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.
- Milosavljevic S, McBride DI, Bagheri N, Vasiljev RM, Mani R, et al. (2011) Exposure to whole-body vibration and mechanical shock: a field study of quad bike use in agriculture. Ann Occup Hyg 55: 286-295 .
- Rehn B, Nilsson T, Olofsson B, Lundstrom R (2005) Whole-body vibration exposure and non-neutral neck postures during occupational use of all-terrain vehicles. Ann Occup Hyg 49: 267-275 .
- Waters T, Rauche C, Genaidy A, Rashed T (2007) A new framework for evaluating potential risk of back disorders due to whole body vibration and repeated mechanical shock. Ergonomics 50: 379-395 .
- Sandover J (1998) Behaviour of the spine under shock and vibration: a review. Clin Biomech 3: 249-256 .
- Peterson BW, Richmond FJ (1988) Control of Head Movement. Inc, New York Oxford 38: 1-322 .
- Keshner EA, Hain TC, Chen KJ (1999) Predicting control mechanisms for human head stabilization by altering the passive mechanics. J Vestib Res 9: 423-434 .
- Forssberg H, Hirschfeld H (1994) Postural adjustments in sitting humans following external perturbations: muscle activity and kinematics. Exp Brain Res 97: 515-527 .
- Vibert N, MacDougall HG, de Waele C, Gilchrist DPD, Burgess M, et al. (2001) Variability in the control of head movements in seated humans: A link with whiplash injuries? J Physiol 532: 851-868 .
- Keshner EA (2003) Head-trunk coordination during linear anterior-posterior translations. J Neurophysiol 89: 1891-901 .
- Masani K, Sin VW, Vette AH, Adam Thrasher T, Kawashima N, et al. (2009) Postural reactions of the trunk muscles to multi-directional perturbations in sitting. Clin Biomech 24: 176-182 .
- Preuss R, Fung J (2008) Musculature and biomechanics of the trunk in the maintenance of upright posture. J Electromyogr Kinesiol 18: 815-828 .
- Zedka M, Kumar S, Narayan Y (1998) Electromyographic response of the trunk muscles to postural perturbation in sitting subjects. J Electromyogr Kinesiol 8: 3-10 .
- St-Onge N, Côté JN, Preuss Ra, Patenaude I, Fung J (2011) Direction-dependent neck and trunk postural reactions during sitting. J Electromyogr Kinesiol 21: 904-912 .
- Kumar S, Ferrari R, Narayan Y (2004) Cervical muscle response to whiplash-type right lateral impacts. Spine 29: E479-487 .
- Kumar S, Ferrari R, Narayan Y (2005) Cervical muscle response to head rotation in whiplash-type left lateral impacts. Spine 30: 536-541 .
- Eklund J, Odenrick P, Zettergren S, Johansson H (1994) Head posture measurements among work vehicle drivers and implications for work and workplace design. (1994) Ergonomics 37: 623-639 .
- Rehn B, Nilsson T, Järvholm B (2004) Neuromusculoskeletal disorders in the neck and upper extremities among drivers of all-terrain vehicles – a case series. BMC Musculoskeletal Disorders 5: 1 .
- Siegmund GP, Sanderson DJ, Myers BS, Inglis JT (2003) Rapid neck muscle adaptation alters the head kinematics of aware and unaware subjects undergoing multiple whiplash-like perturbations. J Biomech 36: 473-482 .
- Blouin JS, Descarreaux M, Belanger-Gravel A, Simoneau M, Teasdale N (2003) Attenuation of human neck muscle activity following repeated imposed trunk-forward linear acceleration. Exp Brain Res 150: 458-464 .
- Donati P, Schust M, Szopa J, Starck J, Iglesias EG, et al. (2008) Workplace exposure to vibration in Europe: an expert review. 1-126 .
- Stenlund TC, Lundström R, Lindroos O, Häger CK, Burström L, et al. (2015) Seated postural neck and trunk reactions to sideways perturbations with or without a cognitive task. J Electromyogr Kinesiol 25: 548-556 .
- Konrad P, Schmitz K, Denner A (2001) Neuromuscular Evaluation of Trunk-Training Exercises. J Athl Train 36: 109-118 .
- Konrad P. The abc of emg. (2005) A practical introduction to kinesiological electromyography.
- Stensdotter AK, Hodges P, Ohberg F, Hager-Ross C (2007) Quadriceps EMG in open and closed kinetic chain tasks in women with patellofemoral pain. J Mot Behav 39: 194-202 .
- Hodges PW, Bui BH (1996) A comparison of computer-based methods for the determination of onset of muscle contraction using electromyography. Electroencephalogr Clin Neurophys 101: 511-519 .
- Granata KP, Slota GP, Bennett BC (2004) Paraspinal muscle reflex dynamics. J Biomech 37: 241-247 .
- Horak FB, Henry SM, ShumwayCook A (1997) Postural perturbations: New insights for treatment of balance disorders. (1997) Physical Therapy 77: 517-533 .
- Nashner LM, Cordo PJ (1981) Relation of automatic postural responses and reaction-time voluntary movements of human leg muscles. (1981) Exp Brain Res 43: 395-405 .
- Siegmund GP, Sanderson DJ, Myers BS, Inglis JT (2003) Awareness affects the response of human subjects exposed to a single whiplash-like perturbation. Spine 28: 671-679 .
- Olafsdottir JM, Brolin K, Blouin JS, Siegmund GP (2015) Dynamic spatial tuning of cervical muscle reflexes to multidirectional seated perturbations. Spine 40: E211-219 .
- Bazrgari B, Shirazi-Adl a, Kasra M (2008) Seated whole body vibrations with high-magnitude accelerations--relative roles of inertia and muscle forces. J Biomech 41: 2639-2646 .
Follow us on Academia.edu
Access denied for user 'root'@'localhost' (using password: YES)