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Changes in Pressure during Static Sitting versus Dynamic Movement While Sitting

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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.

Table 25: Changes in Static and Dynamic Movement

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. 

  • Peak interface pressure is greater for dynamic movement in SCI subjects than static sitting but cumulative loading is comparable between dynamic and static loading for the SCI population.

    Peak pressures appear to be located slightly anterior to the ischial tuberosities.

  • The use and integration of forward reaching into daily life activities can be used as a means to promote regular pressure redistribution. Caution however is needed to ensure the movement is of adequate distance and duration to affect pressure management.