Biomechanical analysis of different dynamic sitting techniques: an exploratory study

Prolonged static sitting in wheelchairs increases the risk of pressure ulcers. This exploratory study proposed three dynamic sitting techniques in order to reduce the risk of developing pressure ulcer during wheelchair sitting, namely lumbar prominent dynamic sitting, femur upward dynamic sitting, and lumbar prominent with femur upward dynamic sitting. The purpose of this study was to analyze the biomechanical effects of these three techniques on interface pressure. 15 able-bodied people were recruited as subjects to compare the aforementioned sitting techniques in a random order. All parameters, including dynamic contact area, dynamic average pressure, and dynamic peak pressure on backrest and seat were measured and compared. In result, when compared with lumbar prominent dynamic sitting, femur upward dynamic sitting and lumbar prominent with femur upward dynamic sitting appeared to yield significantly lower dynamic average and peak pressure on the back part of seat, and significantly higher dynamic average and peak pressure on the front part of seat. This study can serve as a reference point for clinical physicians or wheelchair users to identify a suitable dynamic sitting technique.

prolonged static sitting loads [13][14][15]. Nonetheless, the optimal movement strategy of DST to reduce the risk of developing pressure ulcers is still being debated. Although lumbar supports and cushions are the most common static form of pressure-relieving assistive devices for wheelchair users, studies on their dynamic design and interface pressure measurement are lacking. Therefore, this exploratory study engaged in the dynamic design of above-mentioned assistive devices, and developed three innovative DSTs, namely lumbar prominent dynamic sitting (LPDS), femur upward dynamic sitting (FUDS), and lumbar prominent with femur upward dynamic sitting (LFDS), as illustrated in Fig. 1. The purpose of this study was to analyze the biomechanical effects of LPDS, FUDS, and LFDS on interface pressure.

Subjects
Fifteen able-bodied people were recruited to participate in this exploratory study (8 men, 7 women; age, 22.5 ± 1.8 years; weight, 65.2 ± 10.6 kg; height, 168.5 ± 8.9 cm; body mass index, 22.8 ± 2.6 kg/m 2 ). Subjects with preexisting musculoskeletal disorders and spinal pathologies were excluded. All subjects read and signed an informed consent form that explained the study objective and experimental protocol. This study was approved by the Institutional Review Board of National Cheng Kung University Hospital.

Experimental wheelchair
The researcher of this study designed an experimental wheelchair, which was installed with airbags providing adjustable support for lumbar and femur areas. The size of each airbag was 40 × 23 cm 2 . When filled with air, the airbags were 4 cm thick. A customized microprocessor was used to adjust the extent and cycle period of filling or deflating each airbag to periodically change the sitting postures. A 1-cm-thick foam pad was installed to the backrest and seat to minimize the discomfort caused by the surface discontinuity between the skin contact with the backrest and seat.

Experimental protocol
Before the experiment started, each subject was first asked to rest their upper body on the backrest and relax their arms on both sides. Moreover, they had to keep their thighs parallel to the ground, feet approximately shoulder-width apart and firm on the footrest, and eyes looking straight ahead [16,17]. Afterward, the sequence of three DSTs was randomly drawn by each subject (illustrated in Fig. 1): (1) LPDS: An airbag providing adjustable support for lumbar area was placed at L3 (on the subject). The airbag configuration was periodically switched between deflated (0-cm-thick) and filled (4-cmthick), alternating every 5 min, the total period of experiment lasted for 20 min. (2) FUDS: An airbag providing adjustable support for femur area was placed at the middle of the subject's thighs. The airbag configuration was periodically switched between deflated (0-cm-thick) and filled (4-cm-thick), alternating every 5 min, the total period of experiment lasted for 20 min. (3) LFDS: This technique is a combination of the previous 2 techniques simultaneously. Between each DST experiment, the subject took a 5-min break during which they could stand up and walk around.

Results
The interface pressure measurements on both the whole backrest and the entire seat are shown in Table 1     When FUDS was compared with LFDS, no significant differences in all interface pressure parameters of the back and front part of seat were observed.

Discussion
This study examined and quantified the biomechanical effects of three DSTs, namely LPDS, FUDS, and LFDS, by measuring the interface pressure. The results of the experiments revealed that FUDS and LFDS periodically changed the interface pressure on the buttocks that can be used to reduce the risk of developing pressure ulcers. LPDS yielded the significantly lowest performance in interface pressure on the buttocks. The weight of a wheelchair user is primarily supported by the backrest and seat. In addition, because the buttocks support most of the body weight, the stress of body weight is primarily focused on the ischial tuberosities and the surrounding soft tissues, increasing the likelihood of developing pressure ulcers there [8,9,18,19]. Previous studies have indicated that increasing the stress load between the backrest and the front part of seat can assist in reducing the stress on the ischial tuberosities [8,9]. The results of the interface pressure measurements illustrated that the three DSTs yielded significant differences in B-DAP values. LFDS yielded the significantly highest B-DAP values, LPDS the significantly medium, and FUDS the significantly lowest. Consequently, LFDS is more effective than LPDS and FUDS at shifting the sitting load from the seat to the backrest when dynamic alteration process. Moreover, compared with LPDS, FUDS and LFDS yielded significantly lower BS-DAP and BS-DPP values, and significantly higher FS-DAP and FS-DPP values. Regarding these parameters, there were no significant differences between FUDS and LFDS. Consequently, FUDS and LFDS are more effective than LPDS at shifting the sitting load from the back part of seat to the front part of seat when dynamic alteration process; periodically reduce the interface pressure on the buttocks.
In study limitations, the subjects in this study were able-bodied people rather than people with lower-limb disorders, because we concerned about the imposed physiological loads and danger on people with lower-limb disorders for this experiment that required extended processes and multiple sessions of repositioning. In addition, this study focused on reducing the risks of pressure ulcer from a preventive concept and expected participants who had healthy and functional torsos to reach the experiment target. Therefore, we decided to recruit able-bodied people who were not wheelchair users and eliminated those diagnosed with musculoskeletal disorders and spinal pathologies. If applying the study results to wheelchair users, their different pathological characteristics should be considered to ensure feasibility.
In conclusion, the results of this study suggested that FUDS and LFDS can be used to periodically change the distribution of interface pressure on the buttocks. This study can serve as a reference point for clinical physicians or wheelchair users to identify a suitable DST. Thus, further studies should focus on identifying the most adequate adjustment degree and cycle period of the dynamic alteration process.