Eric B. Weston

The effect of vibration exposure during haul truck operation on grip strength, touch sensation, and balance

Falls from mobile equipment are reported at surface mine quarry operations each year in considerable numbers. Research shows that a preponderance of falls occur while getting on/off mobile equipment. Contributing factors to the risk of falls include the usage of ladders, exiting onto a slippery surface, and foot or hand slippage. Balance issues may also contribute to fall risks for mobile equipment operators who are exposed to whole-body vibration (WBV). For this reason, the National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research conducted a study at four participating mine sites with seven haul truck operators. The purpose was to ascertain whether WBV and hand-arm vibration (HAV) exposures for quarry haul truck operators were linked to short-term decreases in performance in relation to postural stability, touch sensation threshold, and grip strength that are of crucial importance when getting on/off the trucks. WBV measures of frequency-weighted RMS accelerations (wRMS) and vibration dose value (VDV), when compared to the ISO/ANSI standards, were mostly below levels identified for the Health Guidance Caution Zone (HGCZ), although there were instances where the levels were within and above the specified Exposure Action Value. Comparably, all mean HAV levels, when compared to the ISO/ANSI standards, were below the HGCZ. For the existing conditions and equipment, no significant correlation could be identified between the WBV, HAV, postural stability, touch sensation threshold, and grip strength measures taken during this study.

A biomechanical and physiological study of office seat and tablet device interaction

Twenty subjects performed typing tasks on a desktop computer and touch-screen tablet in two chairs for an hour each, and the effects of chair, device, and their interactions on each dependent measure were recorded. Biomechanical measures of muscle force, spinal load, and posture were examined, while discomfort was measured via heart rate variability (HRV) and subjective reports. HRV was sensitive enough to differentiate between chair and device interactions. Biomechanically, a lack of seat back mobility forced individuals to maintain an upright seating posture with increased extensor muscle forces and increased spinal compression. Effects were exacerbated by forward flexion upon interaction with a tablet device or by slouching. Office chairs should be designed with both the human and workplace task in mind and allow for reclined postures to off-load the spine. The degree of recline should be limited, however, to prevent decreased lumbar lordosis resulting from posterior hip rotation in highly reclined postures.

Investigation of human body vibration exposures on haul trucks operating at U.S. surface mines/quarries relative to haul truck activity

Workers who operate mine haul trucks are exposed to whole-body vibration (WBV) on a routine basis. Researchers from the National Institute for Occupational Safety and Health (NIOSH) Pittsburgh Mining Research Division (PMRD) investigated WBV and hand-arm vibration (HAV) exposures for mine/quarry haul truck drivers in relation to the haul truck activities of dumping, loading, and traveling with and without a load. The findings show that WBV measures in weighted root-mean-square accelerations (aw) and vibration dose value (VDV), when compared to the ISO/ANSI and European Directive 2002/44/EC standards, were mostly below the Exposure Action Value (EAV) identified by the health guidance caution zone (HGCZ). Nevertheless, instances were recorded where the Exposure Limit Value (ELV) was exceeded by more than 500 to 600 percent for VDVx and awx, respectively. Researchers determined that these excessive levels occurred during the traveling empty activity, when the haul truck descended down grade into the pit loading area, sliding at times, on a wet and slippery road surface caused by rain and overwatering. WBV levels (not normalized to an 8-h shift) for the four haul truck activities showed mean awz levels for five of the seven drivers exceeding the ISO/ANSI EAV by 9-53 percent for the traveling empty activity. Mean awx and awz levels were generally higher for traveling empty and traveling loaded and lower for loading/dumping activities. HAV for measures taken on the steering wheel and shifter were all below the HGCZ which indicates that HAV is not an issue for these drivers/operators when handling steering and shifting control devices.

Biomechanically determined hand force limits protecting the low back during occupational pushing and pulling tasks

Abstract Though biomechanically determined guidelines exist for lifting, existing recommendations for pushing and pulling were developed using a psychophysical approach. The current study aimed to establish objective hand force limits based on the results of a biomechanical assessment of the forces on the lumbar spine during occupational pushing and pulling activities. Sixty-two subjects performed pushing and pulling tasks in a laboratory setting. An electromyography-assisted biomechanical model estimated spinal loads, while hand force and turning torque were measured via hand transducers. Mixed modelling techniques correlated spinal load with hand force or torque throughout a wide range of exposures in order to develop biomechanically determined hand force and torque limits. Exertion type, exertion direction, handle height and their interactions significantly influenced dependent measures of spinal load, hand force and turning torque. The biomechanically determined guidelines presented herein are up to 30% lower than comparable psychophysically derived limits and particularly more protective for straight pushing. Practitioner Summary: This study utilises a biomechanical model to develop objective biomechanically determined push/pull risk limits assessed via hand forces and turning torque. These limits can be up to 30% lower than existing psychophysically determined pushing and pulling recommendations. Practitioners should consider implementing these guidelines in both risk assessment and workplace design moving forward.

Wheelchair pushing and turning: lumbar spine and shoulder loads and recommended limits

Abstract The objective of this study was to determine how simulated manual wheelchair pushing influences biomechanical loading to the lumbar spine and shoulders. Sixty-two subjects performed simulated wheelchair pushing and turning in a laboratory. An electromyography-assisted biomechanical model was used to estimate spinal loads. Moments at the shoulder joint, external hand forces and net turning torque were also assessed. Multiple linear regression techniques were employed to develop biomechanically based wheelchair pushing guidelines relating resultant hand force or net torque to spinal load. Male subjects experienced significantly greater spinal loading (p < 0.01), and spine loads were also increased for wheelchair turning compared to straight wheelchair pushing (p < 0.001). Biomechanically determined maximum acceptable resultant hand forces were 17–18% lower than psychophysically determined limits. We conclude that manual wheelchair pushing and turning can pose biomechanical risk to the lumbar spine and shoulders. Psychophysically determined maximum acceptable push forces do not appear to be protective enough of this biomechanical risk. Practitioner Summary: This laboratory study investigated biomechanical risk to the low back and shoulders during simulated wheelchair pushing. Manual wheelchair pushing posed biomechanical risk to the lumbar spine (in compression and A/P shear) and to the shoulders. Biomechanically determined wheelchair pushing thresholds are presented and are more protective than the closest psychophysically determined equivalents.

Impact of two postural assist exoskeletons on biomechanical loading of the lumbar spine

This study evaluated loading on the low back while wearing two commercially available postural assist exoskeletons. Ten male subjects lifted a box from multiple lift origins (combinations of vertical height and asymmetry) to a common destination using a squatting lifting technique with and without the use of either exoskeleton. Dependent measures included subject kinematics, moment arms between the torso or weight being lifted and the lumbar spine, and spinal loads as predicted by an electromyography-driven spine model. One of the exoskeletons tested (StrongArm Technologies™ FLx) reduced peak torso flexion at the shin lift origin, but differences in moment arms or spinal loads attributable to either of the interventions were not observed. Thus, industrial exoskeletons designed to control posture may not be beneficial in reducing biomechanical loads on the lumbar spine. Interventions altering the external manual materials handling environment (lift origin, load weight) may be more appropriate when implementation is fesible.

Effectiveness of a vacuum lifting system in reducing spinal load during airline baggage handling

Information on spinal loading for using lift assist systems for airport baggage handling is lacking. We conducted a laboratory study to evaluate a vacuum lift system for reducing lumbar spinal loads during baggage loading/unloading tasks. Ten subjects performed the tasks using the industry average baggage weight of 14.5 kg on a typical two-shelved baggage cart with or without using the lift system (i.e. lifting technique). Repeated measures analysis of variance (2 tasks × 2 shelf heights x 2 techniques) was used. Spinal loads were estimated by an electromyography-driven biomechanical model. On average, the vacuum lift system reduced spinal compressive forces on the lumbar spine by 39% and below the 3400 N damage threshold. The system also resulted in a 25% reduction in the anterior-posterior shear force at the L5/S1 inferior endplate level. This study provides evidence for the potential to reduce spinal loads when using a vacuum lift system.

Biomechanically-Determined Guidelines for Occupational Pushing and Pulling

Background: In an attempt to reduce heavy lifting exposures, the manual materials handling burden has shifted towards pushing and pulling. Pushing and pulling may pose a biomechanical risk due to excessive loads placed onto the lumbar spine, particularly in anterior/posterior (A/P) shear (Knapik and Marras 2009). The only risk limits available in the scientific literature for pushing and pulling were psychophysically-determined, relying on the assumption that subjective perception of an individual’s maximum acceptable external forces corresponds to biomechanical tolerance (Snook and Ciriello 1991). However, individuals are unlikely able to sense biomechanical loading on critical tissues in the spine due to the lack of nociceptors in the intervertebral disc (Adams et al. 1996). As such, the objective of this study was to create a set of biomechanically-determined risk limits for occupational pushing and pulling that are protective of the low back. Methods: Sixty-two subjects (31 male, 31 female) performed occupational pushing and pulling tasks in a laboratory. Subjects performed three types of exertions (one-handed pull, two-handed pull, two-handed push) at three handle heights (32 in., 40 in., 48 in.) and in one of two directions (straight or turn). Subjects pushed or pulled on custom-built hand transducers connected to an overhead braking system via a rig while performing each exertion. To document a wide range of pushing and pulling exposures, the braking system incrementally increased the linear or rotational resistance proportional to the subject’s changes from the initial global position throughout each trial; subjects exerted up to a maximum voluntary exertion. Dependent measures consisted of the magnitude and direction of three-dimensional forces recorded at the hands, turning torques, net joint moments calculated at each shoulder, and three-dimensional spinal loads (compression, A/P shear, lateral shear) at the superior and inferior endplates of each spinal level extending from T12/L1 to L5/S1, as calculated by a dynamic EMG-driven biomechanical spine model (Knapik and Marras 2009; Hwang et al. 2016a; Hwang et al. 2016b). Multiple linear regression techniques correlated spinal loads with hand force or turning torque in order to develop biomechanically-determined hand force and turning torque limits. The values for straight two-handed pushing and pulling were also compared to psychophysically-determined thresholds developed by Snook and Ciriello (1991). Results and Discussion: The independent measures (exertion type, handle height, and exertion direction) and their interactions significantly influenced dependent measures of hand force, turning torque, shoulder moment, and spinal load. In agreement with Knapik and Marras (2009), spinal loads most frequently exceeded tissue tolerance limits for spinal loading (NIOSH 1981; Gallagher and Marras 2012) in A/P shear. The biomechanically-determined limits developed from this work are up to 30% lower than the closest psychophysically-derived equivalents (Snook and Ciriello 1991). Conclusion: Psychophysically-derived hand force limits are not protective enough of biomechanical risk imposed onto the lumbar spine during pushing and pulling. The biomechanically-determined pushing and pulling guidelines proposed herein provide a more objective and conservative indication of risk and should be implemented moving forward.