Research Review By Drs. Demetry Assimakopoulos & Arnold Wong (lead author of this study)©

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Date Posted:

January 2020

Study Title:

Do Different Sitting Postures Affect Spinal Biomechanics of Asymptomatic Individuals?

Authors:

Wong AYL, Chan TPM, Chau AWM et al.

Author's Affiliations:

Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hong Kong Special Administrative Regio; Faculty of Medicine, Memorial University of Newfoundland, St. John’s, Canada.

Publication Information:

Gait & Posture 2019; 67: 230-235.

Background Information:

Low back pain (LBP) is an extremely common problem amongst office workers (1). More specifically, prolonged sitting is thought to be associated with low back and buttock pain, and discomfort associated with sitting may even predict future LBP (2, 3). However, there are controversial findings regarding whether sitting duration or sitting postures are related to the initial development of non-specific LBP (5-7). The discrepancy might be attributed to divergent study designs, sitting durations and/or measurement methods.

Prior biomechanical research has found that slouched sitting may induce viscoelastic creep of spinal tissues, which may desensitize mechanoreceptors in spinal tissues, alter the joint sense, overload passive spinal structures, and result in LBP (8, 9). Importantly, different sitting postures may have differential influences on the activation or deactivation of trunk muscles (stabilizers), which may amplify the adverse biomechanical effects of creep on passive spinal tissues after sitting. However, no studies have comprehensively investigated the short-term effects of common sitting postures on ensuing changes and recovery of pain and spinal biomechanics of asymptomatic individuals. As such, the authors of this study sought to compare the effects of three common sitting postures (slouched, upright and supported sitting with a backrest) on pain, lumbar ROM, proprioception and trunk muscle activity immediately following 20-minutes of sitting and 30-minutes of recovery.

Pertinent Results:

Pain Levels and EMG:
There were no significant demographic differences or significant temporal changes in pain amongst the 3-groups (remember, they were asymptomatic). There were also no significant interactions or temporal changes in surface EMG in the 3 sitting postures (slouched, upright or supported sitting with a backrest). However, significant between-group differences in bilateral internal oblique/transverse abdominus activity were identified. Post-hoc analysis showed that left internal oblique and transverse abdominus activity in slouched sitting and supported sitting was significantly lower than in upright sitting (P < 0.05). Similarly, average surface EMG activation of right internal oblique and transverse abdominus activation in slouched sitting was significantly lower than those measured in upright sitting or supported sitting (P = 0.04). Overall, the lowest surface EMG activity of bilateral internal oblique and transverse abdominus was identified during slouched sitting.

Lumbar flexion and ROM during sitting:
The average lumbar ROM during slouched, upright and supported sitting were on average 44.4%, 9.5% and 14.8% of the participants’ maximum lumbar flexion ROM, respectively. One-way ANOVA showed that the lumbar flexion angle in sitting significantly differed amongst the groups (P < 0.05). Post-hoc analysis showed that the lumbar flexion angle during slouched sitting was significantly larger than other sitting postures (P < 0.05). There was no significant interaction between time and group in lumbar ROM. There was also no group or between-group significant temporal change in lumbar active ROM.

Relative proprioception weighting (RPW):
Compared to people in the supported sitting group, those in the upright sitting group displayed significantly greater relative proprioceptive weighting (RPW; postural/proprioceptive control strategy) when standing on a fixed surface at all time points (more reliance on ankle proprioception than trunk proprioception for standing balance). Conversely, the RPW results while on the foam surface showed no significant interaction, time-effect or between-group effect.

Clinical Application & Conclusions:

This trial evaluated the impact of common sitting postures on spinal biomechanics after 20-minutes of sitting and a 30-minute recovery in asymptomatic individuals. A summary of the pertinent findings is found below:
  1. Slouched sitting elicited the lowest bilateral internal oblique/transverse abdominus muscle activity during sitting.
  2. Right internal oblique/transverse abdominus surface EMG activity during supported sitting was significantly lower than upright sitting.
  3. Participants in slouched sitting maintained the greatest amount of lumbar flexion angle.
  4. None of the sitting postures had a significant impact on LBP intensity, lumbar active ROM or RPW, except upright sitting displayed significantly greater RPW than supported sitting.
  5. Overall, there were no temporal changes in lumbar symptoms or biomechanics immediately following 20-minutes of sitting, or after 30-minutes of recovery.
The results of this study showed significantly lower bilateral internal oblique and transverse abdominus activity during slouched sitting compared to upright sitting. This reduction in activation during slouched sitting may impair lumbar stabilization. Still, in spite of this finding, no between-group differences in creep were identified after prolonged sitting, or after the 30-minute recovery period.

The fact that there were no between-group differences in creep during the 30-minute recovery period should however be interpreted with caution, as other studies have demonstrated a sustained level of elevated creep during recovery from slouched sitting. Slouched sitting participants in this study demonstrated lower percentages of full lumbar flexion during sitting (44.4% as indicated above), thus potentially limiting the accumulation of creep. Since creep is a time-dependent variable, there is the potential that longer durations of slouched sitting may induce these potentially negative changes; future studies should be mindful of these variables.

The authors were not surprised that there were no significant temporal changes in the postural control strategy (RPW) between the sitting groups, given the lack of post-sitting creep in spinal tissues. Their findings highlight that a short-period of slouched sitting is unlikely to affect trunk proprioception. Interestingly, participants in upright sitting relied more on ankle-steered postural control compared to those in the supported sitting group. However, these observed differences might only reflect coincidental variations in participants’ RPW.

This study demonstrated that a short duration (20-minutes) of static sitting in different postures does not appear to alter spinal biomechanics in asymptomatic individuals. Future research must investigate the maximum amount of sitting duration in various spinal postures that can be tolerated without adversely affecting spinal biomechanics in people with or without LBP. These future studies should also investigate the optimal combination of sitting and break durations to help develop ergonomic recommendations for office workers.

Study Methods:

Twenty-one males and 16 females with no current or past history of LBP requiring sick leave in the previous 12-months were recruited from the Hong Kong Polytechnic University. Subjects were excluded if they had scoliosis, spinal surgery, vestibular dysfunction, neurological disorders, or lumbosacral/lower limb MSK pathology over the last 12-months. Self-reported pain intensity was measured on an 11-point numeric pain scale (0 [no pain] – 10 [worst pain imaginable] scale). The participants provided verbal pain ratings at baseline, immediately after sitting and 30-minutes after sitting.

Surface EMG (sEMG) was utilized to measure activity of the bilateral internal obliques, transverse abdominus, external obliques and lumbar erector spinae. sEMG signals from each muscle were expressed as a percentage of maximum voluntary contraction (%MVC). The authors evaluated temporal changes in sEMG by dividing the 20-minute sitting period into 3 intervals of 400 seconds, and the mean normalized sEMG activity in each interval was calculated for the analysis. Two 5-second isometric MVC trials were performed for each target muscle against manual resistance with a 2-minute rest between trials. Following signal processing, root mean squared sEMG signals of each muscle during the middle 2-seconds of MVC was calculated. The highest root mean squared EMG amplitude of each muscle during the MVCs was used for subsequent normalization.

Lumbar kinematics (flexion and extension) were measured by a wireless 3D kinematic analysis system at a sampling frequency of 100 Hz. Two inertial measurement units (IMUs) were attached to the skin overlying T12 and S1 SP’s to calculate the relative lumbar angle by measuring their orientations compared to a reference posture (upright standing with ear lobe aligned to AC joint and feet shoulder width apart).

Proprioception/postural control was measured using centre of pressure (COP) displacement during standing using a piezoelectric force plate. The reliance on trunk or ankle proprioceptive inputs for postural control was evaluated through the use of muscle vibration to stimulate muscle spindles. This stimulation is purported to generate an illusion of muscle lengthening that may alter proprioceptive sense (4). Two pairs of muscle vibrators were placed on the L5 multifidi (LM) and triceps surae (TS). Frequency and amplitude were set at 60 Hz and 0.5 mm respectively to optimize the illusion of muscle lengthening. The vibration of the LM in standing is purported to give an illusion of posterior pelvic rotation, which causes an individual to shift COP forward to avoid falling. TS vibration generates a forward leaning illusion, which causes the person to lean backward. The magnitude of COP displacement indicates the relative importance of proprioceptive signals of the vibrated muscles in informing the brain to maintain balance. The LM and TS were separately stimulated in 4 different conditions described below.

To assess the postural/proprioceptive control strategy utilized, the participant stood in an immobile relaxed barefoot position on the force plate. Feet were 10 cm apart, and their arms hung loosely at their sides. The foot position was marked on the force plate to standardize the placement throughout the repeated measures. The participant was blindfolded and tested 4-times for 1-minute each. The participant stood on the force plate directly in conditions 1 and 2, and on a piece of foam placed on top of the force plate in conditions 3 and 4.

Each participant provided demographic data, LBP intensity, and performed MVCs for the 6 trunk muscles. The blinded examiner measured baseline kinematics in standing and proprioceptive strategy. Following these assessments, participants were randomly assigned to either slouched, upright or supported sitting with a backrest. The slouched position was created by relaxing the trunk into flexion by rotating the pelvis posteriorly, while the upright position involved rotating the pelvis anteriorly to maintain a neutral lumbar lordosis with a relaxed thorax. The supported sitting condition was maintained by leaning the upper body against a backrest with a lumbar support. All participants sat in the assigned postures on an adjustable office chair with hips and knees in 90 degrees of flexion. During the 20-minute trial, trunk inclination angles and trunk muscle activity were measured by IMUs and surface EMG.

Study Strengths / Weaknesses:

Strengths:
  • The researchers clearly stated that the findings of their study were poorly generalizable to clinical populations and encouraged additional research into this domain. In spite of this, they reassured readers that up to 20-minutes of slouched sitting with a relatively small lumbar flexion angle has no ill-effect on the lumbar spine compared to other sitting postures in asymptomatic individuals.
Weaknesses:
  • Small(ish) sample size (recognizing the complexity of measuring what they did!).
  • Since there exists considerable redundancy in trunk muscle recruitment, spatial variability in muscle activity recruitment may have been missed. This might have been corrected by use of high-density surface EMG electrode arrays to monitor temporal and spatial changes in trunk muscle activity during sitting.
  • The participants were asymptomatic, and as such, these recommendations cannot be extrapolated to individuals with back pain (although the next logical step would be to repeat this methodology in that patient population).

Commentary from Dr. Arnold Wong (lead author):

These findings reveal that although the obliquus internus/transversus abdominis muscles were less active during slouched sitting, 20 minutes of slouched sitting, upright sitting, and supported sitting have no significant immediate adverse effects on LBP, lumbar active ROM and proprioception. However, it remains unclear if longer durations of sitting may affect these outcomes. Future research should investigate the effects of different sitting and break durations on people with and without LBP so that proper ergonomic recommendations/ standards for office workers can be established.

Dr. Arnold Wong received his PhD in Rehabilitation Sciences from the University of Alberta and is currently working as an Assistant Professor in the Department of Rehabilitation Sciences at The Hong Kong Polytechnic University. His research interests focus on improving the diagnosis and treatments for people with low back pain, lumbar spinal stenosis, and scoliosis.

Additional References:

  1. Spyropoulos P et al., Prevalence of low back pain in Greek public office workers. Pain Physician 2007; 10 (5): 651–659.
  2. Søndergaard K.H.E. et al., The variability and complexity of sitting postural control are associated with discomfort. J Biomech 2010; 43 (10): 1997–2001.
  3. Hamberg-Van Reenen H.H. et al., Does musculoskeletal discomfort at work predict future musculoskeletal pain? Ergonomics 2008; 51 (5): 637–648.
  4. Cordo P et al. Effect of slow, small movement on the vibration-evoked kinesthetic illusion. Exp Brain Res 2005; 167 (3): 324–334.
  5. De Carvalho D, de Luca K, Funabashi M, et al., The Relationship Between Objectively Measured Sitting time and Back Pain: A Systematic Review and Meta-Analysis. J Manipulative Physiol Therap 2019 [accepted].
  6. Roffey DM, Wai EK, Bishop P, et al., Causal assessment of occupational standing or walking and low back pain: results of a systematic review. Spine J 2010; 10: 262–272.
  7. Waongenngarm P, Areerak K, Janwantanakul P., The effects of breaks on low back pain, discomfort, and work productivity in office workers: a systematic review of randomized and non-randomized controlled trials. Appl Ergon 2018; 68: 230–239.
  8. Gedalia U, Solomonow M, Zhou BH, et al. Biomechanics of increased exposure to lumbar injury caused by cyclic loading: Part 2. Recovery of reflexive muscular stability with rest, Spine 1999; 24: 2461-2461.
  9. McGill SM, Brown S. Creep response of the lumbar spine to prolonged full flexion. Clin Biomech 1992; 7: 43–46.