Comparing non-invasive scapular tracking methods across elevation angles, planes of elevation and humeral axial rotations.

Altered scapular motions premeditate shoulder impingement and other musculoskeletal disorders. Divergent experimental conditions in previous research precludes rigorous comparisons of non-invasive scapular tracking techniques. This study evaluated scapular orientation measurement methods across an expanded range of humeral postures. Scapular medial/lateral rotation, anterior/posterior tilt and protraction/retraction was measured using an acromion marker cluster (AMC), a scapular locator, and a reference stylus. Motion was captured using reflective markers on the upper body, as well as on the AMC, locator and stylus. A combination of 5 arm elevation angles, 3 arm elevation planes and 3 arm axial rotations was examined. Measurement method interacted with elevation angle and plane of elevation for all three scapular orientation directions (p < 0.01). Method of measurement interacted with axial rotation in anterior/posterior tilt and protraction/retraction (p < 0.01). The AMC had strong agreement with the reference stylus than the locator for the majority of humeral elevations, planes and axial rotations. The AMC underestimated lateral rotation, with the largest difference of ∼2° at 0° elevation. Both the locator and AMC overestimated posterior tilt at high arm elevation by up to 7.4°. Misestimations from using the locator could be enough to potentially obscure meaningful differences in scapular rotations.

The effects of police duty belt and seat design changes on lumbar spine posture, driver contact pressure and discomfort.

Police officers spend large amounts of time performing duties within a police cruiser and report a high prevalence of musculoskeletal problems. This study evaluated the effects of driver seat and duty belt design on posture, pressure and discomfort. Ten male and 10 female university students attended two sessions involving simulated driving in a standard police seat (CV) and an active lumbar support (ALS) seat. Participants wore a full duty belt (FDB) or reduced duty belt (RDB) in each seat. Lumbar postures, driver-seat and driver-duty belt pressures and perceived discomfort were measured. Gender × Seat interactions were found for pelvic (p = 0.0001) and lumbar postures (p = 0.003). Females had more lumbar flexion than males and were more extended in the ALS seat (-9.8 ± 11.3°) than CV seat (-19.8 ± 9.6°). The FDB had greater seat pressure than the RDB (p < 0.0001), which corresponded to increased pelvis discomfort. This study supports the use of an ALS seat and RDB to reduce injury risk associated with prolonged sitting in police officers. PRACTITIONER SUMMARY: Police officers report a high prevalence of musculoskeletal problems to the lower back, associated with prolonged driving and further investigation is needed to reduce injury risk. This simulated driving study investigated seat and duty belt configuration on biomechanical measures and discomfort. Seat design had the greatest impact, regardless of gender and males benefited more from a reduced belt configuration.

Evaluation of the influence of mobile data terminal location on physical exposures during simulated police patrol activities.

Prolonged occupational police driving combined with use of an in-vehicle computer elicits awkward, sustained postures in a scenario that lacks the adjustability to accommodate many mobile officer anthropometries and job-specific components. Twenty participants performed simulated police patrol sessions at five mobile data terminal (MDT) locations and using two seats: standard police vehicle seat and modified seat designed for police use. An MDT location self-selected prior to the session reduced perceived discomfort by up to 50% in the low back (p < .0001) and 68% in the right shoulder (p < .0001) compared to other tested locations, including the most common currently used location recorded from a representative police force. Muscle activity was lowest at the self-selected and current MDT locations for all muscles, significantly so for posterior deltoid (p < .0001) and supraspinatus (p < .0001). The modified seat reduced low back discomfort from the standard seat by 28% (p < .0001). Combining a self-selected MDT location and modified driver seat generated lower discomfort and physical loading than the currently used configuration.

Task selection for increased mechanical exposure variation: relevance to job rotation.

It is generally considered that variation in mechanical exposure is beneficial from the viewpoint of preventing musculoskeletal disorders. An approach to quantifying the functional similarity of manual tasks was developed and tested by using three handgrips: power grip (PG); pulp pinch (PP); lateral pinch (LP). Changes in maximal voluntary contraction (MVC) forces were measured during 15 combinations of 2-min duration, 30% MVC isometric contraction in 25 participants. Strength changes in PG were found to be substantially different from LP; when the two grips were alternated, strength dropped to 85.1% MVC whereas after PG alone, strength dropped to 59.3% MVC. The findings suggest that PG and LP are functionally different enough for their alternating use to contribute to increased exposure variation during manual activities. STATEMENT OF RELEVANCE: The paper describes how to determine if two tasks, when performed alternately, are different enough to reduce fatigue, increase work capacity and potentially prevent musculoskeletal disorders. Better understanding of the differences between tasks has the potential to improve job design by allowing better choice of tasks that combine to define a job.

Predictors of perceived effort in the shoulder during load transfer tasks.

The mechanism of muscular effort perception in the shoulder was examined in this experiment. Two shoulder biomechanical models and experimental muscle activity data were used to assess physical exposure for a series of reaching tasks. Effort perception was quantitatively correlated to these measures of physical loading, both at the resultant torque (r(2) = 0.50) and muscle activity model-based muscle force predictions (MFPs): r(2) = 0.42, electromyography (EMG): r(2) = 0.26) levels. Muscle data did not explain variation in effort perception more fully than torque data. The inclusion of subject and task variables improved the ability of each model to explain variability in effort perception (torque: r(2) = 0.74; MFP: r(2) = 0.67, EMG: r(2) = 0.64). These results suggest that effort perception may not be fully explained by only an image of the motor command, but is rather a complex integrative quantity that is affected by other factors, such as posture and task goals, which may be dependent on sensory feedback.