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Seated postural reactions to mechanical shocks: laboratory studies with relevance for risk assessment and prevention of musculoskeletal disorders among drivers
Umeå University, Faculty of Medicine, Department of Community Medicine and Rehabilitation, Physiotherapy.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Professional drivers of off-road vehicles, driving on irregular terrain such as in forestry, agriculture and mining, are exposed to whole-body vibration and mechanical shocks. These driver groups have reported severe musculoskeletal problems in the spine, but the association to seated postural reactions is not fully understood. One assumption is that unexpected shocks may create excessive load on spinal joints. The driver’s posture and exposure to mechanical shocks are required to be included in work risk assessments, but muscle activity and body kinematics are not included. The overall aim of this thesis was to describe and analyse seated postural reactions to mechanical shocks and to evaluate measuring of seated postures with relevance for risk assessment and the prevention of musculoskeletal disorders among drivers.

The thesis includes four studies, all laboratory-based using a repeated-measures design. Postural reactions were recorded from 23 (Paper I) and 20 (Paper II & III) young, healthy male participants who were seated on a movable platform. The platform delivered mechanical shocks with peak accelerations up to 14 m/s2 in lateral directions during different conditions. Furthermore, twenty participants (Paper IV) were tested by four testers for analysis of test-retest reliability within and between testers measuring seated postures. Kinematics were here detected by means of a motion analysis system (MoLabTM) and described for the spine as angular displacements or range of motion (ROM) using a three-segment model of neck, trunk and pelvis (Paper I–III) and as a more specific model (Paper IV). Surface electromyography (EMG) was recorded bilaterally on the following muscles; trapezius upper part, upper neck, erector spinae and external oblique (Paper I–III).

The general findings show that EMG amplitudes normalised to maximum voluntary contractions (MVC) did not exceed 2% in the trapezius, 8% in the upper neck and erector spinae and 18% in the external oblique. The EMG amplitudes and the angular displacements in the neck were significantly reduced from the first compared to the fifth mechanical shock. Adding a cognitive task significantly increased angular displacements. The largest ROM with approximately 20° in each segment was found during a double-sided mechanical shock (shock that changes direction). The reliability within one tester measuring seated postures was mostly considered good and superior to the reliability between several testers, but still insensitive to changes of less than 10°.

Exposure to single-sided or double-sided mechanical shocks with accelerations up to 14 m/s2 seem not to cause postural reactions to such an extent that overload of muscles or joint structures should be expected. There seems to be a quick adaptation that causes an improved readiness. The external obliques were most active when restoring equilibrium and seem important for stabilising the whole spinal column. Stability training, in order to improve neuromuscular control of the external obliques could, therefore, be a possible recommendation. The angular displacement in the neck increases if the subject solves a cognitive task of why such activities should be avoided when driving in difficult terrains. Since accurate descriptions of the spinal posture seems difficult even when advanced technical equipment is used, simpler models seem more appropriate. The results show that postural control is maintained even when exposed to considerable mechanical shocks. On the basis of these results, there is no need to change established risk assessment models.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2016. , 70 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1780
Keyword [en]
Postural balance, Posture, Electromyography, Musculoskeletal pain, Whole-body vibration, Reliability, Kinematics, Biomechanics
National Category
Physiotherapy
Research subject
Physiotherapy
Identifiers
URN: urn:nbn:se:umu:diva-117178ISBN: 978-91-7601-410-3 (print)OAI: oai:DiVA.org:umu-117178DiVA: diva2:905940
Public defence
2016-03-18, Aulan, Vårdvetarhuset, Umeå universitet, Umeå, 09:00 (Swedish)
Opponent
Supervisors
Funder
AFA Insurance
Available from: 2016-02-26 Created: 2016-02-23 Last updated: 2016-02-25Bibliographically approved
List of papers
1. Seated postural neck and trunk reactions to sideways perturbations with or without a cognitive task
Open this publication in new window or tab >>Seated postural neck and trunk reactions to sideways perturbations with or without a cognitive task
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2015 (English)In: Journal of Electromyography & Kinesiology, ISSN 1050-6411, E-ISSN 1873-5711, Vol. 25, no 3, 548-556 p.Article in journal (Refereed) Published
Abstract [en]

Driving on irregular terrain will expose the driver to sideways mechanical shocks or perturbations that may cause musculoskeletal problems. How a cognitive task, imposed on the driver, affects seated postural reactions during perturbations is unknown. The aim of the present study was to investigate seated postural reactions in the neck and trunk among healthy adults exposed to sideways perturbations with or without a cognitive task. Twenty-three healthy male subjects aged 19-36 years, were seated on a chair mounted on a motion system and randomly exposed to 20 sideways perturbations (at two peak accelerations 5.1 or 13.2 m/s2) in two conditions: counting backwards or not. Kinematics were recorded for upper body segments using inertial measurement units attached to the body and electromyography (EMG) was recorded for four muscles bilaterally in the neck and trunk. Angular displacements (head, neck, trunk and pelvis) in the frontal plane, and EMG amplitude (normalised to maximum voluntary contractions, MVC) were analysed. The cognitive task provoked significantly larger angular displacements of the head, neck and trunk and significantly increased EMG mean amplitudes in the upper neck during deceleration, although 10% of MVC was never exceeded. A cognitive task seems to affect musculoskeletal reactions when exposed to sideways perturbations in a seated position.

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
Sideway perturbation, Postural reaction, EMG, Dual task, Neck
National Category
Physiotherapy
Research subject
Physiotherapy
Identifiers
urn:nbn:se:umu:diva-103258 (URN)10.1016/j.jelekin.2015.03.002 (DOI)000354026200018 ()25843010 (PubMedID)
Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2017-12-04Bibliographically approved
2. Adaptation of postural reactions in seated positions and influence of head posture when exposed to a single sideway perturbation: relevance for driving on irregular terrain
Open this publication in new window or tab >>Adaptation of postural reactions in seated positions and influence of head posture when exposed to a single sideway perturbation: relevance for driving on irregular terrain
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2016 (English)In: Journal of Novel Physiotherapy and Physical Rehabilitation, ISSN 2455-5487, Vol. 3, no 1, 022-029 p.Article in journal (Refereed) Published
Abstract [en]

 Background and objectives: Mechanical perturbations in seated positions caused by driving on irregular terrain destabilize the driver which, combined with the drivers’ posture, may cause musculoskeletal disorders. The aim of this study was to investigate adaptation and the effect of different head postures on seated postural reactions caused by perturbations. 

Materials and Methods: Twenty healthy male participants, aged 18-43 years, were tested on a movable platform delivering 15 sideways perturbations (peak acceleration 13.3 m/s2) while the participants held their head in a neutral or a laterally flexed posture. Surface electromyography (EMG) signals were recorded bilaterally in upper neck, trapezius, erector spinae and external oblique, while kinematics were recorded with inertial sensors for the head, trunk and pelvis. EMG amplitudes, muscle onset latencies and angular displacements in the frontal plane were analyzed. 

Results: In the neutral position, the EMG amplitudes and neck angular displacements significantly decreased by 0.2% and more than 1.6° respectively after repeated perturbations. Muscle onset latencies remained unchanged. During lateral flexion of the head, the EMG amplitudes decreased by 0.5% but the muscular onset latencies increased by more than 9 ms. 

Conclusion: The developed neuromuscular strategy seem to prefer a reduced EMG amplitude. The modest size of the postural reactions during the conditions presented here do not by themselves explain the musculoskeletal disorders found in drivers.

Keyword
Postural balance, Posture, Electromyography, Musculoskeletal pain, Neck muscles
National Category
Physiotherapy
Research subject
Physiotherapy; injury prevention
Identifiers
urn:nbn:se:umu:diva-117165 (URN)10.17352/2455-5487.000031 (DOI)
Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2017-05-29Bibliographically approved
3. Greater seated postural reactions are provoked by double-sided compared to single-sided mechanical shocks
Open this publication in new window or tab >>Greater seated postural reactions are provoked by double-sided compared to single-sided mechanical shocks
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Study design: Human volunteers were exposed experimentally to single-sided mechanical shocks (SSMS) and double-sided mechanical shocks (DSMS) while seated.

Objective: To describe seated postural reactions due to SSMS and DSMS in healthy male adults.

Summary of Background Data: Mechanical shocks, caused when driving on irregular terrain are suggested to be hazardous to the spine and may be associated with the reported musculoskeletal disorders of the back and neck among professional drivers.

Methods: Twenty healthy male subjects (18-43 years old) were exposed while seated to 5 SSMS and 15 DSMS in lateral directions. The second acceleration in the DSMS was in the opposite direction to the first acceleration and divided into fast, medium and slow depending on the speed of direction change. Surface electromyography (EMG) was recorded in upper neck, trapezius, erector spinae and external oblique and kinematics were recorded with inertial sensors for the neck, trunk and pelvis. Muscle activity evoked by the shocks was normalized to maximum voluntary contractions (MVC).

Results: The EMG amplitudes were significantly higher (0.6-1%; p<0.001) for the fast DSMS compared to all other shocks. Range of motions of the neck and trunk were greater from the DSMS compared to the SSMS. Muscle activity in the erector spinae and upper neck was sparse while the most intense muscle activity was found in the external obliques > 10% MVC, with elements of co-contraction.

Conclusion: Fast DSMS in lateral directions appear more demanding compared to SSMS and increase seated postural reactions, especially activity in the external oblique muscles. Still, the extent of range of motions in the neck and trunk and muscle activities alone, do not suggest a high risk for musculoskeletal overload.

Keyword
Postural balance, Posture, Electromyography, Musculoskeletal pain, Whole-body vibration
National Category
Physiotherapy
Research subject
Physiotherapy
Identifiers
urn:nbn:se:umu:diva-117167 (URN)
Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2016-02-25
4. Inter- and intra-tester reliability when measuring seated spinal postures with inertial sensors
Open this publication in new window or tab >>Inter- and intra-tester reliability when measuring seated spinal postures with inertial sensors
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2014 (English)In: International Journal of Industrial Ergonomics, ISSN 0169-8141, E-ISSN 1872-8219, Vol. 44, no 5, 732-738 p.Article in journal (Refereed) Published
Abstract [en]

Prolonged awkward sitting postures may be associated with neck or back pain, but it is often unclear which specific postures cause most problems and which mechanisms that may underlie the pain. In order to increase the knowledge in this field, it seems crucial first of all to be able to analyse, in depth, different seated spinal postures. A problem is however the lack of reliable and direct measurement methods of the posture, especially for sitting. Recently developed systems with inertial sensor attached along the spine have potential for this purpose. The aim of the present study was therefore to test the reliability of using such a system to assess various seated postures. Inter- and intra-tester as well as intra-subject relative reliability was estimated with intra-class correlation coefficient (ICC). Absolute reliability was estimated with standard error of measurement (SEM) and smallest detectable change (SDC). Ten + ten healthy subjects and four testers participated. Three standardised unsupported seated postures (lumbar lordosis, lumbar kyphosis and neutral posture) and two standing postures (neutral and lumbar kyphosis) were evaluated using five sensors attached to the head, the thorax (high and low), the lumbar spine and the pelvis. The ICC for intra-tester reliability ranged from 0.37 to 0.90, SEM 2.5-12.0 degrees, and SDC 7.1-333 degrees where the largest measurement error was from the head. Intra-tester reliability was higher than inter-tester reliability but not as good as intra-subject reliability. The intra-tester absolute reliability was nevertheless not considered sufficient to distinguish smaller differences. The low reliability may depend on inertial sensor size and attachment but also on the tester's accuracy. This study shows that assessing unsupported seated spinal postures with inertial sensors could be performed with higher reliability if done by the same, rather than different, testers. Relevance to industry: Prolonged awkward seated postures at work may be associated with back and neck pain and should therefore be analysed. Inertial sensor units is a promising tool to measure spinal posture. Smaller sensors attached by one skilled tester directly onto the body will most likely improve assessment in the future. (C) 2014 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Biomechanics, Human engineering, Observer variations
National Category
Physiotherapy
Identifiers
urn:nbn:se:umu:diva-96963 (URN)10.1016/j.ergon.2014.06.002 (DOI)000344439600015 ()2-s2.0-84906734375 (Scopus ID)
Available from: 2014-12-11 Created: 2014-12-05 Last updated: 2017-12-05Bibliographically approved

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