Introduction

Office work is characterized by long hours of sitting. Occupational sitting is associated with numerous musculoskeletal disorders, such as low back pain [1, 2] and neck pain [2]. Prolonged sitting increases spinal stiffness [3] and low back discomfort [4]. Working with the computer also affects the neck region. In particular, increased muscle stiffness [5, 6] and increased EMG activity [7] of the upper trapezius (UT) muscle may contribute to the development of neck pain. In recent years, several interventions have been proposed to mitigate the negative effects of sitting, including sit–stand desks and other active workstations, such as cycling and treadmill desks [8], as well as performing active breaks [9]. While sit–stand desks and active workstations have the potential to reduce sitting time [10], increase energy expenditure [11, 12] and reduce discomfort [13], there are some associated drawbacks, such as cost, potential difficulties with integration into office environment, and questionable acceptance by some users [14].

The implementation of active breaks is a promising alternative to active workstations. Indeed, a recent review found that active breaks were effective in reducing discomfort and pain, and did not negatively impact work productivity (Waongenngarm et al., 2018). On the other hand, less is known about the effects of active breaks on muscle stiffness. Both muscle stiffness [15, 16] and spinal stiffness [3] have been shown to be increased during sitting. As the possible underlying mechanisms of increased stiffness include restricted blood flow and decreased muscle tissue oxygenation caused by static muscle contractions [17], active breaks with dynamic exercises could be an optimal approach to mitigate the increase in muscle stiffness observed after prolonged sitting. Recently, it has been shown that regular muscular contractions may ameliorate the increase in stiffness of the erector spinae (ES) muscle [16]; however, the contractions were induced by electrical stimulation. Another study reported a similar effect for an 8-min roller massage intervention [15]. Performing short (i.e., ~ 1–3 min) but frequent (e.g., every 20–30 min) active breaks could be a feasible alternative for workers that cannot afford or are not allow to take longer breaks from work. While short active breaks are known to positively affect physical and mental wellbeing of office workers [18], to the best of our knowledge, no study has yet investigated the effect of active breaks on muscle stiffness during sitting.

Previous studies have measured muscle stiffness during sitting work using myotonometry [15, 16]. Although this method has high reliability, its validity has been questioned [19]. In recent years, ultrasound-based shear wave elastography (SWE) has been increasingly used for assessing muscle stiffness [20]. In this method, a shear wave in the tissue is excited with ultrasonic acoustic waves, whereupon the speed of the wave propagation along the tissue is measured [21]. By multiplying the tissue density and the propagation speed of the shear wave, a shear modulus (representing muscle stiffness) is calculated. The shear modulus of the UT muscle has been linked to neck pain [5], but not in all studies [22]. Moreover, the stiffness of the back muscles (ES and multifidus) has been implicated as a potential factor in LBP [23, 24]. However, it is still unknown whether the shear modulus is increased during sitting work and whether short active breaks have the potential to mitigate the increase in muscle stiffness.

The purpose of this study was to preliminary assess the effect of sitting with and without short active breaks on muscle stiffness as measured by SWE (shear modulus). While previous studies have focused on low back muscles [15, 16], we also included the UT muscle due to its potential role in neck disorders [5]. In addition, we also assessed the rectus femoris (RF) muscle (typically not highly active during sitting [25]) to reveal if muscle stiffness may also increase in inactive muscles. We hypothesized that shear modulus in all muscles would increase after sitting, and that this increase would be neutralized or ameliorated with short active breaks.

Methods

Participants

A convenience sample of 10 (7 females, 3 males) young healthy students of Faculty of Health Sciences was recruited for the study (age 24.9 ± 1.2 years; body height = 171.7 ± 10.2 cm, body mass 66.7 ± 13.2 kg). The sample size was calculated using G*Power software, version 3.1.9.2 (Kiel, Germany). Power analysis demonstrated that the sample size needed for the present study were 8 subjects, with a moderate effect size of 0.50, an α error probability of 0.05, and a statistical power of 0.80. The effect size was estimated from previous studies assessing muscle stiffness with SWE during sitting [15, 16]. The inclusion criteria were age > 18 years. The participants who reported musculoskeletal injuries in the last 12 months or having any non-communicable chronic diseases or neuromuscular problems were excluded from the study. Before the measurements, each participant was thoroughly informed about the aims and procedures of the study. The participants were requested to sign an informed consent form prior to participation. The study was approved by the National Medical Ethics Committee of Slovenia (approval number 0120–690/2017/8) and was conducted in accordance with the Declaration of Helsinki.

Study design and experimental conditions

This was a randomized cross-over study, wherein the participants came to the laboratory for two experimental sessions. They were exposed to 1 h of sitting (sitting condition) in one session and to 1 h of sitting with active breaks in the second session. The order of the conditions was counterbalanced across participants, and the two experimental sessions were performed 7–10 days apart, at the same time of the day. The participants were asked to refrain from any vigorous physical activity 48 h before each session. Upon arriving to the laboratory, the participants rested for 10 min in relaxed prone position to dissipate any effect of previous activity on muscle shear modulus. Then, baseline measurements were conducted for erector spinae muscles (ES), with the participants remaining in prone position. Then, the shear modulus was also assessed for upper trapezius (UT) and rectus femoris (RF) muscles, with the participants switching to sitting posture. All measurements were performed only on the right side of the body.

Immediately after UT and RF assessments, the participants were exposed to 1 h of sitting. The participants were asked to bring their own laptops and perform their ongoing work on the computer. They were seated in an office chair (Ergoles Enjoy, Ergoles Ltd., Ljubljana, Slovenia) with back support and hand rests. The seat height was adjusted so that knee and hip angles were ~ 90°. In sitting alone condition, the participants performed their work undisturbed for 60 min, with a very short break at 30 min to measure UT and RF stiffness again. These two muscles could be measured without a need to change the sitting position. At 60 min, UT and RF were measured first, after which the participants switched to lying prone position for the final measurement of ES. In active break condition, two very short (~ 2 to 3 min) breaks (details below) were performed at 20- and 40-min timestamps. Otherwise, the order of the procedures was exactly the same as in the sitting alone condition. Desk height and all chair settings were kept identical in both sessions.

Assessment of shear modulus

Before the measurements, the locations for the placement of the ultrasound probe were determined and marked with a semi-permanent ink. The probe was always positioned in parallel with muscle fibers orientation. Shear modulus for ES was assessed in relaxed prone position (Fig. 1A). The ultrasound probe was placed at the level of L1 vertebrae (which was determined through palpation), 3 cm from the midline. For UT and RF shear modulus assessment, the participants were seated in the same chair use for exposure to sitting. They were instructed to place their hands on the hand rests (to prevent UT activity) and to relax completely while leaning into the backrest (to prevent RF activity which would arise in case of anterior tilting of the trunk and pelvis). For the RF, the location was set at 50% of the distance between spina iliaca superior anterior and the superior aspect of the patella [26] (Fig. 1B). The probe location for the UT was 50% of the distance between the acromion and the C7 vertebra [27] (Fig. 1C).

Fig. 1
figure 1

Measurement sites for erector spinae (A), recuts femoris (B) and upper trapezius (C)

Full size image

We used the Resona 7 ultrasound device (Mindray, Shenzhen, China), set to musculoskeletal mode. The muscle tissue density was assumed to be 1000 kg/m3. A 4-cm linear ultrasound probe (Model L11-3U, Mindray, Shenzhen, China), with a generous amount of water-soluble hypoallergenic ultrasound gel (AquaUltra Basic, Ultragel, Budapest, Hungary) was used. The region of interest was fixed at 1.5 × 1.5 cm for ES, 1.0 × 1.0 cm for RF, and 0.5 × 1.0 cm for the UT. This was done to ensure that only the properties of the muscle tissue were captured. The depth of the region of interest was determined individually for each participant and was kept constant across sessions. As recommended, we ensured that the depth of the region of interest was never more than 4 cm, as the reliability of shear modulus assessment decreases with tissue depth [28]. The outcome measure for each trial was the mean value of eight quick consecutive scans, which is the maximum storage capacity of the device. The device displays the variability of the measurements (standard deviation expressed as % of the mean value). If the variability of the values within the 8 scans was above 15%, the trial was repeated. Three trials were collected at each time point (total 24 scans for UT and RF, and 16 for ES), and the mean value of all scans was taken for further analysis. The device interface displays the mean shear modulus value on the screen, which was transcribed into pre-prepared sheets on a personal computer. In addition, the interface displays scan-by-scan variability, which enabled us to quickly perform an additional trial if the variability exceeded 15%. Figure 2 shows examples of ultrasound scans with regions of interests for ES (Fig. 2A) and UT (Fig. 2B). Previous studies in our laboratory, using the same device and very similar procedures, reported high intra- and inter-session reliability for UT and biceps femoris muscles [29, 30].

Fig. 2
figure 2

Examples of ultrasound scans with regions of interests for ES and UT muscles

Full size image

Active breaks

Active breaks were designed to be easy to do without a need for extra equipment, and to be feasible to perform in a short period of time (~ 3 min). Moreover, versatile exercises were chosen to reflect the real-life situations. Specifically, the active breaks included the following exercises: (a) upper trapezius stretch; (b) hip flexors stretch; (c) front plank with support on the table; (d) latissimus dorsi stretch; (e) 10 squats; (f) briefly walking around the room and performing shoulder rotations. Each exercise was performed for ~ 15 s and repeated twice. The active breaks were supervised by the experiment supervisors, who is an expert in exercise science (i.e., kinesiology graduate).

Statistical analyses

The data were statistically processed in the SPSS 25 software (IBM, New York, USA). Descriptive statistics is presented as mean ± standard deviation. We assessed within-day reliability (using consecutive trials from the first visit) and test–retest reliability (using baseline values from each visit) by calculating intra-class correlation coefficient (ICC; model 2,1), which was interpreted as: fair (0.40–0.59), moderate (0.60–0.74), good (0.75–0.90) or excellent (> 0.90) [31]. The effects of time (i.e., time within the sessions – 0, 30 and 60 min) and condition (sitting alone and with active breaks) were tested with 2-way repeated measures analysis of variance or paired-sample t tests. The level of statistical significance was set at α < 0.05 for all analyses. Given the preliminary nature of this study and the small sample size, we also closely examined the partial eta-squared (η2) to estimate the effect size. The effect sizes were interpreted as small (< 0.06), moderate (0.06–0.14) and large (> 0.14) [32].

Results

Reliability

The data for two participants for RF muscle were excluded as they presented as outliers (values at 33.3–54.1 kPa. This was done because these values are clearly outside the range reported in the previous literature for relaxed RF muscle [26]. Moreover, these values were clear outliers by the statistical standards for detecting outliers [33]. The within-day reliability was excellent for UT (ICC = 0.94) and RF muscles (ICC = 0.93), and good for ES muscle (ICC = 0.88). The test–retest reliability was good for all of the tested muscles (ICC = 0.84 for ES; 0.80 for UT and 0.83 for RF).

Changes in shear modulus after sitting and the effect of active breaks

For the ES muscle, we found a statistically significant main effect of time (p = 0.041; η2 = 0.38), but no statistically significant effect of condition (p = 0.453) or interaction (p = 0.381). Based on the descriptive statistics, shear modulus increased in time (from 15.3 ± 2.7 kPa to 17.1 ± 3.6 kPa in sitting condition; from 15.3 ± 2.4 kPa to 16.0 ± 1.9 kPa in active breaks condition; Fig. 3A. However, the interaction effect was not statistically significant (p > 0.05; η2 = 0.08), which. Across all participants, the maximal value after 60 min of sitting alone was 22.3 kPa and 19.4 kPa after sitting with active breaks.

Fig. 3
figure 3

The changes in muscle shear modulus across 1 h in sitting alone condition (dashed line) and in active breaks condition (solid line). A—erector spinae; B—upper trapezius; C—rectus femoris

Full size image

For the UT muscle, we found no statistically significant main effects nor interactions (p = 0.389–0.948), despite the moderate interaction effect size. (η2 = 0.11). In sitting alone condition, the shear modulus values were relatively constant (pre: 16.4 ± 6.9 kPa; post 30 min 16.7 ± 6.3 kPa; post 60 min 17.1 ± 5.1 kPa). On the other hand, shear modulus tended to decrease during active breaks condition (pre: 18.5 ± 7.3 kPa; post 30 min 16.6 ± 4.4 kPa; post 60 min 15.4 ± 5.4 kPa) (Fig. 3B). For this muscle, a high between-participant variability was present, likely limiting the statistical power.

For the RF muscle, we also did not find any statistically significant main effect or interaction (p = 0.152–0.257) despite large effect sizes were large for all (η2 = 0.25–0.36). Since we had to remove two participants from the RF data, the statistical power for this analysis was further reduced. During sitting alone, the baseline value was 9.7 ± 1.8 kPa, which increased to 11.9. ± 2.9 kPa after 30 min and to 11.5 ± 2.7 kPa after 60 min. In the active break condition, the baseline value was 9.6 ± 2.2 kPa, which increased to 10.2 ± 3.0 kPa after 30 min, but then decreased to 9.4 ± 2.8 kPa (Fig. 3C). Across all participants, the maximal value after 60 min of sitting alone was 15.8 kPa and 12.1 kPa after sitting with active breaks.

Discussion

The purpose of this study was to explore the effect of 1-h sitting on muscle stiffness quantified by shear-wave elastography, and to assess the effect of including short active breaks. While this preliminary investigation is limited by small sample size, the trends indicate that sitting might increase shear modulus across different muscles, and that performing short active breaks during sitting work may mitigate these effects.

Out study demonstrated statistically significant effects of sitting on ES stiffness, but not on RF and UT stiffness, although the trends in the data indicate that increase in stiffness would be confirmed with larger sample. The effects of prolonged sitting on trunk/spine stiffness have been known for a long time [3]. However, the changes on the muscular level have been largely unknown. Indeed, changes in trunk/spine stiffness can have multiple underlying mechanisms, such as increased intrinsic tissue stiffness, baseline muscle activity, and muscle reflex gains [34]. Together with recent studies [15, 16], this study adds to the evidence that prolonged sitting increases muscle stiffness. While further studies are urgently needed to confirm our findings with larger sample sizes, the increases in ES muscle stiffness (+ 11%) were consistent with the results obtained by Kett et al. (2021), who reported even larger (+ 16%) increase after a longer time (4.5 h) of sitting. Note that previous studies have measured muscle stiffness using myotonometry (Kett et al., 2021; Kett and Sichting, 2020), which has its own limitations although it has been recently shown that its outcomes are correlated with shear modulus [35]. The exact mechanisms underlying the changes in muscle stiffness in either muscle are difficult to discern. It has been suggested that static sitting posture negatively impacts muscle metabolism, blood flow, and oxygenation [36], which potentially contributes to the formation of persisting actin–myosin cross-bridge formations [37] and development of trigger points [38], which could increase the shear modulus [39]. Regardless of the exact mechanism, the increased muscle stiffness could contribute to the development of chronic musculoskeletal diseases. Indeed, increased muscle stiffness of UT muscle has already been implicated as a factor in development of neck pain [5, 6].

It is clear that individual, organizational, and environmental interventions are needed to counteract the effect of prolonged workplace sitting [40]. While several studies have investigated the effects of various interventions on workplace sitting time [10, 41], discomfort or pain [9, 13, 42], and energy expenditure [11, 12], fewer studies have involved muscle stiffness assessments. In sport science studies, a decrease in muscle stiffness after cycling and foam-rolling activities has been shown [43]. However, the feasibility to perform such activities during office work is questionable. Apart from the aforementioned studies that confirmed the potential of roller massage [16] and electrical stimulation [15], one recent study explored the effects of prolonged sitting, standing, and an alternating sit–stand on trunk stiffness [44]. While the alternating the two postures was associated with the lowest discomfort, the trunk stiffness was the highest in this condition [44]. Standing alone was not more comfortable than sitting alone, and while research is being done to determine the optimal sit:stand ratio [45] or more generally, work:rest ratio [46], sit–stand stations are not without limitations [14].

Short and frequent active exercise breaks could be a feasible alternative, which was also supported by the results of this studies. Indeed, a recent systematic review reported positive metabolic and mental/cognitive effects of short active breaks in office workers [18]. The mitigation of the increases in muscle stiffness, as observed in this study, could contribute to prevention of musculoskeletal disorders [5, 6] and workplace discomfort [9]. Given that active breaks require the interruption of work, future studies are needed to determine the optimal length and frequency of the breaks to improve the feasibility of the real-life implementation. Although our active breaks are relatively short (~ 2–3 min), the implementation of such breaks might not be always possible.

The main limitation of this study is its preliminary nature. Further research with larger sample sizes will need to be performed to confirm our findings. While our initial power analysis indicated that this small sample should suffice, we only used 1-h sitting exposure (compared to 4.5 h in previous studies), which could result in much smaller effects. Future studies should consider using longer sitting periods to study the effect of active breaks on muscle stiffness. In addition, only a small region of each muscle has been assessed. While this was done to minimize the interruptions of sitting and the time in between sitting exposure and final assessments, region-specific stiffness and its changes within the muscle have been shown with SWE [47]. Investigating region-specific changes could be another interesting avenue for future research. We asked participants to bring their own laptops and perform their ongoing work. While the work they did was generally very similar (typing, interspersed with reading and clicking), and this contributes to ecological validity of the study, the work was not controlled by the examiners, which could contribute to the variability among the participants and between the sessions. Moving the participants out of the chair to take ES measures could also affect the results; although challenging, it should be considered in future to attempt to measure ES while sitting. Finally, muscle shear modulus obtained by SWE is dependent on probe pressure [48]. While extra care was taken to maintain similar probe pressure for all participants and the reliability of the SWE was good to excellent, probe pressure could nevertheless be an additional factor contributing to the variability in the scores.

Conclusions

This study indicates that stiffness in ES muscles is increased during prolonged sitting, and might also increase in UT and RF muscles. Short (~ 2–3 min) active breaks could be effective in mitigating the increase in muscle stiffness; however, statistically significant effects could not be confirmed with this preliminary investigation. This study is the first to investigate muscle stiffness changes with prolong sitting using shear modulus, demonstrating the potential of SWE applied ergonomics research. Future studies with larger sample sizes are needed to investigate the effectiveness of different active breaks on muscle shear modulus through different periods of prolonged sitting.