Quantitative scapular and humeral motion assessments are required to understand shoulder movement and upper limb abilities. Scapular and humeral motion are associated with shoulder injury risk (1–3), and shoulder injuries are prevalent in both the general population (4, 5) and athletic populations (6–8), necessitating further research into how the scapulae and humeri move in daily life. Inertial and magnetic measurement units (IMMUs) present a unique opportunity for quantitative assessment of scapular and humeral motion during functional movements in real-world settings, such as at work or during sport. However, there is a research gap regarding the use of IMMUs to measure humeral and scapular motion outside of the research laboratory. While some in-field work-specific assessments of posture or kinematics have been done for the upper arm (9–12), these assessments generally do not include humeral axial rotation or scapular motion, despite their importance to upper limb function and injury (13). Indeed, even in research lab settings, healthy and pathological scapular kinematics are most frequently measured during planar arm elevation (2, 14–20). To our knowledge, there exists no in-field measurement of the scapula during functional tasks, and humeral assessments are almost solely focused on arm elevation exposures (9–12, 21). Improved measurement of upper limb postures, including externally valid in-field assessments, could better inform interventions to prevent or address upper limb musculoskeletal disorders.

Best practices for IMMU use for measuring the humeri and scapulae during lab-based planar arm elevation have been previously defined (22–24). Recent research from our research group has validated the use of IMMUs for measuring both scapular movement (25) and humeral axial rotation (26) during a range of functional and work-related tasks. However, these studies have been conducted exclusively in lab settings, despite the portability of the IMMUs and their potential for in-field applications. These procedures differ from typical in-field motion capture set-up or analysis (27) due to external calibration tools for scapular tracking (23, 25) and calculations of rotation about the long axis of the arm (25, 28), which may be difficult to implement with field work. IMMUs can also be prone to drift or magnetic disturbance (29, 30); therefore, identifying if IMMUs and the necessary measurement and analysis protocol for measuring humeral and scapular motion can be applied to different field settings is necessary to confirm the utility of these methods or to identify potential sources of error that need to be addressed.

The primary objective of this study was to determine how humeral and scapular motion measured with IMMUs in different field settings (home, work, sport) compares to motion measured in a lab setting (26, 31). It was anticipated that the IMMUs will be able to successfully and accurately measure humeral and scapular kinematics in all in-field settings, as demonstrated by no difference in kinematic outcomes, as well as similar shape and magnitude of waveforms, between the field settings and the lab setting.

Twenty-eight adults (14 in-field, 14 in-lab) participated in the study (Table 2): five in the home setting, four in the work setting, five in the sport setting, and fourteen in the lab. There were no significant differences in age, mass, height, and QuickDASH scores between field participants and lab participants (p > .05) (Table 2). Additional data from two participants collected in the field settings (one home, one work) (not included in the above group of 14 participants) were not incorporated into the analysis due to calibration and collection errors, respectively.

The current study examined whether humeral and scapular kinematic data collected using IMMUs in various field settings (home, work, sport) is comparable to IMMU data collected in a lab setting. Findings indicate that humeral elevation, humeral axial rotation, scapular upward rotation, and, to some extent, scapular tilt measured in-field were not different from data collected in a controlled lab setting. Field data were collected in different home, work, and sport settings suggesting that these methods can be applied for in-field research and may be robust and generalizable across settings. Researchers can begin to use these methods to assess upper limb motion in natural environments, such as sporting arenas or workshops, which may better inform understanding of upper limb musculoskeletal disorders. The ability to capture natural movements, whether in sport, work, or daily life activities, will have positive implications for detailed investigations of how biomechanics influences performance and injury risk.

Given the similarity of humeral waveforms for most WRAFT protocol tasks, some of the observed differences in upper limb motion may be explained by equipment modifications required to implement the protocol in the field settings (37–39). Different set ups between the lab and field settings (Figure 2) may explain greater humeral elevation and external rotation in the lab at the beginning of the Overhead Reach and Lift tasks. Greater humeral elevation magnitudes at the beginning of the Overhead Reach and Overhead Lift tasks may be due to different start positions in the lab and field settings. In the lab setting, participants were not able to sit or stand as close to the shelf in the lab setting as compared to the field settings. Greater humeral external rotation for the Forward Transfer task in the lab setting compared to the field settings may also be due to differences in task set up: in the lab, all participants sat at the same table, while in the field setting, participants either sat at a table that was available in that setting or sat in front of the set of shelves and reached beyond to an “imaginary” table top. The observed differences in upper limb motion are explainable, suggesting that the observed differences may be largely due to equipment or set up modifications and not the measurement system.

The between-subjects' design may have contributed to visual differences in waveform magnitude for scapular upward rotation and tilt outcomes and increased variability in tilt. Between-subjects' variability was considered by matching field participants to lab participants by sex and body height, but because the comparisons are of two different samples, individual variation likely affected the results (40, 41). For scapular tilt, the larger standard deviations for the field data could mask significant differences; therefore, caution should be used when interpreting those results.

The upper limb movements during WRAFT protocol tasks were relatively unconstrained, allowing for self-selected movement patterns following standardized verbal instruction and visual demonstration of each task. Increasing task constraints, such as movement speed (42, 43), could have further minimized the differences between settings. Despite this variation, humeral elevation, humeral rotation, and scapular upward rotation demonstrate minimal differences, similar visual movement trajectories, and similar variability between settings and to previously reported data (i.e., 10°–24°) (16, 31, 36, 44–47). As between-groups comparisons are prevalent in biomechanics, the similarity of the mean and standard deviation of angle waveforms between the lab and field datasets is promising. However, a test-retest study design would increase the robustness of the findings and interpretation.

Scapular internal rotation measurement was not successful in this study. Significant differences are not present due to the very wide standard deviations for the in-field measures, demonstrating the inconsistency of the scapular internal rotation measurements. There may be two primary explanations for this issue: (1) IMMU movement on the skin, and (2) distortion of the IMMUs. First, in the initial lab-based validation of the scapular IMMU measurement methods (25), the tracking IMMU was placed directly on, and likely braced by, the rigid, L-shaped motion capture acromial marker cluster. In the field, the IMMU was placed directly on the skin above the flat part of the acromion (25, 48, 49). Without the support of the motion capture cluster, the tracking IMMU could have moved on the skin around the Y axis, affecting internal rotation outcomes. Second, it is documented that magnetic distortion of IMMUs largely affects rotation about the Y axis (29, 50), which corresponds with scapular internal rotation in this set-up. The error levels in this previous research are considerably lower than the variability in the current study; it is possible that any distortion affects could be compounded as the scapular angle calculations rely on three separate IMMUs (locator, sternum, scapular tracking IMMU). These same issues are not present in the humeral axial rotation, which is traditionally calculated as rotation about the humeral Y axis (34, 51), because the current study leverages the newer “True axial rotation” method, which does not rely on Euler angles. Overall, it may not be possible to accurately analyze scapular internal rotation with the current set-up. Future research will further investigate possible distortion affects and mitigation strategies.

There are some limitations to address with this study. Although we attempted to make the lab and field protocols identical, there were differences in equipment and protocol between lab and field groups. While our study demonstrates that good quality data can be collected over periods of approximately 45 min, it is unclear whether results can be extended to data recording sessions spanning over several hours. Additionally, data may not generalize across all outdoor and indoor settings, as data could differ from what is reported in settings with local magnetic field disturbances. Future research should consider the specific environment when measuring movement with IMMUs. Importantly, due to the between subjects' design, we were not able to use traditional agreement, repeatability, or reliability tests such as Bland-Altman plots, intraclass correlation coefficients, or standard error of measurement. However, as the purpose was to assess if the setting affected IMMU kinematic outputs, the lack of differences for many angles, and the similarity of many waveforms, even for these two separate (matched) datasets, we believe our approach and conclusions are valid. Future research will explicitly explore agreement between settings using a test-retest design with the same participants. Finally, as mentioned scapular internal rotation did not demonstrate acceptability similarity to the lab data. Scapular angles were calculated from Euler angles, suggesting angle calculation crosstalk could also affect upward rotation and tilt, although findings suggest these degrees of freedom were minimally affected. Further work is needed to understand and address this issue.

These findings provide a foundation for future research with IMMUs. Beyond the methodological work to refine these methods, as described above, the next steps should focus on applying these methods to measure real, daily life movements in each environment. Application of IMMUs in different field settings presents an opportunity for interdisciplinary research exploring how biomechanics, sport sciences, wearable technology, and clinical science can work together to address research gaps with modern solutions. Specifically, upcoming research will build on this work to assess sport-specific movements with IMMUs, such as overhead throws and different pitch types on baseball diamonds, or work-specific tasks, such as machinery maintenance and animal care on grain and large animal farms.

Humeral elevation and axial rotation and scapular upward rotation show promise for biomechanical assessments in different field settings using IMMUs with the methods used in this study. However, due to increased variability in field data, scapular tilt outcomes should be interpreted with caution and scapular internal rotation should not be assessed with IMMUs outside of the laboratory with these procedures. These results are encouraging for the evaluation of upper limb motion in natural environments.