a sitting position, namely trunk strength, foot support, large base of support, anterior pelvic tilt, thigh support, back support and wheelchair set-up. Anterior pelvic tilt, thigh support, and a stable sitting surface can all be achieved using an appropriate wheelchair cushion.

Stability of the wheelchair user can be compromised by the cushions ability to resist medial-lateral or anterior-posterior pelvic rotations caused by shifting center of mass. The Jay 2, ROHO High Profile and Varilite Evolution could not create a stable base to perform reach. Vreede found that a compartmented air cell cushion could provide a stable base to perform reach and ADL tasks (Vreede, 2018).

Conclusion: Selection of an adequate wheelchair cushion is of utmost importance to ensure a satisfactory quality of life of wheelchair users. Increase in of postural stability in a sitting position can be obtained by using a compartmented air cell cushion.


1.  Background

  • ​What is postural stability?

Postural stability is the ability to keep or return the center of body mass over the base of support in a position or during changes in position. It is a complex process involving coordinated actions of biomechanical, sensory, motor and central nervous system components (Horak, 1990).

Sitting balance is believed necessary in performing functional activities from a seated position. Several prognostic studies have shown that seating balance is a valid predictor for functional outcomes, such as activities of daily living (ADLs) in patients with brain injury or stroke (Black et al., 2009; Kwakkel et al., 1996; Sandin & Smith, 1990; Wade et al., 1983; Wade & Langton Hewer, 1987). Tidemann found that sitting balance is a relevant factor to define functional sitting position. They found that sitting balance is assumed to be essential for obtaining independence in other vital functions such as reaching, sit-to-stand, and sitting down. Balance is therefore relevant to consider in particular with regards to reaching tasks that might challenge the users’ postural stability (Tidemann et al., 2019).


Impact of postural stability:


  1. Activities of daily living (ADLs)

The wheelchair and/or seating system should enable individuals to perform the activities of daily living (ADLs) that are important to them with minimal to no assistance and with the least amount of energy expenditure. Types of activities can include transfers, personal needs (e.g., bathing, toileting), working, preparing meals, cleaning, and shopping.

The capacity to maintain postural stability in a sitting position is a prerequisite to perform ADLs and deficits in postural stability can severely limit task performance (Dean, 1997; Riley et al., 1995). Adequate sitting stability is positively correlated with performance of ADL tasks (Chen et al., 2003). A lack of postural stability can lead to issues regarding safe transfers, wheelchair skills, physical strain during wheelchair propulsion, ADL, pain, and spasticity (Bolin et al., 2000).

In 2014, Gao et al. found that individuals with a faster reaction on the sitting balance test had better mobility scores in the functional mobility assessment. Similar results were found for participants who could lean more and had better directional control. In ADLs, being able to shift the body’s center of mass controllably in all directions is important and therefore relevant for daily functioning (Gao et al., 2015).


  1. Propulsion and shoulder injury

Lack of postural stability due to impaired abdominal- thoracic musculature has been linked to increased susceptibility to shoulder pain and shoulder injury. Poor seating posture due to improper wheelchair seat position and configuration can further increase this susceptibility (Dyson-Hudson & Kirshblum, 2004). Wheelchair cushions can help in correcting posture. Another study, Sinnott et al, found a high correlation between the prevalence of rotator cuff disorders and level of injury in individuals with SCI. This study also showed a correlation between shoulder injury and decreased trunk control and pelvic stability. 72.7% of individuals with poor postural control were diagnosed with bilateral rotator cuff disorders. This study also reported that more than 70% of all individuals with SCI experience shoulder pain (Sinnott et al., 2000). Prevalence of pain and rotator cuff tears (63%) is higher after paraplegia than age-matched able-bodied controls (15%), highlighting the importance of trunk stability (Akbar et al., 2010).

Seated postural stability also strongly contributes to propulsion and together with strength- generating capabilities of the shoulder, it explains 71.3% of variance observed in propulsion (Gagnon et al., 2016). Triolo et al. found similar results in their study: ‘Stabilizing the pelvis and trunk with low levels of continuous electrical stimulation to the lumbar trunk and hip extensors can positively impact the mechanics of manual wheelchair propulsion and reduce both perceived and physical measures of effort’ (Triolo et al., 2013).

Dyson-Hudson & Sinnot also emphasize the great impact that shoulder injuries can have on peoples independence, ADL’s, and participation in society, which also brings us to the impact on quality of life (Dyson-Hudson & Kirshblum, 2004; Sinnott et al., 2000).


  1. Quality of life

Postural stability is not only important for improvement in functional outcomes such as reach and the ability to perform ADLs, but it is also important for social functioning and satisfaction with the equipment and therefore for quality of life. One of the given reasons for the improvement could be the fact that participants had less difficulty propelling their new wheelchair (Trefler et al., 2004).


  1. Falling

Postural instability is a common problem for wheelchair users that can result in increased risk of falling (Okunribido, 2013; Poojary-Mazzotta, 2016). Okunribido et al. found that during wheelchair transfers the risk of the occupant falling forward and out of the wheelchair is higher when they are sat on a thick soft cushion (100mm castellated Propad HOMEPAD low profile), compared to a hard regular cushion (50mm basic polyurethane (flat foam) high-density), particularly when the feet are unsupported. The risk of falling is increased when the occupant is prone to slouch forward in the wheelchair, due to their medical condition (Okunribido, 2013).


  1. Pressure injuries & other

A lack of postural stability can cause pressure injuries and advance deformations in the spine (Jung et al., 2015). The seated posture affects how loads are redistributed. Postural instability can induce asymmetric loading on for instance the ischial tuberosities. A slouched, kyphotic posture creates extra pressure at the sacrum and coccyx while seated. Body posture and positioning have a direct relationship to loads on specific body sites, which is why posture and stability must be considered when devising pressure ulcer prevention strategies (Sprigle & Sonenblum, 2011).


Furthermore, a lack of postural stability can lead to issues regarding safe transfers, wheelchair skills, pain, and spasticity (Bolin et al., 2000).


  • ​Measuring postural stability:
  1. COP

There are several instruments that can be used to assess postural stability in a sitting position. One frequently used measure to assess stability is center of pressure (COP). Test-retest reliability is moderate to excellent for COP. This test-retest reliability indicates that COP measures of multidirectional seated postural stability may be useful as a clinical measure and similar results have been found in a study focusing on stroke patients. However, COP is not an effective instrument to determine sway, which can occur during quiet sitting (Kerr & Eng, 2002; Näf et al., 2020). Quiet sitting is not sitting completely motionless, usually there is some movement, called sway.

Everyone experiences postural sway to some degree. But in some cases, created postural sway can be an indication of poor balance and coordination.



Figure 1 (right). Centre of Pressure measurements, measured with a pressure plate underneath the buttock and feet of a hemiplegic wheelchair user (T. Sato & Nabeya, 2021).

 Figure 2 (left). Start and end position of the seated reach test. Start position is the stippled gray figure, maximum reach in the forward (dark gray), leftward (light gray) and backward (white) direction. Rightward reach was also tested but is not illustrated (Field-Fote & Ray, 2010).


  1. Reach tests

Another method to measure stability/dynamic sitting is the seated reach test (SRT). The SRT is a test of maximal reach distance forwards, leftwards, rightwards and backwards (see figure 2). Field-Fote et al. demonstrated that the seated reach test has a good to excellent test-retest reliability and a significant correlation with COP. Similar test-retest reliability results were found in a study by Gao et al., indicating that the seated reach test is a valid and reliable method to measure stability (Field-Fote & Ray, 2010; Gao et al., 2015; Sprigle et al., 2003) .


  1. WST

A third comprehensive method to assess postural stability while being seated (May et al., 2003), is a wheelchair skills test (WST) including 30 functional tasks that wheelchair users encounter in daily living. These tasks are scored ranging from 0 (fail) to 3 (advanced pass) on a score form (figure 5 in appendix C). Test-retest and inter-rater reliability for all tasks included in the WST are excellent (May et al., 2003). Available via: forms/


  1. Other methods

Some less used methods to assess postural stability are the Berg Balance Scale (BBS) which evaluates postural control with static and dynamic tasks in 14 sections (figure 6 in appendix C) (Wong, 2014), the Posture And Postural Ability Scale (PPAS) which evaluates posture quantitively and qualitatively (figure 7 in appendix C)(Rodby-Bousquet et al., 2014), the Function in Sitting Test (FIST) which evaluates the level of functionality in sitting with 14 tasks (figure 8 in appendix C) (Abou et al., 2020), and the Trunk Control Test (TCT) which evaluates the control that the subject has over their trunk. However, some of these (BBS, PPAS, TCT & FIST) tests are less reliable since they are dependent on the level of skill and experience of the therapist (Reguera-García et al., 2020). Abou et al. found that the FIST is still a reliable and valid test with an excellent test- retest reliability, and the FIST correlates with the lateral modified Functional Reach Test (Abou et al., 2020).


Frechette et al. described how smartphones could be used as a postural control assessment in wheelchair users. Their pilot study illustrated that smartphone-based accelerometry may be able to provide a valid and reliable assessment of seated postural control and has the ability to distinguish between those with and without impaired postural control (Frechette et al., 2020).


Santamaria et al. validated the seated postural & reaching control test in children with cerebral palsy (CP), it is a reliable and valid test for therapists to objectively examine and quantify seated postural and reaching control (Santamaria et al., 2020).


Sato developed the Shoulder Shifting Test (SST) for individuals with an SCI and difficulty raising their arms. They found that the SST was compatible with the SRT, and the sitting balance of more individuals with SCI could be assessed by the combination of these tests (H. Sato et al., 2022).

In 2021 T. Sato studied the relation between reach distance and COP in patients with hemiplegia. They asked patients to reach maximal toward the front (a), toward the nonparetic side (b), and toward the paretic side (c) (see figure 4). They found that depending on the trunk function there is a moderately strong to strong or lower correlation (T. Sato & Nabeya, 2021).



Figure 3. Multidirectional reach items (T. Sato & Nabeya, 2021). With the arm of the non-affected side is reached to that side (a), followed by a

frontal reach (a) and a reach to the affected side (c).

  1. Cushion tests: Besides testing the stability of the wheelchair user it is also possible to test the stability of the cushion itself, these tests are described in ISO 16840, namely the horizontal stiffness test & tilt test. A short description of these tests can be found in the table below (table 1) and a visual overview in figure 5.





The aims of this review are:

  1. To determine what factors influence postural stability,
  2. To determine what determining factors in a wheelchair cushion are to obtain optimal postural stability,
  3. To determine how this data is relevant for Vicair wheelchair cushions.


1.  Methods

We designed a literature review. To identify articles for the review, we conducted a literature search in PubMed and in Google Scholar in February of 2021. And updated the search in May 2023. The search terms were reach, seated functional reach, wheelchair, cushion, support surface, daily functioning, ADL, thigh support, propulsion, and variations of these search terms. Furthermore, we consulted the references of the literature for relevant articles. The search was limited to studies published in English or Dutch. We did not specify limits on dates, study design, or the age of the participants.


2.  Results

  • ​General description of the studies

A total of 25 studies was included in this review. The study designs were randomized controlled trials/intervention studies, case studies, case-control studies, repeated measures studies, and cross-sectional group studies, among others (Flowdiagram 1, Appendix A & Table 2, Appendix B). All studies were published between 1986 and 2022. A summary of study characteristics is shown in Table 2 (Appendix B).


  • ​Factors influencing postural stability

There are several contributing factors to obtaining and retaining stability in a sitting position, both intrinsic factors and external factors.


  1. Trunk strength

One of the most known contributing factors for stability in a sitting position is trunk strength. Less affected trunk extension and flexion strength is statistically significantly associated with higher levels of reach (Gabison et al., 2014).


  1. Foot support

In 1986, a wheelchair basketball player with bilateral above the knee amputation pointed out that wearing his prostheses allowed him to lean farther forward. This observation led to speculation that the lower limbs, or their prosthetic replacements, might contribute to sitting balance in forward-


reaching tasks by enlarging the base of support. This led to a study to determine the extent of the contribution of one or both legs on sitting balance. The study confirmed the observation of the basketball player (Chari & Kirby, 1986). The results of that study were later supported by a study by Janssen-Potter et al. who assessed the effect of the footrest on sitting balance in paraplegic subjects and healthy individuals. Although healthy participants did show a decrease in reach and displacement of the center of pressure (COP) while seated in a chair with an elastic foot support, these results were not seen in subjects with SCI. The elastic foot support does have influence on how reach is performed by subjects with an SCI , indicating that a solid foot support contributed to postural stability (Y. J. Janssen-Potten et al., 2002).


  1. Anterior pelvic tilt

Another contributing factor to obtain stability in a sitting position is anterior pelvic tilt. This was demonstrated by a study by Amos et al. (Amos et al., 2001). They measured functional reach while seated in a wheelchair using goal-directed tasks since this produces a higher quality of movement and is more concrete and purposeful than a rote (mechanical or habitual repetition of something to be learned) reaching task. Participants sitting with forward pelvic rotation had a significantly greater forward functional reach compared to participants sitting in a wheelchair with a sling seat that allowed backward pelvic tilt. Besides an increase in functional reach, forward pelvic tilt also allows for less head-forward positioning and less shoulder protraction and a more upright trunk position (Amos et al., 2001; Hastings et al., 2003). Rice et al. found in their pilot that anterior tilt in a power wheelchair is also helpful for vertical reach, and with that participants found anterior tilt helpful in performance of reaching tasks (Rice et al., 2019).


  1. Thigh support

Thigh support is also considered a contributing factor to the increase of stability in a sitting position. A study with 12 healthy elderly volunteers showed no statistically significant difference between percentage of thigh support and duration of reaching task (Dean et al., 1999). However, another study that followed the same protocol as Dean et al. found that there was a significant effect of amount of thigh support on velocity and extent of reach during a reaching task in the participants with spinal cord injury (SCI) indicating that thigh support is a contributing factor to stability in a sitting position in disabled individuals. Passive mechanical participation of the paralyzed body is probably reduced when the thigh support is reduced (Ilha et al., 2020).


  1. Back support

A factor contributing to static postural stability that is often overlooked, is back support. In 2020, a study was performed where the wheelchair’s back support of 50 participants with chronic SCI was removed and replaced with a solid back to assess whether a solid back would improve postural alignment, increase forward upward reach, promote respiratory function and facilitate mobility as compared with an upholstery back. Posterior pelvic tilt was significantly reduced by using a solid back. Furthermore, all functional outcome measures were improved when using the solid back support. However, statistical significance was not achieved due to the low number of participants. The increase in vertical forward reach was noted as a clinically significant difference as it is double the minimal detectable change (Presperin Pedersen et al., 2020).

A study by Ukita et al. also underlined the importance of back support shape to contribute to postural stability. However, in this study greater postural stability was achieved by pelvic support (Ukita et al., 2020).


  1. Wheelchair set up

Several studies have investigated the effect of wheelchair configuration or seating systems on reach and upper extremity functional tasks. These studies found results differed depending on the task and the configuration used in the study (Aissaoui et al., 2001; Amos et al., 2001; Bolin et al., 2000; Curtis, 1995; Hastings et al., 2003; Y. J. Janssen-Potten et al., 2002; Y. J. M. Janssen- Potten et al., 2000).


Hastings et al found that persons with complete SCIs exhibited a more erect posture and a greater vertical reach ability while sitting in a wheelchair with posterior seat inclination, low back support height, and acute seat-to back angle (clinically referred to as ‘‘squeeze’’) compared with standard wheelchair configurations (Hastings et al., 2003). Bolin, Bodin, and Kreuter compared changes in seating and wheelchair configuration of 4 wheelchair users with C5-C6 tetraplegia on 8 different variables: balance, transfers, wheelchair propulsion, wheelchair skills, physical strain, spasticity, respiration, and perceived changes by the subjects. They found that the variables tested were affected by the sitting position in an individual manner and that not 1 standard solution worked for all (Bolin et al., 2000). The results of these studies were corroborated by Sprigle et al, who concluded that individuals may perform better with a particular seating system but that no single seating system is best for all people (Sprigle et al., 2007).


  1. Other factors

Other factors that can positively influence postural stability, seated balance, or reach: Exercise (Rose Lee-Hauser et al., 2021), passive trunk orthosis (Rao & Hasan, 2021), virtual reality training in patients with SCI (Nair et al., 2022), epidural electro stimulation in patients with severe thoracic SCI’s (Gill et al., 2020), exoskeleton walking training (Tsai et al., 2021).


  • ​Wheelchair cushion effects on postural stability

Functional reach can be increased by individually configured wheelchairs and seat cushions, according to Brienza (Brienza et al., 2010). The seat cushion, as the base of support for the wheelchair user, affects postural stability by resisting moments when the users’ center of mass (CoM) is displaced. For example; users’ center of mass shifts as they lean during a reach activity or when users encounter a sloped surface. Stability of the wheelchair user can be compromised by the cushions ability to resist medial-lateral or anterior-posterior pelvic rotations caused by shifting center of mass (Delazio et al., 2019).


In 2001, Aissaoui et al. designed a study with the aim to examine the effect of seat cushions on postural stability in sitting during a controlled reaching task performed by individuals with paraplegia. They used an interconnected air cell cushion (3-inch air-flotation ROHO), a contoured foam cushion (3-inch polyurethane HR45, ISCUS) and a flat foam cushion (2-inch polyurethane HR35) . They included 9 wheelchair users with paraplegia by SCI. During the reaching task, the pressure distribution at the body-seat interface was recorded by a force-sensor array. The maximal covered distance (MCD) by the center of pressure (COP), the maximal velocity (MV) of the COP, as well as the surface area underneath the curve defined by the velocity-distance relationship were computed. When a contoured foam cushion is used, the COP covers a larger distance with higher speeds compared to the interconnected air cell cushion or the flat foam cushion. They found that paraplegic subjects were able to increase both the distance covered and the velocity of the COP when a generic contoured cushion was used during reaching. The ability of the COP to cover a large distance with high velocity appears to be linked to the increase in postural stability during reaching (Aissaoui et al., 2001).


Sprigle et al. reported that subjects exhibited greater reach when sitting with a pelvis positioned on a stable base on which upper extremity and trunk movement occurred (Sprigle et al., 2003). In their study they used 3 different cushions:

  1. Jay 2: viscoelastic fluid and polyurethane foam encased in vinyl atop non deforming foam
  2. ROHO High Profile: single-valve adjustable air
  3. Varilite Evolution: bonded multistiffness polyurethane foam encased in single-valve adjustable air cushion.


And found that the posture adopted by wheelchair users is a more important influence on upper extremity than is the cushion (Sprigle et al., 2003). None of these cushions is pre-contoured or consists of compartmented air. Therefor none of these cushions can create the stable base Sprigle refers to.


In 2018, Vreede et al. performed a case-control study where both healthy controls and individuals with a disability who relied on a wheelchair for daily ambulation performed a modified functional reach test, also known as the seated functional reach test (SRT), while sitting on different wheelchair cushions (foam + gel contoured cushion, interconnected air cell cushion, and two compartmented air cell cushions; the Vicair Vector O2 and the Vicair Adjuster O2) and a foam cushion which was considered the gold standard. In healthy controls, upward and horizontal reach are highest in both compartmented air cell cushions. Downward reach is highest in one of the compartmented air cell cushions. In disabled individuals, upward, horizontal and downward reach are highest in both compartmented air cell cushions, because they provide the most stable sitting surface . For both groups, there is a statistically significant difference in reach between the gold standard and the compartmented air cell cushions. Indicating an increase of postural stability when using a compartmented air cell cushion (Vreede, 2018).


In 2013, Nag et al., assessed COP on different seat cushions (cotton, foam and wooden) in 9 healthy participants. When participants sat on the foam cushion (polyurethane foam, covered with rexine), they showed significantly lower COP trajectory length and speed than when sitting on the cotton cushion or the wooden cushion, indicating that they experienced more postural stability on the foam cushion (Nag et al., 2013).

The study of Delazio compared multiple cushions, but only the results of an interconnected air cell cushion with and without closed valve in comparison with a foam cushion are shown. They found that airflow restriction in air cell cushions has a beneficial effect on stability due to a lower tilt response and better pressure distribution (Delazio et al., 2019). Since a Vicair cushion consists of SmartCells, al containing the air, according to the theory of Delazio they should be more stable compared to interconnected air-cell cushions. The comparison with and without a postural insert beneath cushion indicated that postural insert increased stability (Delazio et al., 2019).


  1. Discussion

The aims of this review were to determine what factors influence postural stability in a sitting position, what determining factors in a wheelchair cushion are to obtain optimal postural stability, and how this data is relevant for Vicair wheelchair cushions.


The 4 most important factors determining postural stability that were found in our literature search, were trunk strength, increase of base of support (i.e. using rigid footrests), anterior pelvic tilt and thigh support (Amos et al., 2001; Chari & Kirby, 1986; Hastings et al., 2003; Ilha et al., 2020; Y.

  1. Janssen-Potten et al., 2002). Two of these factors, anterior pelvic tilt and thigh support, can be obtained by selecting the right wheelchair cushion. Two studies found that cushion material greatly contributes to the amount of postural stability that can be obtained (Aissaoui et al., 2001; Vreede, 2018).


While Aissaoui et al. reported that postural stability increased most on a contoured foam cushion, Vreede et al. found that an even larger amount of postural stability could be achieved when a compartmented air cell cushion was used, a type of cushion which Aissaoui did not test (Aissaoui et al., 2001; Vreede, 2018).

The compartmented air cell cushions provided better postural stability than the interconnected air cell cushion and contoured foam cushion that were tested in all three studies, which not only allowed the user to reach further during the seated functional reach test, but allows users more independence in ADLs, social functioning and satisfaction with the equipment. All these factors are contributors to the quality of life experienced by the wheelchair user (Chen et al., 2003; Gao et al., 2015; Trefler et al., 2004).


The influence of back support on postural stability is only present during static seating (in a resting position) when contact between the back support and pelvis is present. However, during reach or dynamic sitting this contact is lost resulting in a negligible effect of the back support. The effect of the wheelchair set-up on stability is individually and task specific.


Although only one study focusing on wheelchair cushions included compartmented air cell cushions (Vicair Vector O2 and the Vicair Adjuster O2), the compartmented air cell cushions were compared to the same cushions (Jay 2, ROHO, Varilite) that were tested by Aissaoui et al. and Sprigle et al. (Aissaoui et al., 2001; Sprigle et al., 2003; Vreede, 2018). So the study of Vreede is not an isolated study and can be compared to the studies of Assaoui and Sprigle. The fact that Vreede found that compartmented air cell cushions are the most stable cushions is in line with what Delazio found, namely that airflow restriction in air cell cushions has a beneficial effect on stability due to a lower tilt response and better pressure distribution (Delazio et al., 2019). Since a Vicair cushion consists of Smartcells, al containing the air, according to the theory of Delazio they should be more stable compared to ROHO cushions.


5.  Conclusion

Postural stability is an important factor for independence and social functioning, which leads to increase of quality of life experienced by wheelchair users. Furthermore, the role of trunk control on shoulder injuries may be an important consideration in mitigating injury and improvement of wheelchair propulsion.

Therefore, selection of an adequate wheelchair cushion is of utmost importance to ensure a satisfactory quality of life of wheelchair users. Increase of postural stability in a sitting position can be obtained by using a compartmented air cell cushion.



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