... and Research Centre, Institute for Kinesiology Research, Koper, Slovenia ..... for health promotion at working place in 2013-2014 by the Health Insurance.
7th International Scientific Conference on Kinesiology, 2014, Opatija, Croatia
COMPARISON OF TRUNK MUSCLES’ ACTIVATION AND PERCEIVED SITTING DISCOMFORT ON A STANDARD OFFICE CHAIR AND A NOVEL ACTIVE CHAIR® Mitja Gerževič1 and Veronika Mikuletič2 University of Primorska, Science and Research Centre, Institute for Kinesiology Research, Koper, Slovenia University of Primorska, Faculty of Mathematics, Natural Sciences and Information Technologies, Applied Kinesiology, Koper, Slovenia
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Abstract The purpose of this study was to evaluate differences in trunk muscles’ activation and perceived sitting discomfort during prolonged office-computer work on a standard office chair and a chair with unstable seat Active Chair®. Fourteen healthy volunteers (6 men, 8 women), aged 22.4 ± 2.2 years, body mass 65.0 ± 11.8 kg; body height 169.9 ± 11.6 m, performed four 15-minute long computerized office tasks. While performing the tasks electromyography (EMG) activity of eight trunk muscles (left and right lower trapezius (LT), erector spinae (ES), rectus abdominis (RA) and obliquus externus (OE)) and the overall level of perceived sitting discomfort at the beginning and after each task were measured and analyzed. The results showed that the average EMG activity of back and abdominal muscles during 1 hour of work on the Active Chair® was only 1.35% of the maximum voluntary contraction (MVC) and was not significantly different from the standard chair (p = .323). On the Active Chair® the right ES activity was significantly lower (p = .002), while the activity of the left OE was significantly higher (p = .026) compared to the standard office chair. The overall level of perceived sitting discomfort on the Active Chair® at the beginning (40.2 ± 26.0 mm; p = .001) and at the end (65.7 ± 25.7 mm; p < .001) of 1-hour work was significantly higher compared to the standard office chair at the end of 1-hour work (9.1 mm). However, the discomfort level on the Active Chair® increased significantly only after 45 minutes (61.0 ± 20.9 mm; p = .024). It could be concluded that sitting on the Active Chair® increases the activity of some muscles, but it is relatively low considering muscles’ activity during MVC. The overall level of sitting discomfort on the Active Chair® is higher than on a standard office chair, which is somehow expected for these types of chairs, yet it could still be improved.
Biomechanics and Motor Control
Key words: active sitting, electromyography, unstable seat, computerized office work
Introduction With the rise in sedentary work and all the negative consequences that could arise from it influencing human locomotor apparatus, the importance of workplace exercise, physical activity and training aids for their implementation are increasingly emphasized. Among many, sitting on stability balls or chairs with unstable seats are often recommended and promoted as “active sitting”, an alternative to a standard office chair and an efficient way to prevent and reduce low back pain as well as to increase muscle work, strength and blood flow. However, little research has been done to evaluate this (Gregory, Dunk & Callaghan, 2006; McGill, Kavcic & Harvey, 2006; Schult et al., 2013). Therefore, it was the purpose of this study to evaluate differences in trunk muscles’ activation and perceived sitting discomfort during prolonged office-computer work on a standard office chair and a novel chair with unstable seat Active Chair®. There were three main objectives: i) to evaluate the differences in the activity of back and abdominal muscles during 1 hour of computerized office work on a standard chair and the Active Chair®, ii) to identify if there are differences in the overall level of perceived sitting discomfort between the two chairs and iii) when the level of perceived discomfort on the Active Chair® during 1-hour working time significantly increases. Methods Fourteen healthy adult volunteers (6 men, 8 women), aged between 19 and 28 years (Table 1), randomly performed four 15-minute long computerized office tasks on the Active Chair® (Figure 1a). The tasks included: i) typing, ii) computeraided design, iii) typing/mouse combined work, and iv) reading from the monitor (Figure 1b). Table 1: Sample’s basic characteristics (average ± 1 SD)
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Gender
n
Age (years)
Mass (kg)
Height (cm)
Male
6
22.5 ± 0.5
76.0 ± 3.2
181.7 ± 4.7
Female
8
22.3 ± 3.0
56.8 ± 8.5
161.1 ± 5.1
Total
14
22.4 ± 2.2
65.0 ± 11.8
169.9 ± 11.6
Fundamental and Applied Kinesiology – Steps Forward
Before and immediately after each 15-minute task the participants rated the overall level of perceived sitting discomfort on a 100-millimeter Visual Analogue Scale (VAS; Hawker et al., 2011), where 0 mm stood for very comfortable and 100 mm for very uncomfortable. During the tasks bipolar surface electromyography (EMG) signal of eight muscles (lower trapezius (LT), erector spinae (ES), rectus abdominis (RA) and obliquus externus (OE)) was recorded on the left and right side. Two self-adhesive disposable Ag/AgCl electrodes Ambu BlueSensor N (Ambu A/S, Ballerup, Denmark) were placed on each muscle’s belly according to SENIAM recommendations (Hermens et al., 1999; Figure 1c) with the reference electrode on the anterior superior iliac spine. Raw EMG signals were sampled at 3000 Hz and band-pass filtered from 10 Hz to 500 Hz with a Butterworth filter using an 8-channel telemetric system TeleMyo 2400T G2 (Noraxon U.S.A. Inc., Scottsdale, USA). The EMG data were then processed off-line, using MyoResearch XP Clinical Application Protocol 1.07 software (Noraxon U.S.A. Inc., Scottsdale, USA): i) passed through a bi-directional Butterworth filter with a cut-off frequency of 2.5 Hz, ii) rectified and ii) smoothed using root mean square (RMS) on a time window of 50 ms to produce a linear envelope for each eight muscles. The data were normalized to the maximum average RMS value on a 500-ms interval during maximum voluntary contraction (MVC) performed in the position for crunches (supine, bended knees, arms crossed on the chest, flexed trunk ~30°), rotation crunches (supine, bended knees, arms crossed on the chest, flexed trunk ~30° and rotated left/right side) and trunk extension (trunk over the bench in line with legs (0°), palms behind the neck, elbows in line with shoulders). Average RMS values (expressed as% MVC) of each muscle and each 15-minute interval, while performing the computerized office tasks were considered for further analysis.
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Figure 1: Representation of a) a novel chair with unstable seat (Active Chair®), b) a computerized office task while measuring EMG activity on the Active Chair® and c) EMG electrodes positions for left and right lower trapezius and erector spinae (upper picture) and rectus abdominis and obliquus externus (lower picture) muscles.
The measuring protocol was carried out along the lines of Gregory, Dunk & Callaghan (2006), who compared muscle activation, lumbar spine posture and perceived discomfort during 1 hour of computerized office work on a therapeutic ball with the standard office chair. Thus, using one-sample t-test, the data of muscles’ activation and the level of perceived sitting discomfort on the Active Chair® were compared to the data obtained for the standard office chair by Gregory, Dunk & Callaghan (2006). Differences in the scores of perceived sitting discomfort on the Active Chair® in time were analyzed with ANOVA for repeated measurements. If there were significant differences Bonferroni correction was used for posthoc analysis. All analyses were performed using the IBM SPSS Statistics 20.0 software (IBM Corporation, Armonk, New York, USA) and the statistical significance was set at the p-level < .05 (two-tailed).
Results All muscle activation parameters (average RMS values) were not normally distributed (p < .05), except the RMS of the right ES muscle. Therefore, instead of a parametric one-sample t-test, a Wilcoxon’s non-parametric one-sample t-test was used. However, the sitting discomfort parameters were normally distributed (p > .05). The analyses showed the overall average muscle activation for all four 15-minute tasks and all eight muscles was 1.35 ± 0.54% on the Active Chair® which was not significantly different (p = .323) compared to the standard chair (1.07 ± 0.48%; Gregory, Dunk & Callaghan, 2006). On the Active Chair® the activation of right ES muscle was significantly lower (p = .002), while that of the left OE muscle was significantly higher (p = .026) in comparison with the standard office chair (Figure 2), but the effect size was large only for the left OE muscle (Cohen’s d = 2.0).
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Biomechanics and Motor Control
a)
7th International Scientific Conference on Kinesiology, 2014, Opatija, Croatia
Biomechanics and Motor Control
Figure 2: Average muscle activation (RMS), expressed as% of maximal voluntary contraction for right and left rectus abdominis (RA), obliquus externus (OE), erector spinae (ES), and lower trapezius (LT) during 1-hour computerized office work on the Active Chair® (dark grey bars) and the standard office chair (light grey bars). Values for the standard office chair were taken from the study Gregory, Dunk & Callaghan (2006). Significant differences are marked with asterisks: * – p < .05, ** – p < .01.
The level of perceived sitting discomfort on the Active Chair® increased with time (Figure 3a). Compared to the initial value (40.2 ± 26.0 mm) the level of discomfort increased significantly after 45 minutes (61.0 ± 20.9 mm; p = .024) and 60 minutes (65.7 ± 25.7 mm; p = .001) of work with large effect sizes (d0-45min = .81 and d0-60min = .98). The discomfort level after 1 hour of computerized office work on the standard chair (9.1 mm) showed to be significantly lower than that on the Active Chair®, both at the beginning (p = .001) and at the end (p < .001) of 1-hour work (Figure 3b).
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b)
Figure 3: Representation of a) the increase in the perceived sitting discomfort with the increase in working time on the Active Chair® before (time 0 min) and immediately after each 15-minute task (time 15, 30, 45 and 60 min) of computerized office work and b) the differences in the perceived sitting discomfort level between the Active Chair® at the beginning (start) and at the end (end) of 1-hour work and the discomfort level on a standard office chair after 1-hour work (end). Values for the standard office chair were taken from the study Gregory, Dunk & Callaghan (2006). Significant differences are marked with asterisks:* – p < 0.05, ** – p < 0.01, *** – p < 0.001.
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Fundamental and Applied Kinesiology – Steps Forward
Discussion and conclusion The present study showed that the overall average muscle activation during 1 hour of computerized office work on the Active Chair® (1.35% MVC) is low and it is not significantly different from the activation on the standard office chair. This is in accordance with similar studies (Gregory, Dunk & Callaghan, 2006; McGill, Kavcic & Harvey, 2006) where sitting/working on standard (stable) chairs was compared with stability balls. The advantage of the latter showed to be only in a reduced pelvic tilt (Gregory, Dunk & Callaghan, 2006), which could be assumed also for the Active Chair®, but unfortunately spine kinematics was not measured in the present study. The Active Chair® showed to induce a significantly greater activation of left OE muscle, which could be a consequence of unilateral mouse work with the right hand (all participants were right-handed) and it was generally much more uncomfortable than the standard office chair already at the beginning of the work. The discomfort increased with working time, reaching significant difference after 45 minutes. These results are in accordance with the before mentioned older studies as well as with the latest one (Grooten et al., 2013), where postural sway, sway velocities and muscle activation, surprisingly, showed to be smaller during “active sitting” (chair without backrest) and standing compared to standard office chair condition. Therefore, it could be concluded that from the muscle activation perspective the Active Chair® do not have any advantages compared to standard office chairs. Namely, muscle activation does not reach even a minimal value typical for normal walking (5–80% MVC). Safety and well-being are also questionable due to the higher level of sitting discomfort on the Active Chair®, which is on the other hand expected for these types of chairs, yet it still could be improved with optional height adjustment of the seat, possibility of fixing the seat as well as changing position during sitting.
References 1. Hawker, G.A., Mian, S., Kendzerska, T. & French, M. (2011). Measures of adult pain: Visual Analog Scale for Pain (VAS Pain), Numeric Rating Scale for Pain (NRS Pain), McGill Pain Questionnaire (MPQ), Short-Form McGill Pain Questionnaire (SF-MPQ), Chronic Pain Grade Scale (CPGS), Short Form-36 Bodily Pain Scale (SF-36 BPS), and Measure of Intermittent and Constant Osteoarthritis Pain (ICOAP). Arthritis Care & Research, 63(Suppl 11), S240–252. 2. Hermens, H.J., Freriks, B., Merletti, R., Hägg, G.G., Stegeman, D., Blok, J. et al. (1999). European recommendations for surface electromyography. Enschede: Roessingh Research and Development. 3. Gregory, D.E., Dunk, N.M. & Callaghan, J.P. (2006). Stability ball versus office chair: comparison of muscle activation and lumbar spine posture during prolonged sitting. Human Factors, 48(1), 142–153.
5. McGill, S.M., Kavcic, N.S. & Harvey, E. (2006). Sitting on a chair or an exercise ball: various perspectives to guide decision making. Clinical Biomechanics, 21(4), 535–560. 6. Schult, T.M., Awosika, E.R., Schmunk, S.K., Hodgson, M.J., Heymach, B.L. & Parker, C.D. (2013). Sitting on stability balls: biomechanics evaluation in a workplace setting. Journal of Occupational and Environmental Hygene, 10(2), 55–63. Acknowledgements This study was part of a project entitled “With active sitting to better health and higher working efficiency” which was financially approved within the Public call for co-financing projects for health promotion at working place in 2013-2014 by the Health Insurance Institute of Slovenia and co-financed by PFEIFER, Boris Pfeifer s.p.
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4. Grooten, W.J., Conradsson, D., Ang, B.O. & Franzén, E. (2013). Is active sitting as active as we think? Ergonomics, 56(8), 1304– 1314.
