Tobias C Stenlund1*, Fredrik Ohberg2, Ronnie Lundstrom2, Ola Lindroos3, Charlotte K Hager1, Gregory Neely4 and Borje Rehn1
1Department of Community Medicine and Rehabilitation, Physiotherapy, Umea University, Sweden
2Department of Radiation Sciences, Biomedical engineering, Umea University, Sweden
3Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences, Umea, Sweden
4Department of Psychology, Umea University, Sweden
Received: 21 June, 2016; Accepted: 01 July, 2016; Published: 02 July, 2016
Tobias Stenlund, Department of Community Medicine and Rehabilitation, Physiotherapy, Umea University, 90187 Sweden, Tel: +46907868040; E-mail:
Stenlund TC, Ohberg F, Lundstrom R, Lindroos O, Hager CK, et al. (2016) 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. J Nov Physiother Phys Rehabil 3(1): 022-029. DOI: 10.17352/2455-5487.000031
© 2016 Stenlund TC, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Electromyography; Musculoskeletal pain; Neck muscles; Postural balance; Posture
EMG: Electromyography; IMUs: Inertial Measurement Units; MVC: Maximum Voluntary Contractions; UN: Upper Posterior Neck Muscles; UT: Upper Trapezius; ES: Erector Spinae Lumbar Level; EO: External Oblique; E0: (Epoch 0) Time Window 150-0 ms before the Perturbation; E1: (Epoch 1) Time Window 0-150 ms after the Perturbation; E2: (Epoch 2) Time Window 150-300 ms after the Perturbation
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.
Mechanical perturbations in vehicles, caused by driving on irregular terrain, are transmitted to the body of the seated driver. For drivers of certain vehicles, such as forest machines and quad bikes, there can be a substantial exposure to perturbations in the sideway directions [1,2]. Perturbations are suggested to be hazardous to the spine [3,4] even though few studies have analyzed and reported adverse consequences.
The spine has to be stable to counteract for mechanical perturbations and in order to maintain equilibrium. The stabilization is achieved by the postural control system which is dependent upon intact sensory information to generate compensatory muscle reactions. If the postural control system does not adapt properly there may be potential hazards for musculoskeletal tissues. Low back pain is frequently reported among professional drivers exposed to perturbations [3,4].
Postural reactions in the neck are more sparsely studied. The head-neck system is a complex biomechanical linkage with at least 20 pairs of muscles rendering a range of opportunities in stabilizing the head . The control mechanism for head stabilization depends on voluntary muscle mechanisms, postural reflexes, and passive mechanical (i.e. inertial, viscous, and elastic) properties [6,7]. The initial detectable movement caused by a perturbation occurs closest to the contact point and propagates further on to more distal body parts [7,8]. Therefore, the movements start in the pelvis segment followed by the trunk and head. It has been suggested that the head and trunk reactions initially rely on passive mechanical properties and signals from segmental proprioceptors .
The muscle reaction from a perturbation has been shown to be direction dependent in the trunk [10-12] and neck [8,13]. The muscle reaction to a sideways perturbation has been suggested to have a reciprocal activation pattern in the neck, starting in the contralateral muscle that stretches first [8,14]. The EMG amplitude in the neck region has been reported to be high, especially in the contralateral splenius capitis and is therefore most likely to be injured . Further, the initial posture has been reported to influence the nature of the postural reactions, e.g. a head rotation reduces the EMG amplitude in the upper neck . A head posture divergent from neutral is common as a result of work demands , with large variations in time spent in that position assumed depending on the work, driver and vehicle . We have not found any study that has investigated the influence of lateral flexion of the head on postural muscles or kinematic reactions from sideways perturbations.
Seated postural reactions, other than those caused by perturbations in fore and aft directions, are scarcely studied. However, sideways perturbations have been suggested to cause two types of reaction strategies, stiff and sloppy. Both reactions were found to be stereotypical, using either muscle co-contractions (stiff strategy) or a reciprocal more relaxed muscle activity (sloppy strategy) . A strict stiff strategy might cause muscle fatigue or myalgia while a relaxed strategy, depending on passive structures, might increase the risk for injury in joint structures and tissues. Contrary to Vibert et al. (2001), who found no or little adaptation in sideways reactions, seated postural reactions in the forward direction have been found to adapt after the first perturbation with decreased EMG amplitude over time [18,19]. If seated postural reactions in sideways directions adapts or not is still an unanswered question. Based on this background we hypothesized that EMG amplitudes would be reduced after repeated perturbations and that different head postures would affect onset latencies and EMG amplitudes.
Therefore the aims of this study were to explore if seated postural neck and trunk reactions in healthy men adapt during repeated sideways perturbations and whether different head postures influence the results.
Materials and Methods
Twenty healthy males, age 27.5 ± 4.1 years, height 1.81 ± 0.07 m, body mass index (BMI) 24.5 ± 4.1 kg/m2, participated. They were recruited among staff and students at Umea University, Sweden. Young male participants were targeted as they are representative of the majority of professional drivers  and to decrease the risk for degeneration and rigidity of the spine. Exclusion criteria were any reported neurological conditions or reduced ability to perform daily routines during the last 12 months because of back or neck problems. Written informed consent was obtained from each participant and the study was approved from Regional Ethical Review Board in Umea (Dnr 2014-228-32M).
This study used a repeated-measurement design with the participants exposed to 15 sideways perturbations in total (Figure 1). All perturbations were delivered from the participants’ right side while the participant sat with the neck either in a neutral position or approximately 15° laterally flexed to the right or to the left.
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