Neck Strength and Endurance and Associated Personal and Work-Related Factors.

OBJECTIVE: The present study aimed to establish a normative database of neck strength and endurance while exploring personal and work-related factors that can significantly influence neck strength and endurance. BACKGROUND: A normative database combining both neck strength and endurance and delineating how they are affected by personal and work-related factors is currently lacking. It is needed for the development of tools and guidelines for designing work requiring head-neck exertions to contain the risk of occupational neck pain. METHODS: Forty healthy participants (20 males and 20 females) performed sustained-till-exhaustion head-neck exertions, while seated, at 50% and 100% of their maximal efforts in anterior, anterior-superior, and posterior-superior directions in neutral, 40° extended, and 40° flexed neck postures. Exertion force and endurance time data from 38 participants were recorded and analyzed using regression models. RESULTS: Overall, multiple regression analyses of the neck strength and endurance database revealed that head-neck posture is the most significant determinant of both neck strength and endurance. The time of day significantly influenced neck endurance. Among the personal factors, a significant sex effect on neck strength and significant age and body mass index (BMI) effects on neck endurance were identified. CONCLUSION: The work-related factors play a more significant role in shaping both neck strength and endurance than personal factors and therefore are more important modifiable factors in meeting the physical demands of work. APPLICATION: The study findings can aid in work design as well as in pre-employment screening to reduce the incidence of neck pain in the workplace.

Lumbar Facet Joint Kinematics and Load Effects During Dynamic Lifting.

OBJECTIVE: To examine the lumbar facet joint kinematics in vivo during dynamic lifting and the effects of the load lifted. BACKGROUND: Although extensive efforts have been dedicated to investigating the risk factors of low back pain (LBP) associated with load handling in the workplace, the biomechanics of lumbar facet joints during such activities is not well understood. METHOD: Fourteen healthy participants performed a load-lifting task while a dynamic stereo-radiography system captured their lumbar motion continuously. Data from 11 participants were included for subsequent analysis. A randomized block design was employed to study the load effect (4.5 kg, 9.0 kg, and 13.5 kg) on bilateral facet joint motions at approximately 60°, 40°, 20°, and 0° trunk-flexion postures. The facet orientations were also examined. RESULTS: Significant load effects were found for the flexion and lateral bending and superior-inferior translation of the facet joints. The L5-S1 displayed greater lateral bending and twisting, which was due to its more posterolateral orientation than the L2-L3, L3-L4, and L4-L5 facet joints. The left-right asymmetry in facet orientation was observed, most prominently at L3-L4 and L5-S1 facet joints. CONCLUSION: The lumbar facet joint kinematics are affected by the magnitude of the lifted load and are dependent on the orientations of articulating adjacent facets. APPLICATION: This study provided new insights into the role of lumbar facet joints in vivo during lifting. Alterations in the facet joint kinematics due to vigorous functional demand can be one of the primary but overlooked mechanical factors in the causation of LBP.

A biomechanical shoulder strain index based on stabilizing demand of shoulder joint.

Work-related shoulder joint disorders contribute considerably to absenteeism in the workplace. To identify the tasks that are stressful to the shoulder joint, a strain index was formulated based on the concept of concavity compression-a shoulder stabilizing mechanism. The magnitude and direction of the shoulder joint reaction forces were used in formulating the strain index. A two phase experiment was conducted. In Phase 1, participants performed 30 different manual handling tasks. The tasks were categorized into low, medium and high strain tasks based on their strain index values. In Phase 2, out of the 30 tasks, repetitive exertions of three tasks (low, medium and high strain index values) were simulated using three external loads (0.91, 1.81 and 2.72 kg). The muscle activity data recorded from eight shoulder muscles showed that tasks with higher strain index values induced significantly greater activation and muscle fatigue than tasks with lower strain index values.Practitioner Summary: The strain index developed in this study is a conclusive estimation of the concavity compression required for shoulder joint stabilization. It can be used to identify the activities that may contribute to the risks of shoulder disorders. Abbreviation BLS Bureau of the Labor Statistics.

Segmental variations in facet joint translations during in vivo lumbar extension.

The lumbar facet joint (FJ) is often associated with pathogenesis in the spine, but quantification of normal FJ motion remains limited to in vitro studies or static imaging of non-functional poses. The purpose of this study was to quantify lumbar FJ kinematics in healthy individuals during functional activity with dynamic stereo radiography (DSX) imaging. Ten asymptomatic participants lifted three known weights starting from a trunk-flexed (∼75°) position to an upright position while being imaged within the DSX system. High resolution computed tomography (CT) scan-derived 3D models of their lumbar vertebrae (L2-S1) were registered to the biplane 2D radiographs using a markerless model-based tracking technique providing instantaneous 3D vertebral kinematics throughout the lifting tasks. Effects of segment level and weight lifted were assessed using mixed-effect repeated measures ANOVA. Superior-inferior (SI) translation dominated FJ translation, with L5S1 showing significantly less translation magnitudes (Median (Md) = 3.5 mm, p < 0.0001) than L2L3, L3L4, and L4L5 segments (Md = 5.9 mm, 6.3 mm and 6.6 mm respectively). Linear regression-based slopes of continuous facet translations revealed strong linearity for SI translation (r2 > 0.94), reasonably high linearity for sideways sliding (Z-) (r2 > 0.8), but much less linearity for facet gap change (X-) (r2 ∼ 0.5). Caudal segments (L4-S1), particularly L5S1, displayed greater coupling compared to cranial (L2-L4) segments, revealing distinct differences overall in FJ translation trends at L5S1. No significant effect of weight lifted on FJ translations was detected. The study presents a hitherto unavailable and highly precise baseline dataset of facet translations measured during a functional, dynamic lifting task.