Changes in Pressure During Static Sitting Versus Dynamic Movement While Sitting
The following studies have explored the effects of dynamic movement on interface pressure. Stinson et al. (2013) examined changes during reaching as compared to static sitting while working at the computer. Tam et al. (2003) and Kernozek and Lewin (1998) both examined interface pressure differences between static and dynamic sitting.
| Author Year
Country |
Methods | Outcome |
| Stinson et al. 2013
Ireland Pre-Post N=14 |
Population: Age range: 23-62 yr; Gender: males=12, females=2; Level of injury: paraplegia=8, tetraplegia=6; Chronicity range: 1-324 mo; able to safely lean forward and computer literate.
Intervention: Investigate pressure relieving behaviours during everyday computer use. Strand A, (1 hr continuous computer use in standard position versus Strand B (reaching forward by 150o of arm length and typing for 5 min, alternated with 10 min of upright sitting). Outcome Measures: XSensor Interface pressure mapping system: [Dispersion Index (DI); Peak Pressure Index (PPI); Total Contact Area (CA)], Frequency of movement (left lean, right lean, push-up, other), Duration in changed position, Trunk angle and questionnaire. |
1. Only 4.9% of movements performed during normal computer use (Strand A) were considered pressure relief movements (they were considered “moderate” unloading – 51-75% reduction in interface pressure).
2. Frequency and type of movement varied greatly (range 0-28 movements; median 5) 30% of which were classified as task related. 84.4% of movements yielded less than 25% reduction in interface pressure compared to normal sitting. 3. During Strand B, DI and angle of trunk tilt were significantly reduced (p<0.05) compared to normal sitting, but it did not significantly affect CA. During Strand B, PPI for both the right and left ischial tuberosity (IT) regions was significantly reduced (p<0.001), which represents an interface pressure reduction of ~52%. 4. Questionnaire results indicated participants preferred to incorporate pressure management movements into regular activities (77%, n=10). |
| Tam et al. 2003
China Prospective Controlled Trial N=20 |
Population: Mean age: 45 yr; Level of injury: L3-T8; Time since injury range: 5-34 yr.
Intervention: 1) Comparison of interface pressure and IT location during static sitting and dynamic propulsion in standard wheelchair with no cushion; 2) Comparison between ‘normal’ group and test group; use of Quickie TNT manual wheelchair and a rigid seat pan; mathematical calculation of IT location. Outcome Measures: Peak pressure, Location of pressure optical motion analysis system. |
1. The magnitude of dynamic average pressure under the ITs did not exceed the mean pressure recorded during static sitting.
2. Peak pressures during static sitting were high with 4/10 people in the normal group and 7/10 in the SCI group reaching saturation pressures of 572 mmHg on the pressure mat. 3. The ratio of minimum peak pressure to maximum peak pressure during dynamic propulsion was 1:4.1 in the normal group and 1:1.8 for the SCI group. 4. No statistical difference between the normal and SCI groups in the location of the peak pressure over left and right ITs with the calculated locations of the ITs projected onto the pressure mat (20.7±11.5mm on left and 24.6±9.9mm on right for normal group and 17.7±13.1mm on left and 13.2±10.5mm on right for SCI group). 5. Pelvic tilting angle (the angle between the pelvic plane and the reference seat plane which accounts for forward and backward rocking during propulsion), was statistically different between the normal and SCI groups (p<0.05, power=0.9); pelvic tilt angle was 11.2°±2.1° for the normal group and 5.2°±1.1° for the SCI group. |
| Kernozek & Lewin 1998
USA Post Test N=15 |
Population: Gender: males=13, females=2; Mean weight=77.5 kg; Level of injury: paraplegia=15; Chronicity=chronic.
Intervention: Wheelchair locomotion using static seating and dynamic seating. Outcome Measures: Novel Pliance pressure mapping system measuring peak pressure; pressure-time integral. |
1. Peak pressure was up to 42% higher within dynamic wheelchair locomotion when compared with static sitting.
2. Static and dynamic seating peak pressure comparison was significant (t=5.4, p<0.025). 3. No difference was found between static and dynamic seating pressure-time integral. |
Discussion
Tam et al. (2003) reported that sitting in a wheelchair has traditionally been considered to be static, however, wheelchair propulsion is recognized as dynamic. In this study, pressure mapping was used to determine the position of the IT during static and dynamic sitting (wheelchair propulsion). It was found that the IT were located 19.2±11.7 mm behind the peak pressure locations suggesting that rocking of the pelvis during wheelchair propulsion has a direct influence on the redistribution of loadings to the supporting tissues.
Kernozek and Lewin (1998) indicated that peak pressures during dynamic wheelchair propulsion were significantly higher than during static sitting by up to 42%. Pressure-time integral indicated that the cumulative effect of the loading was comparable between static and dynamic loading. Pressure-time integral between static dynamic trials was not significant. The author questions the impact dynamic movement has on skin health since peak pressures change throughout the locomotion cycle. The amount of IT travel during functional activities would also be an interesting factor to evaluate, as friction/shear may also have a significant impact on skin health for the wheelchair user.
Stinson et al. (2013) explored changes in interface pressure related to movement during normal computer use. 14 participants were asked to work at a computer for one hour, during which time changes in interface pressure and trunk position were noted as were frequency and duration of movements. Participants were then asked to reach forward (150% times their arm length) to type for five minutes and then return to normal upright sitting to type for 10 minutes, alternating these positions for a total period of 30 minutes. The same outcomes were measured. Results indicated that during regular computer use, frequency of movement varied greatly (range of 0-28 movements; an average of one movement every five minutes, with three participants not moving at all during the hour), with the majority of time spent in a normal upright position. Only 4.9% of the movements during Strand A produced a moderate reduction in interface pressure (51-75%), being ineffective for pressure redistribution. The questionnaire participants completed following the testing period, indicated that most felt they were completing effective pressure redistribution movements throughout the hour. The second part of this study which required participants to reach forward 150% times their arm length found a 52% decrease in interface pressure and a 24° change in trunk angle. Authors note that three of the 14 participants were unable to attain this position, with another three reporting that it was difficult or uncomfortable to attain this position. They also found a weak correlation between trunk angle and reduction in interface position, and suggested that trunk angle should not be used as a predictor of the interface pressure unloading. Despite the small sample size, this study supports the incorporation of dynamic position changes within regular daily activities but also demonstrates that the effectiveness of the movement needs to be assessed to ensure adequate pressure redistribution.
Conclusion
There is level 4 evidence (from one post-test: Kernozek & Lewin 1998) to support that dynamic peak pressures are greater than static but the cumulative loading is comparable between dynamic and static loading.
There is level 2 evidence (from one prospective controlled trial: Tam et al. 2003) to support that peak pressures are located slightly anterior to the ischial tuberosities.
There is level 4 evidence (from one pre-post study: Stinson et al. 2013) to support the use and incorporation of forward reaching into daily activities as a means to promote pressure redistribution, provided the reach distance is adequate for an effective weight shift.
