Is active sitting on a dynamic office chair controlled by the trunk muscles?

Today's office chairs are not known to promote active sitting or to activate the lumbar trunk muscles, both of which functions are ergonomically recommended. This study investigated a newly developed dynamic office chair with a moveable seat, specifically designed to promote trunk muscle controlled active sitting. The study aimed to determine the means by which the seat movement was controlled during active sitting. This was accomplished by quantifying trunk and thigh muscular activity and body kinematics. Additionally, the effect of increased spinal motion on muscular activity and body kinematics was analysed. Ten subjects were equipped with reflective body markers and surface electromyography on three lumbar back muscles (multifidus, iliocostalis, longissimus) and two thigh muscles (vastus lateralis and medialis). Subjects performed a reading task during static and active sitting in spontaneous and maximum ranges of motion in a simulated office laboratory setting. The temporal muscle activation pattern, average muscle activity and body segment kinematics were analysed and compared using Friedman and post-hoc Wilcoxon tests (p≤0.05). Active sitting on the new chair significantly affected the lumbar trunk muscles, with characteristic cyclic unloading/loading in response to the seat movement. Neither thigh muscle activity nor lateral body weight shift were substantially affected by active sitting. When participants increased their range of motion, the lumbar back muscles were activated for longer and relaxation times were shorter. The characteristic activity pattern of the lumbar trunk muscles was shown to be the most likely dominant factor in controlling seat movement during active sitting. Consequently, the new chair may have a potential positive impact on back health during prolonged sitting. Further studies are necessary to analyse the frequency and intensity of active sitting during daily office work.

Introduction

Up to 72% of the population in the western world works predominantly in a seated position [1], which exposes a large proportion to the risk of developing sitting-related musculoskeletal complaints. Partially conflicting findings have been reported for the spine and the lower back. Although a general causal relationship between sitting and low back pain (LBP) has not been confirmed [2-4], prolonged sitting in unfavourable postures [5, 6], as well as static sitting with continuous isometric muscular activity [7, 8], have been found to increase the risk of developing LBP. Sitting-related LBP has been proposed to be caused by prolonged low static trunk muscle activity [7-9], which could lead to deconditioning of the lumbar spine [10]. Patients with LBP typically show an atrophy of the lumbar multifidus [11, 12] and the lumbar trunk muscles are found to be inactive for 30% of the sitting time [10]. As a consequence, patients with LBP and associated spinal problems, e.g. spinal stenosis, disc prolapse and degenerative disc diseases, are known to have a reduced range of motion (RoM) of the lumbar spine [13]. Active motions have been shown to be better for the intervertebral discs and spinal muscles compared to single static postures [6, 14, 15]. Ongoing postural changes result in altered back muscle activity, spinal load and trunk-thigh angle; factors that are thought to be favourable in the prevention of sitting-related LBP, degenerative disc diseases and impaired muscle function [7, 8, 16, 17]. This hypothesis is supported by the fact that 30 to 85% of patients with LBP report that their pain is aggravated by static sitting positions [18, 19] and 34 to 39% report that walking relieves their LBP [18].

Based on these findings, chair manufacturers promoted active sitting. However, conventional dynamic office chairs (Fig 1) have not been found to change a person’s sitting behaviour and muscle activity pattern, or to have a positive influence on the development and management of LBP [7, 20–24]. Active sitting typically provokes considerable upper body motion, which is unlikely to be compatible with the demands of working in an office [7, 16]. Subsequently, chair manufacturers began to design instable chairs (Fig 1), with only a small support base to provoke continuous small body motion for maintaining balance [25]. However, these chairs have also not encouraged active sitting behaviour or positively influenced the development of LBP [23, 26–30]. In fact, research findings have shown the opposite effect, contending that subjects seek to maintain balance by keeping their center of mass within the small base of support [27]: a phenomena that has already been observed in standing [31, 32].

Fig 1

Office chairs.

The construction of the new chair (a) was based on a conventional dynamic office chair (b) with the inverted dynamic principle of an instable chair (c). In contrast to instable chairs, the seat has a stable central position that rests at the lowest point of the arc corresponding to the centre of the seat.

For this reason, our group has developed together with rotavis® (Winterthur, CH) a new dynamic office chair. The dynamic principle of instable chairs (which use a motion axis below the seat level to provoke instability) has been inverted and integrated into a conventional office chair (Fig 1). In a previous study, we investigated the boundary conditions for the new seat movements in the frontal plane and found a physiological placement of the motion axis at the level of the 11th thoracic vertebra of the chair user [33]. An additional study, which made a comparison between an instable and a conventional office chair, demonstrated that the upper body remained within a range considered to be stable during active sitting [34], even though subjects performed a substantial lateral spinal flexion. Accordingly, we concluded that active sitting on the new chair is compatible with the demands of office work and that even the backrest can be used during active sitting.

However, the means by which office workers control active sitting on the new chair is still unknown. The primary aim of this study was to quantify the muscular trunk activity, thigh activity and body kinematics of active sitting. If muscular activity contributes to the control of the seat movement, we would expect to find a cyclic unloading/loading of the responsible trunk or thigh muscles. If body weight shift contributes to the control of the seat movement, we would expect to find substantial lateral motion preceding the seat motion. As a secondary aim, the study analysed the effects of increased spinal motion on muscular activity and body kinematics.

Materials and methods

Participants

This study was part of a larger study on active sitting and the recordings were made during the second session, described in Kuster et al. 2016 [33]. Ten healthy volunteer office workers (four females, six males) were selected prior to their visit to our motion laboratory. Inclusion criteria were: 1) age between 20 to 50 years; 2) height between 1.53m and 1.92m; 3) BMI <30; 4) working hours of >4 hours per day in a seated position over ≥2 years. Exclusion criteria were: 1) chronic complaints of the back and neck region in the past year; 2) previous spine surgery.

The participants’ age was 32.2±7.6 years (mean±SD), body height 1.77±0.09m and body mass 72.1±9.8 kg. Participants worked on average 8.3±0.6 hours a day, of which 6.5±2.1 hours were spent in a seated position, primarily in front of a computer screen. None of the participants had prior experience with the investigated office chair. Ethical approval was granted by the institutional ethics board of the ETH Zurich (EK 2012-N-52) and all participants gave their written informed consent prior to study inclusion. Since no preliminary data were available, a convenient sample of n = 10 was included based on information from similar studies [20, 35].

Chair

The start-up company rotavis® and our research group equipped a conventional dynamic office chair with an additional degree of freedom in the frontal plane [33]. The additional degree of freedom allowed the seat to slide from side to side on a radius with the centre located above the seat level, approximately at the level of the 11th thoracic vertebra of the chair user (Fig 1). Accordingly, the seat was able to rotate in a convex manner around the anterior-posterior axis, resulting in a combined inclination and translation in cranio-lateral direction. Contrary to instable chairs, the seat maintained a stable central position because it rested at the lowest point of the arc corresponding to the centre of the seat. Consequently, no additional muscle activity was needed to keep the seat position centred. The same chair was used for the analysis of both static sitting and active sitting.

Conditions and experimental procedure

Following completion of the questionnaire on demographics, participants sat on the new dynamic office chair and the seat (90° knee angle) and table height (2 cm above elbow level) were adjusted. They were then equipped with reflective body markers and surface electromyography (sEMG) electrodes. To accustom themselves to the seat movement, participants sat for 1 minute on the chair in an active manner, without receiving movement instructions.

All sitting conditions were recorded while the participants performed a typical reading task (Fig 2). They were instructed to maintain their usual reading performance during the measurement process. With the seat of the office chair fixed in the middle position, sEMG and kinematic signals were recorded for 10 seconds for the static sitting condition (STATSIT). For active sitting, three times six motion cycles (seat movement to the left and right side) were recorded as participants used the dynamics of the chair, firstly in a spontaneous RoM (ACTSIT) and subsequently in a maximum RoM (ACTSITmax). The motion cycle frequency for ACTSIT was standardized to 0.5Hz using a metronome. This standardization was necessary because users with no prior experience tend to use the chair dynamics at a much lower frequency than experienced users. Only the frequency was prescribed, not the RoM. For sEMG signal reference purposes, following the measurement of the sitting conditions, each participant walked on a treadmill for one minute at 1 m/s. The treadmill was tilted upwards to its maximum (13.5°) gradient to increase motion of the lateral pelvis and spine [33, 36].

Fig 2

Measurement setting.

Lateral and posterior views of a subject during active sitting, including electrode and marker placement. The three pictures on the right show examples of the central and the maximum left and right positions. Note that subjects placed their palms on the wrist rest of the keyboard.

Instrumentation

Muscle activity was recorded using a wireless sEMG system (myon 320; myon®, Schwarzenberg, CH) with a sampling frequency of 1000Hz and a pre-amplification factor of 1000. Pairs of disposable sEMG electrodes (Ambu BlueSensor N) were fixed to the prepared skin of the participants in accordance with the SENIAM guidelines [37]. The electrodes were placed on the longissimus bilaterally and on the right multifidus, left iliocostalis, right vastus medialis and vastus lateralis unilaterally.

Body kinematics were recorded using a 12-camera infrared light-emitting motion capture system (Vicon MX system; Oxford Metrics Group®, Oxford, GB) with a sampling frequency of 200 Hz. Reflective skin markers (12.5mm diameter) were placed on the participants according to the Plug-in-Gait upper body model (Table 1) and reconstructed using the software Vicon Nexus 1.7.1 (Oxford Metrics Group®, Oxford, GB) [38, 39].

Table 1

Marker placement.

Segment	Marker	Placement
Head	l/r-FHD PIG	over the l/r temple
	l/r-BHD PIG	l/r back of the head
Shoulder	l/r-SHO PIG	l/r acromio-clavicular joint
Thorax	C7 PIG	7th cervical spinous process (0%)
	T10 PIG	10th thoracic spinous process (54.5%)
	CLAV PIG	between articuli sterno-clavicularis
	STRN PIG	xiphoid process of the sternum
Spine	T4	4th thoracic spinous process (21.2%)
	Llow	centred between L4 and SACR (93.2%)
	SACR PIG	centred between l-PSI and r-PSI (100%)
Pelvis	l/r-PSI PIG	l/r posterior superior iliac spine
	l/r-SIDE	centred between l/r-PSI and l/r-ASI
Seat	SEAT	middle rear of the seat
Vectors	Origin		Direction
Thorax	midpoint of C7/CLAV		midpoint of T10/STRN
u/l-Spine	C7/ Llow		T4/ SACR
Pelvis	midpoint of l-PSI/l-SID		midpoint of r-PSI/r-SID

Abbreviations: left and right (l/r), Front of the Head (FHD), Plug-in-Gait (PIG), Back of the Head (BHD), Shoulder (SHO), Cervical (C), Thoracic (T), Clavicle (CLAV), Sternum (STRN), Lumbar (L), Sacrum (SACR), Posterior superior iliac spine (PSI), Anterior superior iliac spine (ASI), upper and lower (u/l).

Marker placement and segment definition according to the Plug-in-Gait (PIG) upper body model [38, 39], with additional markers on the spine and pelvis. The percentages refer to back length [40]. All PIG markers were used to calculate the centre of mass. Marker placement is also shown in Fig 2.

Data processing

Data processing was carried out with MATLAB version 8.3 (The MathWorks Inc.®, Natick, USA). The raw sEMG signals were bandpass filtered (Butterworth 2nd order, 50 to 400Hz), rectified, smoothed over a 0.1s window and expressed in relation to the peak activity in walking. To calculate the latter, the maximum of 12 consecutive double steps was determined, averaged and set to 100%.

For STATSIT, the median muscular activity was determined over the middle 2 seconds of the recording. The median was taken because data were not normally distributed (verified with Lilliefors test). For ACTSIT and ACTSITmax, sEMG and kinematic data were divided into the individual motion cycles in order to analyse the middle four of each recording, resulting in a total of 12 cycles per subject and condition. The average muscular activity was calculated for each subject and condition. To compare the temporal activity profiles of ACTSIT and ACTSITmax with STATSIT, the time in which the sEMG amplitude exceeded, was equal to, or fell below the 95% range of STATSIT was analysed and expressed as a percentage of the time.

To analyse upper body motion, the midpoint of the thorax segment was calculated, the distance between its maximum left and right positions determined and then divided by two to express the RoM of a unidirectional seat movement to one side. The RoM of the centre of mass was calculated according to the Plug-in-Gait model and evaluated in the same way [39]. Since the extremities did not move (feet placed on the floor, hands on the keyboard), they were not considered in the centre of mass determination. The angular RoM of thorax and pelvis were analysed in the same way but using directional vectors (Table 1) instead of the midpoint. The lateral flexion of the spine was determined by calculating the angular difference between the uppermost (C7 to T4) and lowermost (Llow to SACR) spinal segments and analysed similarly at the left and right maximum positions. The average movement speed was calculated by expressing the RoM in relation to the movement cycle duration.

Finally, sEMG, as well as centre of mass, thorax and seat movement, were time-normalized to 200 data points (100 for the movement to each side) using a linear interpolation, averaged over all repetitions and subjects and plotted against time.

Outcome measure

To quantify active sitting, the average muscle activity and its temporal activation pattern were compared to static sitting. The body segment kinematics of the spine, the thorax, the pelvis and the centre of mass were analysed in terms of RoM and movement speed. Additionally, muscle activity and thorax, centre of mass, and seat movements were plotted in relation to the movement cycle. The movement cycle started and ended when the seat was in the central position and included the seat movements to the left and right side. Thus the movements to both sides were analysed.

Statistics

All statistics were calculated using SPSS 23 (IBM Corp., Armonk, USA). Data normality was tested with Lilliefors test and non-parametric statistics was used for positive test outcomes.

To compare sEMG amplitudes between the sitting conditions, a Friedman ANOVA was used. Significant effects were tested by a post hoc Wilcoxon matched-pair test to compare the individual conditions. The time in ACTSIT and ACTSITmax spent below, equal to and above the muscle activity of STATSIT was expressed with the mean ±SD (normal distribution not rejected). Kinematic data of ACTSIT and ACTSITmax were compared using a Wilcoxon matched-pair test. Since no condition effects were found (tested with a Wilcoxon matched-pair test), movement speed was averaged over ACTSIT and ACTSITmax. A p-value ≤0.05 was considered significant and significant effects were additionally described with a 95% confidence interval of the effect.

Results

Temporal pattern

The temporal muscular activity and kinematic motion pattern over the entire movement cycle is plotted in Fig 3. The figure shows a cyclic unloading/loading of all the investigated trunk muscles for both active sitting conditions. During the movement to the right side (highlighted in Fig 3), lumbar trunk muscles on the right side of the body contracted, while those on the left side relaxed. The movement to the left side was characterised by the reverse activity pattern. For the thigh muscles, findings showed no cyclic unloading/loading in ACTSIT and a very attenuated unloading/loading pattern in ACTSITmax. The centre of mass showed an attenuated and oppositely-directed movement pattern compared to the seat, with no relevant changes when changing the movement direction (at 50% and 150% in Fig 3). The thorax moved along with the centre of mass, but with a slightly larger RoM. It also showed a very consistent course.

Fig 3

Temporal activity and motion pattern.

Time normalized muscular activity and kinematic motion pattern during spontaneous (ACTSIT) and maximum active sitting (ACTSITmax) over all subjects. The seat is at 0%, 100% and 200% in the central position and at approximately 50% (left) and 150% (right) in the extremal positions. The muscle activity is given as a percentage of the peak activity in walking, kinematics in millimetres (mm).

Muscular activity in static and active sitting

An overall condition effect was found for the average muscle activity of multifidus and iliocostalis (Table 2). The post-hoc analysis revealed a significantly increased iliocostalis activity compared to STATSIT for ACTSIT, while for ACTSITmax iliocostalis and multifidus activity were significantly increased compared to STATSIT. The average thigh muscle activity was not affected by active sitting.

Table 2

Average muscular activity.

	STATSIT	ACTSIT	ACTSITmax	p-value	Difference to STATSIT
Trunk Muscles					ACTSIT	ACTSITmax
left longissimus	25.9 [19.4]	31.6 [10.8]	31.2 [17.0]	0.497
right longissimus	34.2 [17.9]	31.9 [18.1]	37.4 [17.7]	0.882
multifidus**	25.9 [12.3]	28.4 [15.5]	30.5 [12.5]**	0.003		3.6 [1.1–7.2]
iliocostalis**	43.2 [27.1]	49.5 [38.1]**	48.3 [38.9]**	0.005	5.2 [1.5–23.9]	6.2 [-0.2–18.0]
Thigh Muscles
vastus medialis	10.7 [7.0]	11.0 [7.3]	10.7 [6.3]	0.165
vastus lateralis	12.7 [7.0]	13.0 [23.8]	12.8 [25.3]	0.247

Median muscular activity [inter-quartile range] during static sitting (STATSIT) and active sitting in spontaneous (ACTSIT) and maximum range of motion (ACTSITmax), expressed as a percentage of peak activity in walking. Significant effects marked with asterisks (**:p≤0.01). For significant effects, the last two columns give the median difference with 95% Confidence Interval.

The temporal activity pattern for ACTSIT and ACTSITmax in relation to STATSIT is shown in Table 3. The proportion of the time below, equal to and above STATSIT muscle activity was approximately equal for longissimus on both sides of the body. The muscle activity of multifidus and iliocostalis were higher during active sitting compared to static sitting for ≥49% of the time. The two thigh muscles were equally activated during active sitting and static sitting for most of the time.

Table 3

Temporal muscular activity pattern.

	Trunk Muscles				Thigh Muscles
	left longissimus	right longissimus	multifidus	iliocostalis	vastus medialis	vastus lateralis
ACTSIT
Time below	30.6 ±19.1	30.6 ±19.4	21.8 ±24.9	8.6 ±8.4	13.9 ±8.1	17.4 ±4.0
Time equal	28.7 ±17.7	35.5 ±20.4	28.7 ±18.2	34.5 ±15.1	65.8 ±13.0	60.9 ±7.0
Time above	40.7 ±26.8	34.0 ±22.7	49.4 ±20.0	56.9 ±20.4	20.2 ±6.9	21.7 ±8.0
ACTSITmax
Time below	29.2 ±17.0	25.6 ±15.7	16.6 ±15.8	9.1 ±13.0	14.1 ±8.4	12.1 ±6.1
Time equal	26.6 ±12.5	35.2 ±17.1	28.7 ±14.4	27.5 ±11.8	52.9 ±8.0	51.2 ±14.0
Time above	44.2 ±23.9	39.2 ±20.5	54.8 ±12.2	63.3 ±18.1	33.1 ±15.3	36.7 ±19.0

Indicated is the percentage of the time for spontaneous (ACTSIT) and maximum active sitting (ACTSITmax) below, equal to and above the 95% range of static sitting (mean ±SD).

Kinematics in active sitting

The kinematic comparison between ACTSIT and ACTSITmax is shown in Table 4. Increasing the seat RoM significantly affected all the investigated RoMs, except thorax inclination. Since there was no effect of the active sitting condition on the movement speed, speed data in Table 4 are averaged across ACTSIT and ACTSITmax.

Table 4

Range of motion and movement speed.

	ACTSIT	ACTSITmax	p-Value	Difference
Range of Motion
Spine (Lateral Flexion) [°]	9.6 [6.5]	13.4 [3.7]**	0.007	3.8 [0.8–8.2]
Thorax (Inclination) [°]	0.8 [1.0]	1.4 [0.9]	0.139
Pelvis (Inclination) [°]	6.4 [1.9]	8.4 [0.6]**	0.009	1.9 [0.0–4.7]
Centre of Mass (Translation) [mm]	4.8 [4.3]	8.1 [5.7]**	0.009	2.2 [0.0–5.4]
Thorax (Translation) [mm]	7.8 [4.8]	11.8 [7.4]**	0.009	4.2 [0.4–8.7]
Movement Speed
Spine (Lateral Flexion) [°/s]	37.6 [19.4]
Thorax (Inclination) [°/s]	4.2 [1.5]
Pelvis (Inclination) [°/s]	25.0 [5.0]
Centre of Mass (Translation) [mm/s]	19.9 [18.3]
Thorax (Translation) [mm/s]	29.6 [19.2]

Median range of motion and movement speed [interquartile range] of the investigated body segments in spontaneous (ACTSIT) and maximum active sitting (ACTSITmax). Since there was no difference in movement speed between ACTSIT and ACTSITmax, data were pooled. The difference between ACTSIT and ACTSITmax is indicated by the median [95% Confidence Interval] in the case of a significant effect.

Discussion

The aim of this study was to investigate a dynamic office chair with a special moveable seat and its effect on muscular activity and upper body kinematics during active sitting in a simulated office laboratory environment. We hypothesised, based on theoretical considerations, that three mechanisms could be used to control the movement of the seat: a periodic activation of the trunk muscles; a periodic activation of the thigh muscles; and/or a substantial lateral shift in body weight.

How do office workers control the seat movement?

In the muscular dimension, an overall statistical effect for multifidus and iliocostalis muscular activity between the static sitting and the two active sitting conditions was found. Iliocostalis muscular activity was increased for both active sitting conditions compared to static sitting, while multifidus muscular activity was only increased for ACTSITmax (Table 2). The temporal analysis in Table 3 also showed that both multifidus and iliocostalis muscular activities were significantly increased or decreased during active sitting compared to static sitting for most of the time. The longissimus muscular activity, the third lumbar trunk muscle under investigation, showed no overall condition effect and a more balanced temporal activity pattern compared to static sitting. Thus, it seems that the movement of the seat is primarily effected by the activity of iliocostalis and multifidus, supported by longissimus (Fig 3). This conclusion corresponds to our expectations based on the anatomical muscle orientation. While longissimus has the most cranially directed fibre orientation, multifidus and iliocostalis have a more laterally directed fibre orientation that is likely to be more efficient in generating the investigated lateral seat movement. Accordingly, iliocostalis showed the highest activity compared to walking. Nevertheless, all recorded lumbar trunk muscles showed the expected temporal variation. During the right movement of the seat (from left to right maximum seat position, highlighted in Fig 3), we observed increased activity of the right multifidus and right longissimus, together with decreased activity of the left iliocostalis and left longissimus. The movement to the left side was characterized by the reverse pattern. The average thigh muscle activity was not affected by active sitting (Table 2) and remained within the same activity range for static sitting most of the time (Table 3). Thus, the thigh muscle is unlikely to have caused the seat movement. However, the results of the temporal analysis in Fig 3 show a slight variation in the thigh muscle activity level for ACTSITmax. This indicates that participants supported the trunk muscles with the thigh muscles to some degree to increase the RoM.

If the lateral body shift controls the seat movement, we would have expected to find a substantial displacement of the centre of mass or the thorax, or at least a body sway preceding the lateral seat motion. However, the centre of mass RoM in ACTSIT (4.8 mm) lay below the centre of mass sway limits reported for a typical typing task (5.3 to 6.9 mm, [27]). Also, Fig 3 shows that the centre of mass path did not precede the seat movement. For ACTSITmax, the centre of mass path even seems to pursue the seat movement. This makes the centre of mass movement very unlikely to have caused the seat movement. The same is true for the thorax movement, which also did not change substantially (0.8 mm in ACTSIT) or precede the seat movement (Fig 3).

Due to the stable upper body posture, the observed trunk muscle activity pattern is likely to have been the dominant factor controlling the seat movement. The trunk muscles are not needed to stabilise the upper body, because the upper body remains stable and upright during active sitting.

How does active sitting affect the trunk muscle activity?

When comparing active sitting with static sitting, only a small increase in the average muscular activity amplitude was observed. This means that the increase in muscular activity during the movement to one side was compensated by a decreased activity of the same muscle during the movement to the other side. Consequently, the average muscular load was not affected by active sitting, except for iliocostalis that appeared to be the most important muscle in controlling the seat movement. When the range of motion was increased, a similar effect was also noticed for multifidus. The temporal analysis of all trunk muscles clearly showed a substantial variation in muscular activity (Fig 3 and Table 3), as ergonomically recommended for the prevention of sitting-related LBP [7, 8, 16, 17]. For this reason, when the movement is performed regularly during everyday office activities, the cyclic variation of lumbar back muscle activity is hypothesized to counteract the atrophy found in patients with LBP [11, 12], as well as spinal deconditioning [10]. However, the intensity and frequency required to cause an effect for lumbar back muscles, as well as the feasibility of implementation during daily office work, remains a subject for future investigations.

How does active sitting affect the trunk kinematics?

To quantify the kinematic activity during active sitting, we present the movement speed of the various body segments. Unfortunately, a direct comparison between active sitting and walking was impossible for us because our treadmill had to be placed outside the measurement area of our motion capture system. However, a comparison to data on walking from existing literature appears to be useful because both active sitting and walking are performed with a very similar pelvis RoM [33, 36, 41, 42]. The observed speed of the lateral spine flexion (38°/s) and pelvis inclination (25°/s) were clearly higher than those reported for walking (lateral spine flexion: 27–33°/s, pelvis inclination: 10–21°/s [36, 41, 42]). A similar movement speed can be found in double step stair ascent (lateral spine flexion: 36°/s, pelvis inclination: 26°/s [36]). However, the thorax inclination (4°/s) was slower than in walking (6°/s [43]), meaning that although the spine and pelvis moved faster, the thorax remained more stable. These observations are in line with the aim of our new chair: to promote trunk controlled active sitting while maintaining upper body stability, so that office workers can focus on the work task. Due to the very similar upper body kinematics of active sitting and walking, a future study should analyse whether patients with LBP, whose back pain is relieved by walking, would also benefit from active sitting [18]. If so, the chair might allow patients with LBP to get pain relief without interrupting their office work.

Participants significantly increased their lateral spine flexion (+3.8°) with expansion of the range of motion, through increasing pelvis inclination and, to a smaller extent, thorax inclination, whilst keeping the same speed of movement. Despite this, the thorax movement remained ≤12 mm and ≤1.4°, within the range observed in real office situations using conventional dynamic office chairs (30 mm translation [7]); 2.8° lateral inclination [20]. Comparing the results of this study (centre of mass speed of 20 mm/s) to those in Grooten et al. 2013 (centre of pressure speed of 27 mm/s for standing and 45 mm/s for a conventional office chair) implies that our active sitting condition was less active, even though we observed a lateral spine flexion faster than in walking [27]. We therefore recommend future studies to be cautious when using the centre of mass path or speed as a measure of activity. Even the difference in methods, kinetic in Grooten et al. 2013 [27] versus kinematic (this study), does not explain the difference in conditions with such low centre of mass speeds (<50 mm/s; [44]). The authors of another study [45] observed a difference between centre of mass and centre of pressure of about 35 mm in a standing balance task of 30 seconds that explains differences of up to 1.2 mm/s. We agree that the centre of mass and centre of pressure are useful for quantifying stability [46, 47], but not for quantifying activity however. Active sitting on the new chair was found to be stable (slow centre of mass motion), but highly active (fast lateral spine flexion). Our recommendation, in general, is to evaluate segmental movement speeds in order to quantify the kinematic activity level. Moreover, we recommend analysing the energy expenditure during active sitting on the new chair in order to quantify the activity level of active sitting for public health purposes (sedentary behaviour).

Critical appraisal

This study has carefully considered some critical issues. The study included only ten office workers, who were unfamiliar with the dynamic chair. To investigate their spontaneous active sitting behaviour, they were given only a short time to familiarise themselves with the chair. Although the sample size is similar to that of previous studies in this field of research [20, 35], it limits the generalisability of the observed results and may partially explain the quite large variation observed in the presented results. In a subsequent study, it would be of great interest to analyse whether office workers who are already familiar with the new seat motion show a different muscular activity pattern. Furthermore, it would be very interesting to repeat this study with a novel dynamic chair that has a second additional degree of freedom in the sagittal plane [48]. Such a future study should use the data presented in this study to calculate the sample size, in order to draw more generalised conclusions.

This study focused on three superficial back muscles to analyse trunk muscle activity. Since previous studies had reported no differences between body sides [25, 49, 50], only one muscle was analysed bilaterally: the findings for the left and right longissimus in this study confirm the previous results. We considered the lumbar back muscles investigated in this study to be the most important for trunk stabilization in sitting [20, 51] and for the management of LBP [11, 12]. We were also limited to the study of superficial muscles due to our use of sEMG sensors. It would be of great interest to analyse additional muscles using invasive EMG techniques, e.g. the role of the iliopsoas [11]. Unfortunately, sEMG data for iliocostalis and r- longissimus were not available for 1 and 2 participants, respectively. Since no difference between r- longissimus and l- longissimus was found, we assume that this did not influence the results.

To compare sEMG data on the study population level, we referenced data to the average peak value observed in upwards walking. Other studies have referenced sEMG data to a (sub) maximum voluntary contraction [20, 27, 49, 51] or to a standardized posture [10]. However, due to the lack of standardisation of experimental protocols, comparison of sEMG data from different studies is limited, even if the same reference method is used [52]. It is also debatable whether sEMG data gathered from dynamic movements can be referenced to an isometric contraction performed in a different body posture [53]. For these reasons, we referenced our sEMG data to another well-known dynamic motion with similar lateral spine flexion that met our expectation for active sitting (stair ascent [33, 36]), although this limits the comparability of the reported values to other investigations. For practical reasons, we finally decided on a treadmill at its maximum inclination (13.5°). It must be noted that these reference values had no effect on the statistical analysis conducted in this study. However, the absolute values presented in Table 2 would have been lower if a reference with higher activity was taken (e.g. maximum voluntary contraction).

The step detection for walking was performed manually using video recordings because the treadmill was situated outside the measurement area of our motion capture system. Consequently, we were unable to compare the kinematics of active sitting and walking. This is the reason that we have discussed the movement speed in sitting compared to the literature data on walking. Corresponding to the active sitting conditions, we also used 12 double steps to calculate the average peak value in walking. Shiavi et al. 1998 demonstrated that 6 to 10 cycles are sufficient to produce a representative activity pattern [54]. We concluded from this data that the investigated 12 repetitions per condition were entirely sufficient to study the muscular and kinematic activity for both active sitting and walking. All sEMG data were analysed at their recorded time scales [53, 55] and we only used time normalization for the visual presentation of the results (Fig 3). Lastly, we analysed the lateral body motion only with markers placed on the torso, with no consideration of the extremities. With feet on the floor and hands on the keyboard, the lateral torso motion is a very close approximation to the lateral whole-body motion. It is very unlikely that non-recorded leg or arm movements were used to control active sitting. In conclusion, we are confident that the above-mentioned limitations to this study are fully acceptable when answering the proposed research questions.

Conclusion

This study shows that active sitting on the investigated office chair is primarily controlled using a very characteristic activity pattern of the lumbar trunk muscles. Active sitting requires a cyclic unloading/loading of iliocostalis, multifidus and, to an attenuated degree, of longissimus. Neither thigh muscle activity nor lateral shift of body weight were substantially affected by active sitting. It is proposed, therefore, that the new chair could have a positive impact on back health during prolonged sitting activities. A future study should further investigate the use of the chair in a real office setting under normal working conditions.

22 Jan 2020

PONE-D-19-28644

Active sitting on a dynamic office chair - Is it trunk muscle controlled?

PLOS ONE

Dear Mr Kuster,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

We would appreciate receiving your revised manuscript by Mar 07 2020 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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Juliane Müller, PhD

Academic Editor

PLOS ONE

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2. Thank you for stating the following in the Competing Interests section:

I have read the journal's policy and the authors of this manuscript have the following competing interests: One of the authors (DB) is the inventor of the new seat motion and owner of rotavis AG who holds the right for commercial use of the intellectual property right. The intellectual property right belongs to ETH Zurich which has approved the present study.

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Additional Editor Comments:

The study addresses a very interesting and highly relevant topic. Clearly, the authors are experts in the field and have conducted a sound experiment. Nevertheless, there are some major issues that needs to be revised before consideration for publication in PLOSOne. Please carefully edit the manuscript and copy-edit it prior to resubmission. Please see the specific details within the reviewer comments.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: No

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Manuscript PONE-D-19-28644

Title: Active sitting on a dynamic office chair - Is it trunk muscle controlled?

General Comments

The goal of this study was to investigate a new dynamic office chair with moveable seat developed to promote trunk muscle controlled active sitting. The study aimed to determine the strategy by which subjects control seat motion during active sitting (through trunk or thigh muscle activity, or by a lateral shift in body weight), to quantify muscular activity and body kinematics during active sitting, and to analyse the immediate effect of a spinal range of motion training on muscular activity and body kinematics during active sitting. While the study addresses interesting questions the following major shortcoming of this study require additional information and revisions.

Major comments

1. Although the authors state that they consulted assistance for writing the manuscript, the manuscript contains several grammatical errors and some sentences are difficult to understand. The manuscript should be carefully copy-edited prior to resubmission.

2. The study design is not fully suitable for answering the study objective as the mechanism of controlling sitting on the dynamic chair cannot be experimentally elucidated. Nonetheless, observed difference in study parameters between conditions are highly relevant. The conclusions should be revised accordingly.

3. The authors postulate that there are three different mechanisms for achieving seat motion. Why is it either or? It is also possible that it is a combination of these mechanisms. How can you unlink these mechanisms? Trunk muscle activity is necessary to initiate trunk motion.

4. The authors state in their second objective that the study quantifies active sitting on the new chair regarding muscular activity and body segment kinematics, and analyses the immediate effect of a spinal RoM training on muscular activity and body segment kinematics. This is the main aim of the study. The first objective is a mere interpretation of these results and would be appropriate in the discussion section. To proof these interrelationships would necessitate a musculoskeletal model where each of the stated factors is modulated systematically.

5. The authors may consider restructuring their manuscript. For instance, shouldn’t the outcome parameter be the result of the data processing? Please rearrange the methods section to follow a logical flow.

6. The authors repeatedly mention spinal ROM training yet miss to describe what they mean. Moreover, this study did not involve any training and hence all statements regarding training should be removed.

Minor Comments

1. Abstract: The objective should be rephrased as specified in the comments to the introduction section.

2. Abstract: The conclusion is not supported by the data presented in this study. See also comments in the introduction and discussion sections.

3. Line 43: to develop -> of developing

4. Line 45: colons are followed by lower case. While -> while. Please check throughout the manuscript as this mistake is repeated several times in the text.

5. Line 47: to develop -> of developing

6. Line 49: Better: This may lead to deconditioning of lumbar spine muscles?

7. Line 49: LBP patients. Not defining patients by their disease is more respectful of the person. Please change to patients with LBP throughout the manuscript.

8. Line 65: having only -> with only

9. Line 66: in order to -> to

10. Line 67 and entire manuscript. Please carefully review the literature - for instance, this paper is missing: Effects of a Dynamic Chair on Chair Seat Motion and Trunk Muscle Activity during Office Tasks and Task Transitions.

11. Line 76-77: “…that the upper body remains during active sitting within a range considered to be stable” -> “…that during active sitting the upper body remains within a range considered to be stable”

12. Line 81: it was so far not known -> to date it is unknown

13. Line 82 and entire manuscript: carefully check the tense. The purpose statement should be in past tense.

14. Line 82: to uncover -> better: to determine

15. Line 82-84: Why is it either or? It is also possible that it is a combination of these mechanisms. How can you unlink these mechanisms? Trunk muscle activity is necessary to initiate trunk motion.

16. Line 83: latter – this is not the correct term. The intended meaning is unclear.

17. Line 92: Opposed to instable chairs -> In contrast to other instable chairs

18. Line 93: without muscular effort – without a user? What is your intended meaning?

19. Line 97: Participants of this study…. -> This study was part of a larger study on….

20. Line 98: The – delete

21. Line 99: Please include inclusion and exclusion criteria for the participants of this study.

22. Line 100: Please provide SI units -> m

23. Line 100: body weight -> body mass

24. Line 101: better: … of which 6.5 +/- 2.1 hours were spent in a seated posture

25. Line 102: Better: None of the participants had any experience with the investigated office chair prior to the study.

26. Line 103: by institutional -> by the institutional

27. Line 106: Please provide more information on sample size calculation.

28. Line 110: The additional degree…. – phrase differently: slide side to side on a radius with a centre…

29. Line 114: better: … because it will rest at the lowest point of the arc corresponding to the centre of the seat…

30. Line 115: better: …no additional muscle activity should be needed to…

31. Line 116. Split sentence: The same chair…

32. Line 118: start with: Muscle activity was recorded using…

33. Line 121: Please include citation for the seniam guidelines.

34. Lines 122ff: There is no reason for abbreviating muscle names. These abbreviations make the text more difficult to read and should be removed throughout the manuscript. Moreover, please use lower case for all muscle names consistently throughout the manuscript, e.g. multifidus muscle….

35. Line 124: start with: Body kinematics were recorded using…

36. Line 126: reflective body markers -> reflective skin markers

37. Line 127: Please provide information on marker size and reference to the PIG model.

38. Table 1: Please define all abbreviations in the table footnotes.

39. Line 132: on spine -> on the spine

40. Line 134: additionally -> also

41. Line 140: was -> were

42. Line 141: to left -> to the left (did you only analyse seat motion from left to right?)

43. Line 143: average muscle activity: here, data processing was not explained yet. The average EMG signal will be close to 0. I assume you full-wave rectified the data and then computed the average? Please make sure that this is explained in the right order.

44. Line 146: The authors may consider moving this section to right after the chair description.

45. Line 150: one minute -> 1 minute (numbers with units time always as digits)

46. Line 151: better: Participants did not receive any movement instructions.

47. Line 154: Muscular -> sEMG

48. Line 54: static sitting condition: Was the chair fixed in this position? Please clearly concisely the experimental conditions.

49. Line 168: How do you know that this is the optimal motion frequency?

50. Line 161: When was this done? Before or after the seating conditions? Please specify.

51. Line 165: Lateral and back view on… -> Lateral and posterior view of…

52. Line 167: extremal seat positions -> maximum left and right positions

53. Line 176: why did you use median and not mean?

54. Line 177: data was -> data were (data is the plural of datum)

55. Line 181: statistical significance? This procedure is not clear. One should always choose clinical relevance over statistical significance.

56. Line 183: body segment thorax -> thorax segment

57. Line 184: two maximum positions (to left and right) -> its maximum left and right positions

58. Lien 184: halved -> divided by two

59. Line 185: Was the movement symmetrical to both sides or did some subjects move more to one side than the other? This may influence muscle activity. The subject in Figure 2 has more spine curvature to the left than the right. It seems that this subject is not placed in the centre of the seat. This could largely influence the data. How did you control for this?

60. Line 191: How were the spinal segments defined? Please specify.

61. Line 199: against normality -> for normality (Please also specify the test you used to test for normality)

62. Lines 201ff: The statistical approach only addresses the second objective. How did you address the first objective?

63. Line 210: Figure 3 was unreadable – and hence I cannot comment on the data presented here.

64. Line 229: Please do not use abbreviated muscle names as stated above.

65. Line 230: significantly higher ILIO activity – compared to what?

66. Line 232: Please provide statistical results of the post-hoc tests, ideally as 95% confidence interval of the difference.

67. Table 2: How do you explain the large variability in the data presented in this table?

68. Line 241: MULT and ILIO were…. – please rephrase this and the subsequent sentence as the intended meaning is not clear.

69. Line 248: Did the difference in left and right muscles occur during the same part of the activity? Why is the 50% mark important? Please show muscle activity patterns. Greater muscle activities in periods of greater muscle activity would be functionally more relevant.

70. Line 253: data in Table 4 is -> data in Table 4 are

71. Table 4: Pleas report the 95% confidence interval of the difference.

72. Line 264: What about a combination of these mechanisms? See also comment to the introduction section.

73. Line 266: better: In particular, ILIO (don’t use abbreviation) was increased for both active sitting conditions compared to…

74. Lines 268-273: Please do not repeat results in the discussion section. It is ok to summarize the results at a very high level.

75. Line 277: preferred -> more efficient?

76. Line 282: by a vice versa activity pattern -> by the opposite pattern?

77. Line 283: How do you know that it is not the result of the motion? Additional activity might be necessary to stabilize the trunk when it is off centre.

78. Line 284: This may be because the thigh muscle activity is much lower during sitting than during walking. Again, here one would need to see the muscle activity patterns because a very small impulse might be sufficient for initiating the motion.

79. Line 288: One would need to look at the timing of the activity. Unfortunately, Fig 3 is not readable, so it is unclear if this statement is supported by the data.

80. Line 293: How large is the variability between subjects? Did these patterns look the same for all subjects? Again, seeing Fig. 3 would be helpful.

81. Line 295: preceding -> precede

82. Line 296: post-hoc regression analysis -> correlation analysis (should also be explained in the statistics section)

83. Line 297: Please include scatter plots showing data point for both sides and all subjects.

84. Line 301: “…this observation is not an indication that the seat motion is controlled…” – Why not?

85. Line 304: What is spinal ROM training?

86. Line 307: By… -. When…

87. Line 307: we notice -> we noticed

88. Line 311: Again, here one would need to see the activation patterns to get an impression of the timing of greater and smaller activity.

89. Line 328: Was this the case for all subjects?

90. Line 330: Why now stair walking when you compare your values to treadmill walking?

91. Line 339: This sentence is incomplete.

92. Lines 340ff: This section does not relate to the study goals. The authors should focus on discussing the results of their study.

93. Line 342: The authors did not report centre of mass speed in the results section. Please do not report additional results in the discussion section.

94. Line 345: does -> do

95. Line 355: Please define what spinal range of motion training is. Here, the authors report acute differences in the outcome parameters yet lack to evaluate the efficacy of such training. Maybe patients with LBP would not even be able to complete these tasks?

96. Line 361: This speculation goes far beyond the objective of this study. Patients with LBP often have fatty atrophy that may be caused by neurological changes rather than by changes in physical activity.

97. Line 363: Again, one would need to consider the timing of these periods of increased and reduced muscle activity.

98. Line 374: Please rephrase this sentence.

99. Line 381: What would such training look like?

100. Line 420: One limitation is that only ten subjects were enrolled in this study. Large variability in some results suggest limited generalisability of the results.

101. Lines 422ff: Again, it is likely a combination of these. To conclusively answer this question, one would need to implement a musculoskeletal model where the different parameters are altered systematically.

102. Line 427: This sentence does not relate to the results of this study.

Reviewer #2: The presented study investigated active muscle contributions during sitting on a newly designed dynamic office chair. The authors highlight the importance of new approaches to counteract prolonged (inactive) seating which is known to be associated with the occurrence of low back pain. They also acknowledge that existing “active Chair” solutions often do not lead to the intended effects, such as ease of pain occurring during prolonged sitting. Therefore the authors comprehensively investigated a newly developed “unstable” office chair, intending to promote active muscle contributions including the lower back muscles, by deploying novel movement characteristics in lateral movement directions.

The manuscript is very well written and clearly structured. The experimental design is thoroughly described, allowing to potentially replicate the performed procedures. Following a detailed presentation of the acquired results, the authors critically discuss their results and also highlight the involved limitations of this investigation.

Major issues:

- No major issues are raised for publication of the manuscript.

Minor issues/comments:

- How long did the participants on average sit on the “active chair” during testing?

- Have subjective responses/comments been assessed, on how sitting on this chair feels? Or whether participants would assume that sitting in this instable situation might be suitable for longer office working tasks?

- While the authors thoroughly discuss their results and carefully state the implications of their findings, it might be recommended to highlight that the actual benefits of this newly developed chair will need to be further tested under situations resembling more adequately real life circumstances (e.g. prolonged sitting for several hours, arm movements, changes from sitting to standing, turning while sitting,...). Furthermore its value will need to be assessed within the target population of people with low back pain.

Miscellaneous remarks:

- Line 185: a typo?: …express the RoM of an unidirectional seat motion...

- Figure caption 3: it is recommended to indicate that the data shown is “exemplary/typical data of one of the participants”

**********

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Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

20 Mar 2020

see response to reviewers file

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

15 Apr 2020

PONE-D-19-28644R1

Is active sitting on a dynamic office chair controlled by the trunk muscles?

PLOS ONE

Dear Mr Kuster,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

We would appreciate receiving your revised manuscript by May 30 2020 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Juliane Müller, PhD

Academic Editor

PLOS ONE

Additional Editor Comments (if provided):

After next round of evaluation the reviewer and I feel that the quality of your manuscript hat improved.nevertheless, there are still some language-based issues, that are recommended by one reviewer, to be revised. Please do so.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Manuscript PONE-D-19-28644

Title: Active sitting on a dynamic office chair - Is it trunk muscle controlled?

General Comments

I would like to congratulate the authors for addressing all points adequately resulting in a very clear revised manuscript. The language has much improved, however there are still some awkward sentences especially in the introduction section that require attention.

Minor Comments

Line 18: delete “, which was”

Line 19: muscle-controlled -> muscle controlled

Line 21: This was accomplished by quantifying the muscular trunk activity, thigh activity and body kinematics. -> This was accomplished by quantifying trunk and thigh muscular activity and body kinematics.

Line 38: could -> may

Line 48: It has been proposed that sitting-related LBP might be… -> Sitting-related LBP have been proposed to be…

Line 55: for the health of the spine – Please be more specific

Line 56: variations in -> altered

Line 59: some 30% to 85% -> 30 to 85%

Line 60: 34-90% -> 34 to 90%

Line 62: made great efforts to promote -> promoted

Line 68: (Fig 1), with only a small support base, to… – delete commata

Lines 102- 105: Please use lower case for all points in the list.

Lines 106-109: Delete “average” because mean is specified in brackets.

Line 109: of the -> with the

Line 100: delete “prior to the study”

Line 129: sEMG -> (sEMG) – then use only abbreviation throughout the remainder of the manuscript

Line 268ff: multifidus and iliocostalis between -> multifidus and iliocostalis muscular activity between – please also add muscular activity to all subsequent mentions of individual muscles.

Line 310: If the shift in lateral body weight controls… -> If the lateral body shift controls…

Line 339: How does active sitting affect the trunk kinematics? – Is this a subheading? If so, please correct the formatting.

Reviewer #2: The aim of this study was to investigate a newly developed dynamic office chair with a moveable seat, specifically designed to promote trunk muscle-controlled active sitting. The authors emphasize the importance of new approaches to counteract prolonged (inactive) seating. The investigations are based on the quantification of muscular trunk/thigh activity and related body kinematics. The results are clearly presented and critically discussed afterwards.

The manuscript was given a comprehensive revision. It is well written and still very clearly structured. All main comments/questions of the previous submission have been addressed.

One minor comment

Line 102: inclusion criteria for height between 1.53 and 1.92 cm. Was this an inclusion criterion or it just happened to be the range of the respective participants?

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

29 Apr 2020

See attached point-by-point response file (Response to Reviewers.docx)

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

11 Nov 2020

Is active sitting on a dynamic office chair controlled by the trunk muscles?

PONE-D-19-28644R2

Dear Dr. Kuster,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Juliane Müller, PhD

Academic Editor

PLOS ONE

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no further

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Comments to the Author

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Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

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Reviewer #1: Yes

Reviewer #2: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: I congratulate you on this very nice manuscript on a very interesting study and novel results with many implications.

Reviewer #2: All comments/questions of the previous submission have been addressed.

The work has further been tweaked by minor changes all over the manuscript.

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Reviewer #1: No

Reviewer #2: No

17 Nov 2020

PONE-D-19-28644R2

Is active sitting on a dynamic office chair controlled by the trunk muscles?

Dear Dr. Kuster:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Juliane Müller

Academic Editor

PLOS ONE