Changing body positions frequently throughout the day to address discomfort, sitting pressures, fatigue and to adjust posture occur naturally and frequently. However, for people with a spinal cord injury, these position changes can be challenged by changes in their ability to physically move their own body, or to sense pressure. It is further complicated by the need for increased frequency of position changes to address issues associated with prolonged sitting. The primary concern for people with spinal cord injury is the risk of pressure ulcer development resulting from increased pressure on the sitting surface and, decreased blood flow and tissue perfusion associated with prolonged sitting. Teaching individuals with spinal cord injuries to shift their weight regularly while seated is a common and intuitive recommendation for pressure injury prevention as it is hypothesized that this redistributes pressure on at risk tissues and allows for recovery of blood flow and oxygenation to these affected tissues (Bogie et al 1995; Consortium for Spinal Cord Medicine 2000; Coggrave & Rose 2003; Makhsous et al. 2007a).
Dynamic positioning devices such as tilt, recline and standing, have been identified as effective tools for assisting people to manage sitting pressures. However, the amount of position change required to offset the negative effects of sitting pressure is unclear. In the recent few years many studies have been conducted to determine the optimal position change using interface pressure, blood flow and tissue perfusion. Of equal concern in the determination of the amount of position change required to affect sitting pressure is that of the required duration of the position change. This is important as the amount of position change movement is often large, which canhave functional or esthetic implications for the person using these types of devices in their dialy life.
The studies outlined in Table 23 have examined a variety of permutations of positions changes with outcome measures of interface pressure, blood flow and tissue perfusion to examine the effects of intentional position changes including lateral leaning, forward leaning and vertical push up and the use of positioning devices within the wheelchair frame including tilt, recline, tilt/recline combination and standing devices. The findings from each study are synthesized into the following sections where relevant; position changes of leaning and push-up, effects of wheelchair frame set-up, position change using recline only, position change using tilt only, and positions change using combinations of tilt, recline and standing.
Position Changes: Leaning and Push-up
Summarized Level 5 evidence studies:
Yang et al. (2009) completed an observational study with the intent of describing the sitting behaviours of 20 people (18 men, two women) with spinal cord injury who use a manual wheelchair as their primary means of mobility and live in the community. Data was collected using a data logger and six force sensor resistors on the seat of the participants’ own wheelchairs to track sitting contact on the wheelchair seat over a one-week period of time. The results indicated that on average these participants lifted off the seat surface once every one-two hours, sat for 9.2 hours a day, and sat for long periods of time without shifting weight (range of 97 minutes to 3.7 hours). The duration of the lift-off was reported so the benefit in relation to pressure management is not clear.
Sonenblum et al. (2016) also found that of the 28 people monitored in their everyday lives, that the weight shift frequency to meet the clinical guideline recommended criteria for pressure management, none of them met the criteria of every 15-60 minutes. Participants sat on average for 140 minutes +/- 84 minutes without shifting their weight at all. They also reported great variability in weight shifts across participants and for the same participant across days.
Similar results were found by Sonenblum et al. (2018c) in their study of 29 adults who were 2 years post SCI. Participants were grouped based on pressure injury history. Findings indicated that weight shift movements that had potential to affect pressure management were performed less than once every 3 hours for both groups, with the no pressure injury history group completing slightly more pressure management weight shifts that the group with pressure injury history. Weight shifts that did not fully offload and in-seat movements occurred more frequently (1-2.5 and 39.6-46.5 times per hour respectively). Variability was noted as considerable for all movements across participants as well as for the same participant across days.
The effectiveness of an intentional change in position by leaning or pushing up to lift the body from the seat surface is often determined by the ability to hold the position for an optimal duration of time. The following studies have examined weight shift behaviour, pressure changes associated with leaning and push-ups as well as the blood flow changes to determine the optimal duration of a position change.
In 1992 Hobson evaluated pressure changes during lateral trunk leaning to 15°, forward flexion to 50°, backrest recline to 120° and full body tilt to 20° (results from the tilt and recline positions are reported later). A 32% to 38% decrease in average pressure on the opposite side was found to occur during lateral trunk leaning. Moving into these alternate positions influenced the location of the maximum pressure, which was identified in the study as the ischial tuberosity location. An average 2.4 cm to 2.7 cm posterior shift occurred with forward trunk flexion. Maximum reductions in tangentially induced shear forces were also noted as occurring with forward trunk flexion of 50º. Hobson also noted that for the SCI population a 10% increase in pressure was observed up to 30° of forward flexion before the reduction began to occur.
Henderson et al. (1994) compared average pressures under the ITs and in a 71 mm x 71 mm area centered around the ITs in four different postures; upright resting posture, 35° and 65° tilt and 45° forward lean (participants were assisted into this position). The results of the tilt and recline positions are reported later. Forward leaning demonstrated a statistically significant (p<0.05) reduction of maximum point pressure to below 60 mmHg for eight out of 10 subjects and for seven out of 10 subjects below 32mmHg.
In a retrospective chart review of 46 SCI subjects seen in a seating clinic, Coggrave and Rose (2003) assessed the duration of various pressure relief positions required for loaded transcutaneous oxygen tension (tCPO2) to recover to unloaded levels. Results indicated that it took approximately two minutes of an intentional position change to raise tissue oxygen to unloaded levels for most subjects. This length of pressure relief was more easily sustained by the subjects leaning forward, side to side or having the wheelchair tipped back at >65º compared to a push-up lift.
Similar to Coggrave and Rose (2003), Makhsous et al. (2007a) demonstrated full recovery of tcPO2 with the dynamic protocol in the off-loading configuration but it took >two minutes to achieve this result. Those individuals with paraplegia using a wheelchair push-up were only able to sustain the lift for 49 seconds leading to incomplete recovery of tissue perfusion.
Lin et al. (2014) examined weight relief raises (WR) and shoulder external rotation protocol activities (ER) in relation to the subacromial space of the shoulder from an unloaded neutral position and the space before and after one minute of each of the above tested tasks. The repetition of 30 WR was suggested to be similar to that performed each day if the recommendations for weight shifting every 15 minutes were followed (of note, the study did not examine duration of the WR). While they did no find a difference in subacromial space pre-post, there was a significant narrowing during the WR. Additionally, they found that participants with increased years of SCI had a greater percentage of narrowing.
The results from the study by Smit et al. 2013, indicate that bending forward, leaning sideways and push-ups reduced interface pressure at the ITs and increased oxygenation at the sub-cutaneous level and increased blood flow. (The study also examined the effects of electrical stimulation on oxygenation which is addressed in an earlier chapter.) The authors propose that the results of this study further support that push-ups should no longer be recommended due to the impact on shoulder integrity, due to the equal benefits of bending forward and leaning to the side for decreasing IT pressure and increasing blood flow and oxygenation.
The results from the study by Sonenblum et al. (2014) also indicated that of five body position changes examined (small, intermediate and large forward lean and intermediate and large sideward lean) only the small forward lean did not have a significant effect on increasing blood flow and decreasing interface pressure at the IT. The effects were the same on all three cushion types tested (foam, gel and air), however they did find a difference in interface pressure on each cushion in upright sitting, with the foam cushion being significantly higher than the gel and air, but no significant difference between the gel and air cushions. The authors suggest that these findings indicate that body changes, (except small forward lean) are effective on any cushion type.
Wu and Bogie (2014) also found that changing body position such as leaning to the side, resulted in improvements in blood flow and tissue oxygenation and, reduction in interface pressure at the IT, however the benefits were not sustained, thus requiring regular and frequent repetition of the movements.
These studies suggest that changing body position by leaning to the side, forward or using a push-up result in decreased interface pressure to the un-weighted sitting area and increased blood flow to that of unloaded levels. Greater effect is seen if the position is sustained for greater than two minutes, which was not achieved by participants when using the push-up technique. Additionally, Lin et al. (2014) suggest that decreases in the subacromial space occur during the push up support limiting use of the vertical full body push up as a strategy for pressure management.
There is level 2 evidence (from one prospective control trial, one case control study, two pre-post study and three case series studies; Hobson 1992; Makhsous et al. 2007a; Sonenblum et al. 2014; Wu and Bogie, 2014; Smit et al. 2013; Coggrave & Rose 2003; Hendersen et al. 1994) to support position changes to temporarily redistribute interface pressure at the ischial tuberosities (IT) and sacrum by leaning forward greater than 45° or to the side greater than 15°.
There is level 4 evidence (from one case series study by Coggrave & Rose 2003; and two Pre-Post test studies by Smit et al. 2013; Hendersen et al. 1994) to support that a minimum two minute duration of forward leaning, side leaning or push-up must be sustained to raise tissue oxygen to unloaded levels.
There is level 3 evidence (from one case control study, two pre-post studies and one case series study; Makhsous et al. 2007a; Lin et al. 2014; Smit et al. 2013; Coggrave & Rose 2003) to support limiting the use of push-ups as a means for unweighting the sitting surface for pressure management.
Leaning forward at least 45° (elbows on knees position) or lateral trunk leaning to 15° reduces pressure and increases blood flow and tissue oxygenation at the sitting surface; it is important to be able to return to the original upright sitting position.
For most individuals with SCI, the use of a push-up/vertical lift is unlikely to be of sufficient duration to be beneficial for managing sitting pressure and has potential to contribute to repetitive strain injuries and a reduction of subacromial space.
Effects of Wheelchair Frame Set-up
Only two studies examined how the set-up of the wheelchair frame influences sitting pressures. The first study by Maurer and Sprigle (2004) pressure mapped a common wheelchair frame configuration often used by the SCI population in which the front seat to floor height is higher than the rear seat to floor height while keeping the same back angle (“squeeze”). In this study, the difference between the front and rear seat to floor heights was measured in degrees or in inches; that measurement was used to identify how much squeeze there was in a wheelchair frame. The study found that there were no changes in peak pressures at the IT. The study also found that there was less pressure concentrated under the ITs, as the rear seat to floor height decreased but the total force on the seat increased. As part of the study protocol the participant was seated with their sacrum up against the back support for all measures, but back support interface pressures were not measured.
Makhsous et al. (2007b) compared interface pressures on the seat and back between normal upright sitting and normal upright sitting alternated with partial ischial support removed. The results indicate a shift in interface pressure towards the middle and anterior seat when the posterior support is partially removed reducing the ischial pressure by as much as 40% for subjects with tetraplegia. Simultaneously, an increase in back support pressure was noted as the peak pressures and average pressures increased at the back support, suggesting a shift of interface pressure to the back support as well as to the anterior and middle aspects of the cushion.
These two studies suggest that the back support plays an important role in supporting the pelvis such that the area of pressure distribution can include the back.
There is level 4 evidence (one pre-post study Makhsous et al. 2007, one pre-post test study Maurer & Sprigle, 2004) to suggest the back support plays an important role in supporting the pelvis thereby increasing the area for pressure redistribution through the inclusion of the back surface.
There is level 4 evidence (one pre-post study and one pre-post test study; Makhsous et al. 2007; Maurer & Sprigle 2004) that sitting surface interface pressure decreases at the posterior aspect of the buttock as it is un-weighted however there is an increase in total force on the seat.
The back support plays an important role in pressure management on the sitting surface.
Position Change: Recline only
In addition to examining maximum sitting pressure in relation to forward and lateral flexion, Hobson (1992) also examined changes in maximum sitting pressures in back support recline alone to 120º and full body tilt to 20º. The results for recline are reported here and results for tilt are reported in the associated section later in this chapter. When the back support was reclined to 120°, a 12% decrease in average maximum pressure occurred. However, this position influenced the location of the maximum pressure, which was identified in the study as the location of IT. The largest shift was 6 cm with back support recline to 120° with an increase in tangentially induced shear forces by 25% as compared to an average 2.4 cm to 2.7 cm posterior shift occurred with forward trunk flexion and a decrease in TIS.
There is level 4 evidence (one post-test, Hobson 1992) to suggest that back support recline to 120° decreases average maximum pressure in the ischial tuberosity area but also causes the greatest ischial tuberosity shift (up to 6 cm) and a 25% increase in tangentially induced shear forces.
Backrest recline alone to 120° decreased average maximum pressures in the ischial tuberosity area but also causes the greatest ischial tuberosity shift (up to 6 cm). Further research on the effect of friction/shear on the sitting surface in relation to the ischial tuberosity shift is required to determine if there is benefit in using backrest recline alone.
Position Changes: Tilt only
Early studies in the examination of the effectiveness of position changes in managing sitting pressures tended to primarily use interface pressure mapping as the outcome measure. As noted in the previous sections, as part of the larger study Hobson (1992) also examined changes in maximum sitting pressures in full body tilt to 20º. The study reported an 11% decrease in maximum sitting pressures with maximum reductions in tangentially induced shear forces.
As noted above, Henderson et al. (1994) pressure mapped 10 SCI subjects and recorded pressures at the ischial tuberosity and the weight bearing surface area around the IT in four different postures; upright resting posture, 35° and 65° tilted position and 45° forward lean (the latter was discussed in the previous section). The results indicated that no significant changes in pressure occurred with the 35° tilt, but for wheelchairs tipped back 65º statistically significant pressure reduction at the IT and weight-bearing surface area (p<0.05) was demonstrated. It is worth noting here that the forward lean showed the greatest reduction (78% reduction at IT, 70% reduction on the weight-bearing surface area). Even with these significant changes in pressure, the pressure levels for only one subject reached 32 mmHg and only 3/10 subject’s maximum point pressures were below 60mmHg.
Spijkerman et al. (1995) assessed interface pressure while individuals were tilted at 5°, 15° and 25° from horizontal. Results indicated that body tilt had a significant effect on mean pressure (p=0.003) with the lowest overall mean pressure (82.91 mmHg) being demonstrated at 25° tilt.
Geisbrecht et al. (2011) examined tilt using a manual tilt-in-space wheelchair. He found that compared to the upright position with back recline of 100° (baseline) there was a significant reduction in peak pressure index for the sacrum at 30° of tilt and greater. Geisbrecht et al. (2011) also compared participants with paraplegia to those with tetraplegia, with the only significant difference being that sacral pressures for participants with tetraplegia were significantly higher. For both groups, the peak pressure index at the ITs was significantly reduced at 30° of tilt and greater. Generally, a significant change in IT pressure was found starting at 30° of tilt with increasing amounts of tilt, resulting in greater the reduction in pressure at the sitting surface. The findings from this study are consistent with the changes in pressure findings in the study by Sonenblum and Sprigle (2011c).
Using participant’s own wheelchair, Sonenblum and Sprigle (2011c) examined changes in interface pressure and blood flow on IT during varying degrees of tilt. Each tilt position was measured from an upright position (range of 0°-5° tilt). Small tilts of 15° did result in significant blood flow changes (8% increase) while interface pressures changed but did not reach a level of significance. Blood flow increased with each test situation of upright to 15°, upright to 30° and upright to 45°. Tilting from 15° to 30°, did not result in an increase of blood flow, however interface pressure decreased. While blood flow increased at all degrees of tilt from an upright position, the amounts were variable across participants. Maximum blood flow increase was noted to be 10% which was achieved at 30° for four of the 11 participants, whereas others achieved a 10% blood flow increase at tilt greater than 45°. The authors noted a weak correlation between the increase in blood flow and pressure changes in tilt less than 30°, suggesting that there may be other mechanisms affecting blood flow other than pressure from the sitting load. An important factor noted by the authors is the need to consider the influence of the cushions used by the participants. Cushion type may influence blood flow and pressure loading of the buttocks on the seat surface. In this study, the participants used their own air floatation or gel cushions.
There is level 2 evidence (one randomized control test study by Sonenblum & Sprigle 2011c, one pre-post test study by Giesbrecht et al. 2011, one post-test Hobson 1992, two pre-post test studies Henderson 1994 and Spijkerman 1995 ) suggesting that there is an inverse relationship between tilt angle and pressure at the sitting surface and that significant reductions in interface pressure begins around 30° of tilt with maximum tilt providing maximum reduction of interface pressures. The amount of reduction realized was variable by person.
There is an inverse relationship between tilt angle and pressure at the sitting surface. Significant pressure redistribution realized was variable by person but on average started around 30° of tilt with maximum tilt providing maximum pressure redistribution.
Position Change: Combinations of Tilt, Recline and Stand
Summarized Level 5 Evidence studies:
Yang et al. (2014) completed an observational study (n=24) with SCI individuals to investigate the shear displacement between the body and backrest/seat, ROM and force acting on the lower limb joints during sit-stand-sit transitions by operating an electric-powered standing wheelchair. Each study subject completed three cycles of sit-to-stand, stand-to-sit with a one-minute break between cycles. Assessments conducted during the testing cycles included measuring the anterior and vertical forces acting on the knee restraint, degrees of sliding on the backrest and seat, and ROM of the hip, knee, and ankle.
The study revealed that the forces acting on the knee restraint were significantly higher during the sit-to-stand transition compared to the stand-to-sit transition (p=0.01). The maximal and average anterior forces on the knee restraint was significantly greater during the sit-to-stand transition (p<0.01) but downward forces were significantly greater when returning to the sit position from standing (p=0.01). The range of sliding and displacement along the backrest was significantly larger during sit-to-stand transition (p<0.01) compared to stand-to-sit. During the stand-to-sit transition, the range of sliding and displacement along the seat was significantly larger (p=0.01) than the sit-to-stand transition. There were no significant differences reported between sit-to-stand and stand-to-sit in respect to hip ROM (p=0.59), knee ROM (p=0.71) and ankle ROM (p=0.78).
Mattie et al. (2017) observed how a group of 8 participants used the “on the fly” adjustable seat elevation and back support angle on an ultralight manual wheelchair. The findings identified that the back-support angle was infrequently adjusted, and the seat height use varied a great deal across participants as well as per participant across days. The study authros suggest there is a need for adjustable features on manual ultralight wheelchairs for function.
Sprigle et al. (2010) examined tilt, recline and standing using power positioning devices.
Sprigle found a 46% reduction of seat pressure in 55° tilt, and a 61% reduction in pressure in full recline (180°) as well as in 75° of standing. The authors acknowledged that recline and standing offers a larger range of movement which likely contributes to the increased pressure reduction. The authors also noted there are contraindications in use of recline and standing that need to be considered before provision as a method to manage sitting pressure.
Similar to Sonenblum and Sprigle (2011c) (in tilt only section), Jan et al. (2010) also examined blood flow at the IT, however, did so during specific combinations of tilt and recline. Tilt at 15°, 25° and 35° were each combined with 100° and 120° of recline and compared to an upright position (0° tilt, 90° recline) to determine changes in blood flow. For 100° of recline, significant changes were found only in combination with 35° of tilt, which is not consistent with the study by Sonenblum and Sprigle (2011a) who found significant changes at 15° of tilt from upright. The combinations of 120° recline with 15°, 25° and 30° tilt produced significant changes in blood flow. The authors noted a significant increase in blood flow between the combinations of 120° recline with 15° tilt and 120° recline with 35° tilt; however, these comparisons are both from an upright position not moving from 15° to 35° of tilt so this finding needs to be applied carefully in daily life tilting situations. This is the same for the findings in which changing recline from 100° to 120° at both 25° and 35° tilt produced a significant increase in blood flow. The authors noted that results should only be generalized to tilt/recline in combination with foam cushions. This difference in cushion type may explain in part some of the differing results for blood flow between this study and the Sonenblum and Sprigle (2011c) study which used the participants’ own air inflation and gel cushions.
The study by Jan and Crane (2013) found that sacral skin perfusion did not change significantly in any of the six variations of tilt/recline combinations as described in the above study by Jan et al (2010). The authors suggested that the expected increase in pressure over the sacrum was instead redistributed across the lumbar and thoracic area. However, it is worth noting that due to the small number of participants, care must be taken in generalizing the results of this study. It is also worth noting that the posture of the pelvis during testing was not described; the potential impact on pressure management by the effect the back support has the position of the pelvis has been noted earlier in the studies by Makhsous et al. (2007b) and Maurer and Sprigle (2004). Further research is required to make any recommendations.
The study by Jan et al. (2013a) compared muscle perfusion and skin perfusion during six different test positions of tilt and recline combinations. Larger amplitudes of tilt-recline combinations enhance skin perfusion over the ITs, but less perfusion is seen in the muscles during the same tilt-recline combinations. The authors indicate that this may suggest that muscle may be at greater risk for ischemia than skin if regular, adequate pressure redistribution is not achieved. Significant perfusion changes for skin or muscle were found for 15°, 25° and 35° tilt with120° recline and 35° tilt with 100° recline but no other combinations. It is worth noting that the risk of shear and friction often associated with recline use was not addressed in this study. It is also worth noting that testing was done on foam cushion with a standard power wheelchair not participants’ own wheelchair and seating. As noted by Sonenblum and Sprigle (2011c) above differences in blood flow may be attributable to cushion type.
The study by Jan et al. (2013b) examined duration of position change. Results suggest that the duration of time spent in 35° tilt and 120° recline as part of a pressure management routine influences skin perfusion, with 3 minutes producing significantly higher skin perfusion than lesser times of one- or zero-minutes in. The study results also found the skin perfusion to be significantly higher during the second ischemic sitting period, but the study did not compare these results to the first ischemic period. This may be helpful to assist in determining the optimal time between pressure redistribution movements. It is worth noting that seven of the nine participants were an AIS C level of SCI injury so consideration needs to be given to the varying autonomic levels of function and the effect this may have on cardiac function and skin blood flow.
Lung et al. (2014) was part of the above studies but examined the effect of the various position configurations in relation to displacement of the peak pressure index (PPI), the displacement of the centre of pressure and the interface pressure mapping (IPM) sensel size used to capture this data. The authors related displacement to pelvic sliding, finding that PPI displacement ranged from 3.3cm to 6.6 cm, during the various position configurations. Based on these findings, the authors suggest the sensel window size needs to either be large enough (preferably 7×7) to capture displacement or it should be shifted to account for the displacement. They also did not find significant differences in PPI displacement between the positon configurations, suggesting that a particular angle does not necessarily produce a certain amount of PPI displacement. However, centre of pressure displacements was significantly different between the various position configurations to which the authors suggest may indicate differences in biomechanical changes for understanding individual differences in skin perfusion responses in different configurations of tilt and recline.
Inskip et al. (2017) explored the effects of seat elevation (similar to moving towards standing), seat lowering (similar to seat dump) and standard seat position as a means to determine impact on orthostatic hypotension. The findings suggest that those people who sustained an autonomically-complete SCI experience cardiovascular changes with positional changes, particularly moving into the elevated position. The authors suggest that there should be concern for cumulative burden of hypotension for this particular group especially for long periods of time. Conversely, findings suggest that moving into the lowered position from the elevated positin improved cardiovascular outcomes.
There is level 2 evidence (from three RCT Jan et al. 2010; Jan et al. 2013a; Jan & Crane 2013) to suggest that larger amounts of tilt alone or 15° tilt and greater in combination with 100° or 120° recline result in increased blood flow and decreased interface pressure at the ischial tuberosities (IT). There is inconsistency in the minimum amount of tilt needed to significantly increase both blood flow and interface pressure reduction. There is also limited evidence related to impact of shear forces with use of recline.
There is level 2 evidence (from two RCT studies Jan et al. 2013b; Sonenblum & Sprigle 2011c) to suggest that it cannot be assumed that changes in interface pressure through use of recline and/or tilt equates to an increase in blood flow at the IT or the sacrum.
There is level 2 evidence (from two RCT studies Jan et al. 2013b; Sonenblum & Sprigle 2011c) to suggest that muscle perfusion requires greater amplitudes of body position changes than that required for skin perfusion.
There is level 4 evidence (from one pre-post study; Lung et al. 2014) to suggest that peak pressure index, which is a common metric used in interface pressure mapping, displaces up to almost 7 cm during tilt and/or recline, therefore consideration for the size of the sensel window used to capture this data should either be large enough (7×7) or the location adjusted to ensure the data is fully captured.
There is level 2 evidence (one prospective controlled trial study by Inskip et al. 2017) that for people who sustained an autonomically complete SCI, that movement into a standing position for periods of time can make them vulnerable to severe orthostatic decreases in blood pressure.
It cannot be assumed that a change in interface pressure through use of tilt/recline equates to an increase in blood flow at the ischial tuberosities (IT).
The variability in blood flow and interface pressure changes associated with tilt/recline, supports the need for an individualized approach to education around power positioning device use for pressure management.
The type and duration of position changes for pressure management must be individualized
More research is needed to determine the parameters of position changes in relation to interface pressure and blood flow at the sitting surface tissues to help prevent pressure ulcers post SCI.
While power positioning technology including combinations of tilt, recline and stand, offer many health-related benefits, individualized assessment and thorough consideration of contraindications are required to ensure safe and appropriate use.
To mobilize knowledge related to pressure, and muscle/skin perfusion into clinical practice further research is needed to determine: 1) the influence of cushion type on muscle and skin perfusion; 2) the effects of friction and shear on skin and muscle perfusion and pressure during use of recline and/or tilt and/or standing; 3) the influence of postural deformities/tendencies on perfusion levels on both of the above and; 4) the effects of duration of large amplitudes of position changes within participants’ regular daily routines of position changes.