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Greater seated postural reactions are provoked by double-sided compared to single-sided mechanical shocks
Umeå University, Faculty of Medicine, Department of Community Medicine and Rehabilitation, Physiotherapy.
Umeå University, Faculty of Medicine, Department of Radiation Sciences.
Umeå University, Faculty of Medicine, Department of Radiation Sciences.
Institutionen för skogens biomaterial och teknologi (SBT), Sveriges lantbruksuniversitet, Umeå.
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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 [en]
Postural balance, Posture, Electromyography, Musculoskeletal pain, Whole-body vibration
National Category
Physiotherapy
Research subject
Physiotherapy
Identifiers
URN: urn:nbn:se:umu:diva-117167OAI: oai:DiVA.org:umu-117167DiVA: diva2:905685
Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2016-02-25
In thesis
1. Seated postural reactions to mechanical shocks: laboratory studies with relevance for risk assessment and prevention of musculoskeletal disorders among drivers
Open this publication in new window or tab >>Seated postural reactions to mechanical shocks: laboratory studies with relevance for risk assessment and prevention of musculoskeletal disorders among drivers
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
Postural balance, Posture, Electromyography, Musculoskeletal pain, Whole-body vibration, Reliability, Kinematics, Biomechanics
National Category
Physiotherapy
Research subject
Physiotherapy
Identifiers
urn:nbn:se:umu:diva-117178 (URN)978-91-7601-410-3 (ISBN)
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

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Stenlund, Tobias
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PhysiotherapyDepartment of Radiation SciencesDepartment of Psychology
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