Impact damping test for wheelchair cushions: effect of calculation methods and new test metric
Sandra Arias-Guzmán, David Brienza, Patricia Karg, Alexandra Delazio, Keerthana Kalliat, Jenna Freedman
Department of Rehabilitation Sciences and Technology, School of Health and Rehabilitation Sciences, University of Pittsburgh (Pittsburgh, PA)
Wheelchair cushions play an important role in the activities of daily life of wheelchair users. A wheelchair cushion must be capable of distributing loads to reduce the concentration of pressure in tissues, providing adequate supporting properties to optimize seated posture, improving comfort and absorbing shock during propulsion . Shock loads may result from rolling over uneven surfaces or dropping off curbs. Transferring these forces to the tissues is associated with increased risk of developing pressure injury and discomfort.
The International Organization for Standardization (ISO) standard for Wheelchair seating Part 2 (ISO16840-2) and the harmonized RESNA WC-3, Section 2 have published laboratory test methods to characterize the performance of a cushion related to tissue integrity [2, 3]. The document includes the impact damping under normal loading conditions test (IDT) that characterizes the ability of a wheelchair cushion to absorb impact loading on tissues and to help maintain postural stability by means of accelerations profiles.
After the document was first published, researchers investigated the results of this test for cushions of different materials . Based on these results, significant changes have been made to the test method and metrics reported in the standard. These modifications include: a) placing the accelerometer in a fixed position for a more consistent result, given that the acceleration depends on the axis of rotation , b) calculating impact accelerations instead of rebounds since the impacts represent a higher energy to transfer to the tissues , and c) removing the number of rebounds as it was redundant with the impact ratio . A recent literature review explored comparisons of the IDT parameters from ISO 16840-2:2007 and the proposed changes . Although the 2018 version of the standard has made modifications to test parameters and calculations, the rationale is the same .
In addition to the modifications in the standard, we found that the procedure for calculating the parameters reported by the standard is not clear, and the published studies do not specify how they calculated the impacts. We hypothesized that the way of calculating the parameters has an influence on the results. The purpose of this study was to clarify the methodology described by the ISO standard, to evaluate the repeatability of the IDT protocol and to investigate whether various interpretations of the calculation method affect the results. In addition, we explored a new metric, damping time, and any relationships with the current metrics.
The IDT test rig was built according to ISO 16840-2:2018. It requires an impact damping rigid cushion loading indenter (IDRCLI) with a uniformly distributed weight of 500 N ± 10 N, which represents the inferior surface of buttocks and thighs when a person is seated. A rigid plate was used to support the cushion and the IDRCLI. A supporting block was fabricated to raise the rigid plate to 10° for the test. An accelerometer was mounted in the mid-line on the top of the flat surface of the IDRCLI. For this study, the 3-axis GP1 Accelerometer (Sensr, USA) at 10g was used. The GP1 has an analog 2 pole Butterworth low pass filter at 45Hz and samples at 100 samples/s.
Impact damping test method
Impact damping measurements following the ISO 16840-2:2018 protocol  were made on 21 commercially available wheelchair cushions of different materials such as foam, air, gel, honeycomb and a combination of materials. The IDT involved preconditioning of the cushions maintained at ambient temperature (23°C ± 2°C and 50% ± 5% relative humidity) and applying a load of 830 N ± 10 N with the IDRCLI for 120-180 seconds prior to all testing. The supporting block was positioned under the rigid plate to achieve an angle of 10° ± 1° between the horizontal testing surface and the rigid plate. The IDRCLI was aligned with the hinged edge of the plate using a laser to ensure the correct position. In addition, we included an alignment board to prevent sliding of the cushion and IDRCLI prior to testing (Figure 1). Finally, the support block was removed allowing the cushion and IDRCLI to drop. After the test, the cushion was unloaded for a minimum of 5 minutes. Three repetitions for each cushion were performed and testing was done randomly by 3 operators. The acceleration was recorded prior to dropping the plate for 180 s ± 10 s and stopped when it had diminished to 1 % of the maximum.
A MATLAB program was created to take the data recorded from the accelerometer and calculate the parameters specified in the standard (see Figure 2), such as the magnitude of the first impact, the magnitude of the second impact, mean values of Impact 1 and Impact 2 of the three repetitions and the ratio of the second impact to the first impact as a percentage. In addition, we proposed to calculate a new metric that measured the damping time. Damping time was defined as the amount of time for the acceleration to be diminished to 1 % of the maximum.
The results were analyzed in three aspects: 1) repeatability of the three trials, 2) the influence of the baseline in the measuring of the impacts, 3) the relation between the parameter proposed with the parameters currently reported.
Intraclass Correlation Coefficients (ICC) were calculated: single ICC (S) and average ICC (A) for each parameter. The single ICC verifies the reliability of the test with only one trial. The average ICC verifies test reliability when the average of the three trials is reported as the final result.
To evaluate the influence of the baseline considered for the calculations, we computed each parameter using the following baseline adjustments: a) initial baseline- this is the initial static position with the rigid plate at 10°, b) final baseline- this is the baseline seen after dropping the rigid plate, and c) no baseline adjustment- this is the value taken when the accelerometer is in a flat position aligned one with gravity). We performed paired t-tests to assess if the choice of baseline produced different results for each test parameter.
To evaluate the relationship of the damping time (proposed parameter) and the three impact damping measurements, Pearson’s correlation coefficients were computed for each test parameter.
Reliability of the protocol
The results of the evaluation of repeatability are presented in Tables 1.a. and 1.b. The ICC’s are very high and almost equal to 1, which indicates that no trend of systematic differences was found between replications of the test when considering the different baseline adjustments for the measurements. The ICCs from impact 2 were a little higher than those for impact 1. The results of the ICCs of the damping time were also high too. This metric is independent of the baseline.
Influence of the baseline on the measurements
The results for each paired t-test comparing measurements using different baselines are presented in Table by examining the p-values of t-tests. The values showed significant differences when comparing the initial baseline either with the final baseline or no baseline adjustment, but no significant differences when comparing the final baseline and no baseline adjustment.
Relation of the damping time with the current parameters
The Pearson’s correlation coefficients for the comparison between damping time and the current parameters are presented in Table 3. The results showed that the damping time is not related to the impact 1, but there is a moderate relation with the impact 2 and a good relationship with the ratio, whether considering any of the baseline adjustments.
Comparison of results with reported studies
Table 4 shows a comparison of the overall results of the IDT from this study with other reported studies. The values of impact 1 were very similar to other reported studies when considering any baseline adjustment, but the ratio and the impact 2 were higher. When comparing these to the typical results ranges observed in Annex B of the ISO standard, our results were higher, and some cushions were out of the range. The results showed a difference depending on the baseline adjustment considered for the calculations. The initial baseline results produced more cushions within the ISO ranges.
One of the changes to the IDT standard since last evaluated in the literature is the fixed placement of the accelerometer relative to the axis of rotation. We evaluated the repeatability of the protocol in this study by means of ICCs. The high ICC’s attests to the repeatability of the protocol and indicates that the results are consistent within the 3 trials.
The standard stipulates to normalize the values to gravity, ideally, the accelerometer gives a value of 1.0 g in an axis aligned with gravity. The standard does not specify the subtraction of this value to obtain the impact magnitudes, but it does imply it in the graph presented as a typical response. One of the modifications that have not been clearly described is the calculation of the impacts. The ISO 16840-2:2018 implies that the impacts should be calculated with respect to a baseline adjustment when the signal is stabilized after dropping , which we have referred to as ‘Final baseline’ in this study. The 2018 revision to the RESNA standard  specifies that the baseline adjustment must be taken with the accelerometer placed on the indenter with its vertical axis aligned with gravity with the indenter placed level on the cushion on a flat surface; which is a close to what we called ‘No baseline’ in this study. However, we found in the literature that some groups also considered the baseline before the dropping, called ´Initial baseline´ in this study. We computed the metrics taking into account different baseline adjustments and compared the results to evaluate their influence. Our results showed significant differences when comparing the test metrics of the ‘Initial baseline’ adjustment with the ‘Final baseline’ or with ‘No baseline’ adjustment, but we did not find significant differences when comparing the ‘Final baseline’ and ‘No baseline’ adjustment, Table 2.
The results of this study showed a similar impact 1 but higher impact 2 and ratios than other reported studies. The number of cushions in this study is higher than in other groups. Previous studies with larger number of cushions used the rebounds and the ratio of the rebounds as metrics, as indicated in the old version of the standard, therefore we did not compare the results with those studies . Our results indicated that the baseline adjustment used for the calculations of the impacts the results not only affects the values of the ratios but also in whether these values fell within the typical ranges specified in the standard. Our results showed that only 11 to 14 (depending on the baseline adjustment) out of the 21 cushions tested were in the ISO ranges (Table 4). Therefore, we recommend that the standard should specify clearly how the impacts are calculated, and the typical range of the impact ratios specified must be adjusted to include this larger set of cushion data.
We proposed to measure the damping time as a metric. We found that the damping time is correlated with the ratio and the second impact. This could be related to the number of rebounds reported previously in the old version of the standard, but further studies must be done to confirm that. Groups that aimed to classify cushions by performance have found that the second impact or the second rebound is good classification parameters. The significance of the impact 2 in the rationale of the IDT must be further investigated.
The interpretation of the standard could vary depending on whether manufacturers, clinicians or researchers apply the ISO standards. Improvements in the documents have been made over the years and the importance of a clear understanding of the application of the standards will achieve better results.
In this study, we applied the IDT for assessing the performance of 21 cushions of different materials and constructs. Our results showed that the way that the parameters are calculated affected the results in both the measurements and whether a cushion fell within the ranges specified in the standard as typical. The results of this study contribute to the understanding of the protocol and the metrics calculated. The standards are voluntary, and the test is intended to characterize the performance of cushions, though the clinical efficacy is implied, thus far there are no studies that confirm the clinical relevance of the mechanical properties of the cushions.
The contents were developed under a grant from the National Institute on Disability, Independent Living, and Rehabilitation Research (NIDILRR grant number 90REGE0001). NIDILRR is a Center within the Administration for Community Living (ACL), Department of Health and Human Services (HHS). The contents do not necessarily represent the policy of NIDILRR, ACL, or HHS, and you should not assume endorsement by the Federal Government.
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