Is what you see what you get? Standard inclinometry of set upper arm elevation angles
Graphical abstract
Introduction
It is generally accepted that estimates of working postures become increasingly more correct as one moves from subjective assessment to observational methods to direct measurement tools (van der Beek and Frings-Dresen, 1998, Winkel and Mathiassen, 1994). Thus, a measurement hierarchy has emerged in which data from higher echelon measurement tools are inherently considered to be ‘better’, that is, more precise (repeatable) and less biased (closer to the actual value) than data from measurement tools at lower levels. From a precision standpoint, this hierarchy is well supported: tools at lower levels are often associated with a larger methodological variability than those at higher levels. For instance, considerable variability has been reported within and between observers rating postures from the same video recordings (Mathiassen et al., 2013, Rezagholi et al., 2012), whereas direct measurement tools, for example, inclinometers, have excellent reported repeatability (Hansson et al., 2001, Hansson et al., 2006). However, the ability of a device to produce a faithful measurement depends on the conditions and contexts in which the measurements are made, and not simply on the technical attributes of the measurement device itself (Portney and Watkins, 2009). Thus, if a device interacts with the system it is intended to measure, measurements may be biased. For example, when using an inclinometer to measure joint angles, measurement bias may be introduced if the inclinometer moves relative to the underlying skeleton. By extension, careful consideration is required regarding whether exposure estimated using different methods are directly comparable; for example, postures assessed using inclinometers and observation. If inclinometer measurements do not agree with estimates of the same joint angle obtained using standard practice observation, it must be considered how to transform measured values into a common scale – ideally, one which comes as close as possible to ‘true’ values. If disagreement is considerable, comparison between studies based on observations and inclinometry can be severely compromised, and thus the need for a bias-corrected common scale is vital.
In 1937 (translated to English in 1972), Goldmeier demonstrated that subjects could always correctly visually identify a 90° angle when presented with simple drawings of 87°, 90°, and 93° angles provided that the drawings were presented in a normal orientation. This excellent ability of humans to correctly identify normal right angles has been dubbed the ‘Goldmeier effect’ (Ferrante et al., 1995). Applying the Goldmeier effect to the human body, it would be anticipated that, if a person were asked to identify a 90° shoulder abduction angle, the selected elevation angle would look approximately like that shown on the left side of Fig. 1. Similarly, a 90° shoulder flexion angle is expected to look approximately like the angle shown on the right side of Fig. 1. This expected consistency between observers suggests inherent construct validity to observation, at least at 90° angles: arguably, this could therefore be considered a ‘true’ 90° angle. By extension, this would mean that direct technical measurement methods should also assess this angle to be 90°. Findings published by Genaidy et al. (1993) showing that observers were fairly proficient and almost unbiased (throughout the 0–180° range of shoulder flexion) at correctly judging ‘true angles’ from still frames taken from video shot perpendicularly to the worker support this notion.
Over the last decade, tri-axial accelerometers have frequently been used as inclinometers to assess upper arm elevation angles with respect to the line of gravity (for example: Bernmark and Wiktorin, 2002, Delisle et al., 2006, Fethke et al., 2011, Hansson et al., 2010, Korshøj et al., 2014, Leijon et al., 2005, Wahlström et al., 2010, Veiersted et al., 2008). Inclinometers are attractive direct measurement devices due to their relatively low cost (Trask et al., 2013), ease of use, and highly portable characteristics.
A 2002 paper by Bernmark and Wiktorin investigated the validity of utilising tri-axial accelerometers as inclinometers (INCs) for the purpose of measure arm postures and movements. To achieve this goal, INC angle measurements were compared to angle measurements recorded from an optoelectronic (OPT) measuring system at fixed arm elevation angles of 0, 45, 90, 135 and 180° that were determined by meticulous, but unassisted, observation by a single researcher (personal communication – Eva Bernmark). The paper presented a figure showing a strong correlation between the INC and OPT measured arm elevation angles (Bernmark and Wiktorin, 2002). The figure also showed that both systems underestimated the expected inclination angles at, and above, 90° arm elevation. The authors have generously shared the data behind their original figure, and we re-plotted the INC measurement data with respect to the expected angle data to highlight this additional finding (Fig. 2). Non-published data from within our research team have also suggested a similar trend of underestimated arm elevation angles across multiple brands of INC systems and INC mounting protocols. This underestimation occurs despite the excellent accuracy of tri-axial accelerometer INC systems during static testing in a rigid rack; for example, the Logger Teknologi AB system (Logger Teknologi HG, Åkarp, Sweden) has a reported ‘accuracy’ of 1.3° and a ‘reproducibility’ of 0.2° during static testing (Hansson et al., 2001), and angles measured using the Virtual Corset system (Microstrain Inc., Vermont, USA) were reported to deviate no more than 2° from correct values (Amasay et al., 2009).
The purpose of the current study was therefore to evaluate the extent to which arm elevation angles measured using standard tri-axial inclinometry systematically underestimate ‘true’ elevation angles, as determined from meticulous, assisted observation positioning of the arm in different postures, and whether a possible bias could be effectively adjusted for using regression models. Two different INC mounting locations on the upper arm were investigated to determine whether anatomical placement has an effect on estimation accuracy.
Section snippets
Subjects
A total of 19 participants (12 males, 7 females) were recruited by advertisement at three educational institutions in Gävle, Sweden (Table 1). Subjects were excluded if they had any ailment preventing them from comfortably moving through a full range of shoulder motion (arms at side to arms overhead) or to their maximum trunk flexion angle (trunk data not reported in the current study). The study was approved by the Regional Ethical Review Board in Uppsala, and all subjects signed an informed
Results
Data from the Cr-VM INC were successfully collected for all subjects and days. A technical issue with the data logger for the Ca-LT INC resulted in loss of data for 6 of the 19 subjects on day 1.
Data from one representative subject are given in Fig. 5, showing the relationships between the set angles, unadjusted INC measurements, and calibrated INC data for both arm abduction and arm flexion. Increased bias at higher elevation angles for the unadjusted INC data is clearly shown for the INCs in
Discussion
Our data demonstrate that arm elevation angles recorded using standard inclinometry systematically underestimate what we argue to be a ‘true’ assessment of arm elevation angle, i.e. by meticulous, assisted observation, particularly at elevation angles greater than 60°. By extension, our results question the notion that different measurement tools assess upper arm postures according to a common scale, and further, whether un-calibrated inclinometers provide the most accurate data possible. Our
Conclusion
A systematic underestimation of upper arm elevation measured using standard inclinometry was found, particularly at larger elevation angles. The bias could be adjusted for by applying regression calibration models; the most effective model found was a subject-specific, two-point linear approach that utilised the slope from the origin to the mean of inclinometer recorded values at 90° arm abduction and 90° arm flexion. This model outperformed quadratic models using group-level data as input to
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