Lumbar posture and muscular activity while sitting during office work

Journal of Electromyography and Kinesiology 23 (2013) 362–368

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Lumbar posture and muscular activity while sitting during office work Falk Mörl a,⇑, Ingo Bradl b,c a

Forschungsgesellschaft für angewandte Systemsicherheit und Arbeitsmedizin mbH, Dubliner Straße 12, 99091 Erfurt, Germany German Social Accident Insurance Institution for the Foodstuffs and Catering Industry, Department of Prevention, Biomechanics, Dubliner Straße 12, 99091 Erfurt, Germany c University Hospital Jena, Clinic for Trauma-, Hand- and Reconstructive Surgery, Division for Motor Research, Pathophysiology and Biomechanics, 07740 Jena, Germany b

a r t i c l e

i n f o

Article history: Received 20 June 2012 Received in revised form 11 September 2012 Accepted 2 October 2012

Keywords: Lumbar spine Long term EMG Sitting Office work Lordosis Lumbar posture

a b s t r a c t Purpose: Field study, cross-sectional study to measure the posture and sEMG of the lumbar spine during office work for a better understanding of the lumbar spine within such conditions. Scope: There is high incidence of low back pain in office workers. Currently there is little information about lumbar posture and the activity of lumbar muscles during extended office work. Methods: Thirteen volunteers were examined for around 2 h of their normal office work. Typical tasks were documented and synchronised to a portable long term measuring device for sEMG and posture examination. The correlation of lumbar spine posture and sEMG was tested statistically. Results: The majority of time spent in office work was sedentary (82%). Only 5% of the measured time was undertaken in erect body position (standing or walking). The sEMG of the lumbar muscles under investigation was task dependent. A strong relation to lumbar spine posture was found within each task. The more the lumbar spine was flexed, the less there was activation of lumbar muscles (P < .01). Periods of very low or no activation of lumbar muscles accounted for about 30% of relaxed sitting postures. Conclusion: Because of very low activation of lumbar muscles while sitting, the load is transmitted by passive structures like ligaments and intervertebral discs. Due to the viscoelasticity of passive structures and low activation of lumbar muscles, the lumbar spine may incline into de-conditioning. This may be a reason for low back pain. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Office work and sitting at a desk for longer periods is common for people in western civilisation. Orthopaedists and physical therapists assume de-conditioning of the trunk and lumbar spine structures due to long-term sitting without longer active periods of standing, walking or running. This de-conditioning may be a reason for low back pain and accelerated degeneration of lumbar spine structures. Looking at the incidence of low back pain and the inability to work because of low back pain in office workers confirms this assumption (Burdorf et al., 1993; Hemingway et al., 1997; Janwantanakul et al., 2008; Juul-Kristensen and Jensen, 2005; Juul-Kristensen et al., 2004; Riihimäki et al., 1989, 1994; Spyropoulos et al., 2007; Törner et al., 1991; Videman and Battie, 1999). In summary, there is high prevalence of low back pain in office workers with the risk of getting low back pain comparable to more demanding work. However, there is currently little information available about the behaviour of the lumbar spine over long periods because of a lack of adequate measurement devices. In laboratory settings, no coherence of lumbar flexion angle and lumbar ⇑ Corresponding author. E-mail address: [email protected] (F. Mörl). 1050-6411/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jelekin.2012.10.002

muscle activity was found (O’Sullivan et al., 2006; Callaghan and Dunk, 2002). One study documents different movement patterns of the lumbar spine during sitting for low back pain developers and asymptomatic subjects (Dunk and Callaghan, 2010). Only one field study measured global angles of trunk and thighs and correlated these posture measurements to the activation of lumbar muscles (Mork and Westgaard, 2009). They did not show clear correlations (.44 < r < .80) because the measurements used for trunk posture are not precise enough. The load on lumbar discs while sitting is not to be underestimated and is greater than in erect positions like standing or reclined (Nachemson, 1966). Newer studies support this data on the whole (Wilke et al., 2001). Further, the flexion–relaxation phenomenon is present in flexed postures of the trunk, so there is no active muscular support or stabilisation while resting in such positions (Schultz et al., 1985; Sihvonen et al., 1988). This means activation of the lumbar muscles is not required in such body positions and it would seem that this is comparable to the situation in sedentary work. Most studies on the flexion–relaxation phenomenon only show global angles for trunk inclination. For the lumbar spine, however, it is assumed that the curvature (lordosis, flat or kyphosis) is the main impacting factor on activating the lumbar muscles. There have been no studies providing this information as yet.

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Low back pain patients show atrophy of lumbar muscles (Hadar et al., 1983; Cooper et al., 1992; Hides et al., 1994; Danneels et al., 2000; Barker et al., 2004; de-las Peñas et al., 2008). Since physical inactivity and a lack of activation of muscles are reasons for atrophy (Hayashi et al., 1992; Salminen et al., 1993; Bloomfield, 1997; Hodges et al., 2006; Hides et al., 2007; Hyun et al., 2007; Belavy´ et al., 2008) de-conditioning or atrophy of lumbar muscles can be assumed due to long periods of sedentary work without active leisure. Physical changes are not only present in the muscles. Passive structures like ligaments or intervertebral discs are characterised by viscoelasticity. This means that (also low) cyclic but long-lasting loading leads to the creep of discs or ligaments (Solomonow et al., 1998; Adams and Dolan, 1996). The main function of passive structures, which is to guide the kinematics of a joint, decreases because of the decrease in stiffness (Solomonow et al., 1998; Adams and Dolan, 1996). Moreover, ligaments have neural connections to muscles and are mechanical receptors for critical situations (Solomonow et al., 1987; Johansson et al., 1991). Due to the creep of the passive structures, the mechanism that triggers reflexes decreases and finally disappears (Solomonow et al., 1999, 2003). It should be noted that recovery of ligaments takes more than 8 h of total rest (Gedalia et al., 1999). Both changes (de-conditioning of lumbar muscles and creep of passive structures) may be characterised as detuning of the lumbar spine. For example, an important reflex is triggered too late and within a wrong joint-angle and the adynamic muscle is not able to protect the joint. With this in mind, the purpose of this paper was to measure the normal behaviour of the lumbar spine during two hours of office work. Healthy subjects were investigated. The lumbar posture and sEMG of lumbar muscles were recorded. As a more precise measure for lumbar posture (than pelvic angle and trunk angle), the curvature of lumbar spine was deduced. The aim of this paper is to investigate the coherence between lumbar curvature and lumbar muscle activity. The results may provide the first impetus for further discussion on whether sedentary work is disadvantageous or unhealthy.

2. Materials and methods 2.1. Subjects and procedure Thirteen subjects (8 $; 5 #) were recruited from an insurance company (n = 4), a software development company (n = 2) and a health care company (n = 7). All the subjects were investigated while undertaking their normal sedentary work at a desk (Table 1). Inclusion criteria was no period of acute low back pain during the 12 months before measurement. Exclusion criteria were acute low back pain, acute pain or injury of lower extremities, deformation of the spine and known protrusion or prolapse of an intervertebral disc. Before work, the subjects were equipped with a small and portable measuring device. At the start of data collection, a calibration procedure lasting approximately 1–2 min was carried out (see Section 2.2).

Table 1 Age, body height, and body weight as mean (standard deviation) for all investigated subjects. Gender (n)

Age (years)

Body height (cm)

Body weight (kg)

Female (8) Male (5)

36.0 (7.0) 41.2 (11.7)

168 (3.2) 182 (4.2)

61.8 (6.1) 81.2 (8.4)

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All the subjects were investigated for a minimum of 2 h. During the measurement, different tasks were manually marked by the researcher and written online to the data file for later identification. The observed tasks were standing, walking, unsupported sitting, supported sitting on backrest of the chair, long-lasting periods of keyboard use and telephoning. Each subject performed each of these tasks at least once, as triggered by the work to be done. The backrest of the chair did not affect the positions of the EMGelectrodes and motion sensors. None of the subjects reported restrictions from the measuring device. All other office-typical tasks and periods in which the subjects could not be viewed by the researcher were summarised using the ‘‘miscellaneous’’ marker (Table 2). All the subjects were volunteers and gave written informed consent under the terms of the Declaration of Helsinki. The experimental protocol was reviewed and approved by the local ethics commission. 2.2. Measurements Using the PS11-UD measuring device (Thumedi, Jahnsbach, Germany) the posture of the lumbar spine, the sEMG of selected lumbar muscles and the cardiogram (for identification within EMG signals) were measured synchronously. It is possible to measure and collect bipolar sEMG and posture data for up to 8 h with this device. Lumbar posture was monitored by three gravity-based sensors. The selected lumbar muscles were the longissimus muscle at lumbar level 1 and the multifidus muscle at lumbar level 4 bilaterally. The motion sensors (size: 24  24  10 mm) were applied on the lumbar spine at level L5, L3 and L1 using hypoallergenic doublesided adhesive tape. Each motion sensor measures spatial orientation with an accuracy of 0.1° within the field of gravity (e.g. inclination from vertical axis) and is comparable to devices described in the literature (Aminian and Najafi, 2004). The angular difference between the sensors in the sagittal plane was calculated as a measure of the lumbar spine curvature (Mörl and Blickhan, 2006). Abrasive lotion was used for skin preparation for bipolar sEMG and ECG measurements. Where there was pronounced growth of hair at the application position, the subjects were shaved prior to skin preparation. After this the skin was fumigated and dried. The electrodes used were Ag/AgCl-electrodes (H93SG, Tyco Healthcare, Germany) with a circular uptake area of 10 mm and an interelectrode distance of 25 mm. The electrode positions of the four investigated muscles were in line with the recommendations of SENIAM (Hermens et al., 1999). Before data collection a calibration procedure was carried out for identification and elimination of the ECG signal from EMG signals, for spinal posture offset-adjustment and for normalisation of the EMG. The first step of calibration was to measure the raw ECG signal on each EMG channel while the subject sat relaxed and supported by the back of the chair. While occurrence of the ECG signal (detected by ECG channel) during data collection, the device eliminates the main part of crosstalk by subtracting the ECG signal from the sEMG signal for each single EMG channel (Mörl et al., 2010). The second step of calibration was to eliminate the angular offset for lateral bending and axial rotation of the spine (not discussed in this paper) due to inaccurate sensor application. The normal shape of the lordosis in standing position was not offset-adjusted and was given in normal angular positions. The golden standard for EMG normalisation are records during maximum voluntary contractions (MVC). These measurements are laborious and depend on the subject’s motivation. MVC measurements are nearly impossible at the workplace, especially for lumbar spine muscles. A special normalisation posture was therefore undertaken by the subjects during the third step of the calibration

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Table 2 Summary of distribution [%, relating to individual measuring time] of different tasks during office work for all subjects. Subject

Standing

Walking

Uns. sitting

Sup. sitting

Keyboard

Phoning

Misc

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13

1.3 4.1 5.0 3.6 0.1 7.0 2.1 11.2 6.7 2.7 – 2.6 0.1

1.4 2.4 2.7 2.9 1.5 1.1 1.6 5.2 2.6 1.7 1.3 0.1 2.7

1.4 23.7 3.6 60.2 65.5 75.2 59.6 60.0 38.9 21.4 55.2 19.0 44.0

0.1 41.3 0.1 3.4 23.7 2.8 5.3 0.4 28.5 22.3 25.8 54.9 16.1

54.2 0.1 49.0 22.2 0.1 0.1 14.2 6.7 9.6 37.4 – 15.0 12.9

20.7 4.1 6.5 3.0 0.1 8.3 20.6 5.8 8.5 14.7 – 6.8

18.9 23.5 32.5 3.7 6.8 4.6 0.8 10.0 12.6 4.7 0.1 6.9 15.9

procedure. For this, the body position of the subjects was as follows: standing position, slightly bent knees and hips about 45°, the trunk tilted about 45°. The researcher advised the subjects to maintain lordosis of the lumbar spine. Within this posture the torque of the tilted trunk has to be compensated for mostly by the lumbar muscles under investigation. Activation was moderately comparable to an exercise in gymnastics. Subjects had to hold this position (isometric contraction) for about 30–60 s. Repeated measurements carried out previously in the laboratory showed adequate reliability and validity. All given sEMG data was normalised to this activation (i.e. fractions of this activation). 2.3. Data processing and analysis The PS11-UD measures and stores the raw sEMG data (4– 650 Hz) at a sampling frequency of 4096 Hz. Amplification of the signal was selected to get a resolution of 688 nV per bit. The raw signals were high pass filtered (16 Hz), low pass filtered (1 kHz) and band pass filtered (moving average at multiplies of 50 Hz) to repress crosstalk from power supply lines. During measurement, the device processes and stores the rectified and averaged activity (RMS, root mean square) at 8 Hz by

ð1Þ

with the gain k, frequency limits fl/u, amplitude A and frequency f for amplitude information. It also calculates and stores the median frequency (not discussed in this paper). The angular data of the motion sensors on the spine were also stored synchronously at 8 Hz. The lumbar flexion angle was calculated by

aflex ¼ ðaS1  aS2 Þ þ ðaS2  aS3 Þ

3. Results 3.1. Occurrence of office tasks The majority (over 82%) of office work was sedentary. Only standing and walking occurred in an erect body position and these accounted for just 5% of the total office time. In the sitting posture, unsupported sitting (41%) made up the main part, with supported sitting only accounting for 17%. Other tasks like keyboard use (16%) and phoning (8%) were not that frequent. In summary, office work was very passive, with 5% undertaken in an erect body position and only 2% walking (Table 2). 3.2. Different tasks and lumbar muscle activation This passivity was accentuated by the differences in lumbar muscle activation within different seating postures. Neither type of sitting activated the lumbar muscles a quarter as much as normalisation posture (Fig. 1). In unsupported sitting, the longissimus muscle showed up to 20% of the normalisation posture whereas

lumbar muscular activity during sitting 1

with inclination angle from vertical axis aSn of each single motion sensor (S1 at lumbar level L1 . . .S3 at level L5) in sagittal plane. All the data presented here is based on the calculated RMS and angular position data. The measurements taken were summarised for each task observed and presented as percentages for all subjects. For the tasks ‘‘unsupported sitting’’ and ‘‘supported sitting’’ the lumbar curvature angle in the sagittal plane was plotted by a defined histogram. Bins of a range of 5° within the total range from 40° (maximum lordosis) to 20° (maximum kyphotic posture of lumbar spine) were used. Sporadic outliers below 40° or over 20° lumbar flexion angle were allocated to the first or last bin respectively. The coincidental RMS measurements of the investigated muscles were presented as medians and quartiles over the occurrence of lumbar posture. The tasks ‘‘standing’’, ’’walking’’, ‘‘keyboard use’’ and ‘‘telephoning’’ were excluded from this analysis because of high variation in muscle activation due to movements of the arms.

Mu L4 ri Mu L4 le Lo L1 ri Lo L1 le

0.9

ð2Þ

0.8 0.7

rel RMS

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z fu 1 RMS ¼ k A2 ðf Þdf fu  fl fl

To detect statistical differences between the different office tasks, the Mann–Whitney U-test was used. For variations within different postures during one task the Kruskal–Wallis-test (nonparametric version of classical one-way ANOVA) was used. The significance level for all tests was P = .05. Both nonparametric tests were used because there was no normal distribution of sEMG data.

0.6 0.5 0.4 0.3 0.2 0.1 0

unsupported sitting

supported sitting

Fig. 1. Median of normalised lumbar muscle activation (LoL1 – longissimus muscle at lumbar level L1, MuL4 – multifidus muscle at lumbar level L4, le – left and ri – right side, respectively) during sitting. In comparison to normalisation posture the activation of lumbar muscles is low (P < .01, for all muscles).

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lumbar posture and muscular activity in unsupported sitting 30

1.5

rel RMS

1

15

0.5

occurence [%]

occurence LoL1 le LoL1 ri MuL4 le MuL4 ri 22.5

7.5

0

0 −40

−30

−20

−10

0

10

lumbar flexion angle [°]

Lordosis

20

Kyphosis

Fig. 2. Median and quartiles of lumbar muscle normalised activation (LoL1 – longissimus muscle at lumbar level L1, MuL4 – multifidus muscle at lumbar level L4, le – left and ri – right side, respectively) and distribution of lumbar posture (grey bars, right y-axis) during unsupported sitting. The activation of all lumbar muscles within lordotic postures is moderate but very low within kyphotic curvature. Lumbar muscle activation depends on posture (P < .001, for all muscles).

the multifidus muscle had lower activity at the level of 10% of normalisation. Supported sitting was effectively passive. The longissimus muscle showed up to 12% of normalised activation. The activity of the multifidus muscle was near the limit of the resolution of the measurement device at only 5%. This low activation of multifidus muscle was below 5 lV (non-normalised value) for most cases. The differences between the described tasks are highly significant (P < .01).

3.3. Lumbar muscle activity and dependency on posture The main outcome is the dependency of lumbar muscle activity on lumbar posture. In walking, as a reference, the lumbar spine had

lumbar posture and muscular activity in supported sitting 1.5

30 occurence LoL1 le LoL1 ri MuL4 le MuL4 ri

rel RMS

15

0.5

occurence [%]

22.5

1

7.5

0

−40

Lordosis

−30

−20

−10

0

lumbar flexion angle [°]

10

20

0

Kyphosis

Fig. 3. Median and quartiles of lumbar muscle normalised activation (LoL1 – longissimus muscle at lumbar level L1, MuL4 – multifidus muscle at lumbar level L4, le – left and ri – right side, respectively) and distribution of lumbar posture (grey bars, right y-axis) during supported sitting on the backrest of the chair. In comparison to Fig. 2 (unsupported sitting) the distribution of postures is shifted right. There is very low activation of all lumbar muscles within kyphotic curvature. Lumbar muscle activation depends on posture (P < .001, for all muscles).

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lordotic curvature (negative flexion angle). A permanent change from low (10°) to pronounced lordosis (up to 40°) was found with regard to the gait cycle. Thus, there is little tilting of the trunk forwards (10°) or backwards (40°) during the gait cycle. In contrast, the lumbar posture was flat or in kyphotic curvature in sitting. The lumbar spine showed 47% lordotic curvature in unsupported sitting and only 30% in supported sitting. Both kinds of sitting showed only low lordotic angles (at minimum 25°) in comparison to walking. Further, the highest lumbar muscle activity was to be found within the lordotic posture of the lumbar spine (Figs. 2 and 3). The statement ‘‘the more the lumbar spine is flexed, the lower the lumbar spine’s muscular activity’’ is true for both kinds of sitting. The dependency of lumbar muscle activation on lumbar posture is highly significant for all investigated muscles (P < .001). In supported sitting, the main curvature of the lumbar spine was pronounced kyphosis (>15°, 30% of time). For this, step by step, the pelvis slid onto the front edge of the seat and was tilted backwards, whereas the thighs were not supported. Within this posture, the activation of lumbar muscles was very low and below the resolution limit of the measurement device (<5 lV) most of the time. 3.4. Gaps in muscular activity during sitting In phases of sitting with kyphotic lumbar posture, long periods of very low (near the resolution limit) or no activity of lumbar muscles were found. In unsupported sitting, the longissimus muscle at L1 showed 28% and the multifidus muscle 36% of no activity, respectively. In supported sitting this increased for the multifidus to 45%. The online filter for removing the ECG signal from the sEMG signal of the muscles does not work perfectly. With this in mind it is appreciable that there is more time without lumbar muscle activation: for the longissimus muscle, the periods increase to 32% in both tasks; for the multifidus muscle this increases to 41% in unsupported sitting and to 51% in supported sitting. In summary, during sedentary office work, the lumbar muscles were deactivated about 40% of working time. 4. Discussion Inactivity of spinal paravertebral muscles during trunk flexion has been described for decades (Floyd and Silver, 1951, 1955; Ahern et al., 1988; Dolan and Adams, 1993; McGill and Kippers, 1994; Newman and Gracovetsky, 1995). What is new in this study is the accurate measurement of postures during long-term sitting. There was found to be a strong dependency of lumbar muscular activity on lumbar spine posture in the sedentary body position. Only one study currently documents comparable data for women (Mork and Westgaard, 2009). In contrast to the cited study, here the angle of curvature of the lumbar spine in the sagittal plane was deduced as a more precise measure of lumbar spine posture. Furthermore the lumbar curvature has an impact on the sEMG of lumbar muscles. Only global angles for trunk and thighs were measured in the earlier study (Mork and Westgaard, 2009). It is possible to incline or bend the trunk with the lumbar spine in slight lordotic curvature. This may be the reason why the activity of lumbar muscles is more affected by the pelvis inclination angle and less by the trunk inclination angle. A flexed lumbar spine (kyphotic posture) leads to very low activity or phases of no activity of the lumbar muscles. This means that the flexion–relaxation phenomenon is present in sitting. In laboratory settings, examining the flexion–relaxation phenomenon during sitting showed no clear effect on the lumbar muscles (O’Sullivan et al., 2006; Callaghan and Dunk, 2002). The measurement times in the cited studies were very short in comparison to the data documented here. In the field

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setting, while doing their own work at their own workplace, the subjects focused on the task in hand and their lumbar muscles relaxed. Because the load on the lumbar spine in the sitting posture is not small, and there is no adequate muscular support, passive structures like ligaments and intervertebral discs or passive muscle properties have to carry the load (Nachemson, 1966; Wilke et al., 2001; Schultz et al., 1985; Sihvonen et al., 1988). When sitting supported by the backrest of the chair, stress on the disc is reduced, whereas intradiscal stress is about three times greater in unsupported flexed sitting (Wilke et al., 1999). There is no knowledge about stress on the ligaments during sedentary work in vivo currently. Biomechanical models may give hints, but depending on the optimisation parameter used and components included (e.g. ligaments, muscles) different results were predicted (McGill, 1986; Cholewicki et al., 1995; McGill et al., 1994; Arjmand and Shirazi-Adl, 2005, 2006; McGill et al., 2006; Brown and McGill, 2008). Due to their viscoelasticity and the long-lasting loading caused by sitting, creep of passive structures can be predicted (Adams and Dolan, 1996; Solomonow et al., 1998, 1999). Step by step, this may lead to changes in mechanical properties and dysfunction of these structures and also to muscular dysfunction (Solomonow et al., 2003). The load on the human lumbar spine during sitting is probably not directly comparable to the cited experiments, but consideration of the daily reiterative longlasting periods of sitting during office work (around 8 h) and the long times for recovery of passive structures may support this assumption (Gedalia et al., 1999). First studies also give evidence of this hypothetical behaviour in humans (Olson et al., 2009; Shin et al., 2009). In reality, low back pain does not develop after some hours of office work, but the fact that life in western societies is more and more physically passive (sitting at desks, sitting to drive cars, resting in an elevator, sitting on the couch and watching TV, etc.) may contribute to the longterm hypothesis described. Distinct passivity due to bedrest and low back pain are associated with atrophy or fatty replacement of paravertebral muscles (Hadar et al., 1983; McConnell and Daneman, 1984; Mattila et al., 1986; Cooper et al., 1992; Hides et al., 1994, 2007; Bloomfield, 1997; Ng et al., 1998; Danneels et al., 2000; Kader et al., 2000; Yoshihara et al., 2001, 2003; Barker et al., 2004; Hyun et al., 2007; Belavy´ et al., 2008; Lee et al., 2008). In an animal study, the main reason for atrophy of lumbar muscles was nonexistent neural activation (Hodges et al., 2006). Only one study documents the coherence of physical inactivity, low back pain and atrophy of paravertebral muscles in children (Salminen et al., 1993). It is currently speculative to assume that years of office work without active leisure lead to atrophy of paravertebral muscles. However, the increase in the cross sectional area of back muscles due to training in patients suffering from low back pain would support this assumption (Rissanen et al., 1995). If there is atrophy because of years of office work, the muscle function to stabilise the spine is reduced and may be a reason for the high prevalence of low back pain. Longitudinal studies on the cross sectional area, function or force of lumbar muscles are necessary to investigate this further. Paravertebral muscles and passive structures of the spine are not independent from each other. Changes in the mechanical properties of both structures and the disappearance of reflexes due to long-lasting (also low) loading, can cause the whole spine to be out of tune (Solomonow et al., 2003; Panjabi, 1992a,b). In this condition of dysfunction, increased degeneration and low back pain can be assumed. The sample of 13 subjects is small and limits the explanatory power of this study. More subjects may lead to more variation in

lumbar posture and lumbar muscle activation within the investigated tasks. In general, significant deviations from the results presented here should not be assumed: Firstly, low or no activation of the lumbar muscles in flexed postures of the trunk also seem to be mechanically linked in lifting (Schultz et al., 1985; Toussaint et al., 1995). Secondly, the medians of lumbar muscle activation presented here are (near) zero within kyphotic lumbar posture during sedentary work. Thus, 50% of time within such postures there is low or no activation of the lumbar muscles. 5. Conclusions In conclusion, to reduce the high prevalence of low back pain in sedentary work, reasonable prevention is necessary. Considering the low activation of lumbar muscles in the sitting posture, the use of instable seats or special office chairs to protect the spine or to train the paravertebral muscles will fail. This is because lumbar muscle activation depends more on the task than on the office chair used (van Dieën et al., 2001). Further, lumbar muscle activation does not differ when seated on an exercise ball, different dynamic office chairs or on a reference chair (McGill et al., 2006; Ellegast et al., 2012). Again the spine has to be seen as a complex of muscles, passive structures and neural control: In the sitting posture, the passive structures are stretched/loaded, the muscles only have low activation within the stretched condition, and the lumbar spine is in flat or kyphotic curvature. A natural way to activate the paravertebral muscles within the normal length conditions within the lordotic curvature of the lumbar spine may be breaks for walking, working in standing position and active leisure in erect body position. Conflict of interest The authors have no conflict of interest, no funding. Acknowledgments Thanks to Carol Keelan for language assistance. Thanks to Sabine Franke for data collection. References Adams M, Dolan P. Time-dependent changes in the lumbar spine’s resistance to bending. Clin Biomech (Bristol, Avon) 1996;11(4):194–200. Ahern DK, Follick MJ, Council JR, Laser-Wolston N, Litchman H. Comparison of lumbar paravertebral EMG patterns in chronic low back pain patients and nonpatient controls. Pain 1988;34(2):153–60. Aminian K, Najafi B. Capturing human motion using body-fixed sensors: outdoor measurement and clinical applications. Comput Anim Virtual Worlds 2004;15(2):79–94. Arjmand N, Shirazi-Adl A. Biomechanics of changes in lumbar posture in static lifting. Spine 2005;30(23):2637–48. Arjmand N, Shirazi-Adl A. Role of intra-abdominal pressure in the unloading and stabilization of the human spine during static lifting tasks. Eur Spine J 2006;15(8):1265–75. Barker KL, Shamley DR, Jackson D. Changes in the cross-sectional area of multifidus and psoas in patients with unilateral back pain: the relationship to pain and disability. Spine 2004;29(22):E515–9. Belavy´ DL, Hides JA, Wilson SJ, Stanton W, Dimeo FC, et al. Resistive simulated weightbearing exercise with whole body vibration reduces lumbar spine deconditioning in bed-rest. Spine 2008;33(5):E121–31. Bloomfield SA. Changes in musculoskeletal structure and function with prolonged bed rest. Med Sci Sports Exerc 1997;29(2):197–206. Brown SHM, McGill SM. How the inherent stiffness of the in vivo human trunk varies with changing magnitudes of muscular activation. Clin Biomech (Bristol, Avon) 2008;23(1):15–22. Burdorf A, Naaktgeboren B, de Groot HC. Occupational risk factors for low back pain among sedentary workers. J Occup Med 1993;35(12):1213–20. Callaghan JP, Dunk NM. Examination of the flexion relaxation phenomenon in erector spinae muscles during short duration slumped sitting. Clin Biomech (Bristol, Avon) 2002;17(5):353–60.

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Falk Mörl received his Master in Sport Science from the University of Jena in 1999 and his PhD in 2004. From 2000 to 2001 he has been postgraduate at the Chair of Motion Science at the University of Jena. Since 2002 he has been a scientist at FSA mbH. His research focuses on passive properties of the lumbar spine and biomechanical modelling.

Ingo Bradl studied Physics and received his PhD (theoretical physics) in 1987 from the Technical University ‘‘Otto von Guericke’’, Magdeburg. Since 1995 he works at the Biomechanics group within the Prevention Department of the German Social Accident Insurance Institution for the foodstuffs and catering industry. Since 2003 he is the head of the Biomechanics group. He is guest scientist at the University Hospital Jena, Clinic for Trauma-, Handand Reconstructive Surgery, Division for Motor Research, Pathophysiology and Biomechanics. His research focuses on the prevention of occupational induced musculoskeletal disease and on the development and application of related mobile measurement methods.