Assessing the effects of sway-back posture on global and regional spinal sagittal angles using inertial measurement units

Abstract

BACKGROUND:

Sway-back posture in the sagittal profile is a commonly adopted poor standing posture. Although the terms, definitions, and adverse health problems of sway-back posture are widely used clinically, few studies have quantified sway-back posture.

OBJECTIVE:

To investigate spinal sagittal alignment in sway-back posture while standing based on global and regional angles using inertial measurement units (IMUs).

METHODS:

This cross-sectional study recruited 30 asymptomatic young adults. After measuring the sway angle while standing, the participants were divided into sway-back and non-sway-back groups (normal thoracic group). Each participant stood in a comfortable posture for 5 seconds with IMUs at the T1, T7, T12, L3, and S2 levels. Then, we measured the global and regional lumbar and thoracic angles and sacral inclination in the standing position.

RESULTS:

Although there was no difference in the global lumbar angle, there was a difference in regional lumbar angles between the two groups. The normal thoracic group had balanced lumbar lordosis between the upper and lower lordotic arcs, whereas the sway back group tended to have a flat upper lumbar angle and increased lower lumbar angle.

CONCLUSION:

It is useful to assess the global and regional angles in the spinal sagittal assessment of individuals with sway-back posture.

Keywords

Assessment lordosis lumbar posture sway back

1. Introduction

Clinicians often highlight the comparison of spinal posture assessment before and after treatment, and for preventing pain and deformity [1, 2, 3]. Optimal spinal posture is commonly accepted to include minimal amounts of stress, strain, and energy expenditure and maximal mechanical advantage during daily activities [4]. However, a poor sagittal profile is strongly correlated with adverse health outcomes such as low back pain, deformity, and gait abnormalities [3, 5, 6, 7].

Swayback posture in the sagittal plane is a commonly adopted faulty standing posture [2, 8, 9]. It is characterized by posterior displacement of the trunk relative to the pelvis, long thoracic kyphosis, flattened or reduced lumbar lordosis, posterior pelvic tilt, and hyperextended hip and knee joints [4, 8, 10]. Potential adverse health arising from swayback posture includes the lower back and hip pain. Examining the hip angles and moments during gait in three postures, including sway back, Lewis and Sahrmann [11] reported that individuals with swayback posture had greater maximal hip extension angle and hip flexor moment of anterior hip structures and suggested that these may contribute to anterior hip pain. Other studies have reported that individuals with swayback posture may exhibit changes in the thickness of the postural stabilizer muscles, such as the lumbar multifidus and transversus abdominis, which can lead to lower back pain by reducing the stability of the spine [6, 9, 12, 13].

Although the terms, definitions, and adverse health problems of swayback posture are widely accepted clinically, few studies have quantified swayback posture. Studies that have assessed swayback posture have relied on skin surface curvature measurement tools such as a flexible ruler [14, 15]. For this type of measurement, the curve of the spine is transferred to paper, and the length and depth of the curve are calculated to objectively quantify the curve of the spine. This measurement method is proven, reliable [16] and economical, however, it is not easy to isolate the desired angle. On the other hand, the inertial measurement unit (IMU) has been proven to be valid and reliable for analyses of the trunk, posture, and dysfunctional movement. It has many advantages, including ease of use, wireless data collection, portability, accuracy, and continuous recording ability, which makes it suitable for frequent spinal curve evaluation and lordosis angle monitoring [17, 18, 19]. Some studies on swayback posture have resulted in conflicting results. Simorgh et al. [14] analyzed joint alignment in swayback posture and reported the presence of increased lumbar lordosis but no significant differences in thoracic kyphosis or pelvic anterior tilt compared to the normal group. They also reported increased flexion in the hip and knee joints of the swayback posture group. In comparison, Abdolvahabi et al. [15] reported increased thoracic kyphosis, pelvic anterior tilt, and hip and knee joint extension in subjects with swayback posture compared to normal subjects.

We believe that these conflicting results are due to the wide ranges of the global thoracic and lumbar angles. Previous studies have reported only global lumbar and thoracic angles [14, 15]. In addition, the flexible ruler measurement method has a disadvantage in that it is difficult to measure only specific angles. Roussouly et al. [3] divided lumbar lordosis into four types based on the sagittal upper and lower lumbar lordosis contours. In addition, several studies have separated the upper and lower regions of the spine when evaluating posture [3, 20, 21]. Thus, we believe that, when assessing sagittal spinal posture such as swayback posture, it is more efficient to evaluate the whole spine and then divide it into parts. Therefore, this study investigated spinal sagittal alignment in swayback posture based on global and regional angles using IMUs.

2. Materials and methods

2.1 Subjects

This cross-sectional study recruited 30 asymptomatic younger volunteers (7 males, 23 females) from universities in the Republic of Korea. The sample size in this study was determined from a pilot study with 10 subjects. G-power 3.1.2 software (Franz Faul, University of Kiel, Germany) calculated a required sample size of 16 subjects (group 1 $=$ 8 subjects, group 2 $=$ 8 subjects) with a significance level of 0.05, power of 0.95, and effect size of 1.96 (calculated using mean and standard deviation from the pilot study). The participants had no history of surgery, traumatic injury, or musculoskeletal pathology during the 6 months before the study, and had no functional restrictions, respiratory or neurological disorders, or pain in the spine or lower limbs. Table 1 summarizes the subjects’ characteristics. Ethics approval was granted by University Ethics Committee for Human Investigations, and informed consent was obtained from all participants (Approval Number: 2020-07-010).

Table 1
General characteristics of the participants ( $N=$ 30)

Variables	Sway back group ( $n=$ 15)		Normal thoracic group ( $n=$ 15)		Range		$p$
Age (years)	23.3	$\pm$ 1.1	23.6	$\pm$ 1.0	22	$\sim$ 25	0.389
Height (cm)	164.6	$\pm$ 7.4	163.5	$\pm$ 7.6	151	$\sim$ 180	0.681
Weight (kg)	58.7	$\pm$ 11.0	58.6	$\pm$ 8.2	45	$\sim$ 86	0.967
BMI (kg/m ${}^{2}$ )	21.5	$\pm$ 2.5	21.8	$\pm$ 1.8	17	$\sim$ 26.8	0.672
Sway angle ( ${}^{\circ}$ )	168.5	$\pm$ 2.0	174.5	$\pm$ 1.7	163.3	$\sim$ 178.4	$<$ 0.001 ${}^{\ast}$

All values are mean $\pm$ standard deviation. Abbreviation: BMI, body mass index. Sway angle, among the acromion, greater trochanter, and lateral malleolus. Sway group, a sway angle $<$ 170 ${}^{\circ}$ (among the acromion, greater trochanter, and lateral malleolus angle); Normal thoracic group, 170 ${}^{\circ}$ $<$ sway angle $\leqslant$ 180 ${}^{\circ}$ . ${}^{*}p<$ 0.05.

2.2 Photographic imaging analysis system

The BodyStyle posture analysis system (BodyStyle S-8.0; MZEN, Seoul, South Korea) and computerized photographic analysis were used to measure the sway angle in the sagittal plane. The system consists of a camera 2.5 m from a grid screen, and a footboard. The participant was asked to stand in a comfortable posture, looking ahead, with the feet on a footboard with foot shapes drawn on it. Next, we attached markers to the subject’s right side on the acromion, greater trochanter, and later malleolus using double-sided tape and photographed the subject’s right side. The sway angle was defined as the angle made by the acromion lateral tip, midpoint of the femoral greater trochanter, and tip of the lateral malleolus (Fig. 1).

Figure 1.

Measurement of sway angle.

2.3 Inertial measurement unit (IMU) systems

We used five wireless IMUs to measure global and regional angles. Each IMU consisted of a transmitter (model EBIMU24G, E2BOX, Seoul, South Korea), a receiver (39 $\times$ 26 mm, EBRF24G3CH; E2BOX), three gyroscopes, three accelerometers, three magnetometers, and Kalman filters. The transmitters measured 32 $\times$ 21 $\times$ 6.5 mm and weighed 7.85 g, including the attached lithium polymer battery. The IMU data were transmitted via radio (Visual FoxPro, Microsoft, Redmond, WA, USA) to obtain real-time orientation at a sampling frequency of 100 Hz. The sensitivities of the gyroscope, accelerometer, and magnetometer were 250–2000 dps, 2–8 g, and 1.3–8.1 gauss, respectively. The static accuracy was $<$ 0.5 ${}^{\circ}$ and the dynamic accuracy was $<$ 2 ${}^{\circ}$ .

The T1, T7, T12, L3, and S2 sagittal angles were acquired in the standing posture using Visual FoxPro in the Eulerian angle coordinate system (i.e., in a roll-pitch-yaw angle sequence) [17]. All reported angles are the mean of three 5-second measurements. Excel 2010 (Microsoft) was used to calculate mean angles from the acquired Eulerian data. The global thoracic kyphosis (GTK), upper thoracic (UTx), and lower thoracic (LTx) angles were calculated between T1 and T12, T1 and T7, and T7 and T12, respectively. The global lumbar lordosis (GLL), upper lumbar (ULx), and lower lumbar (LLx) angles were calculated between T12 and S2, T12 and L3, and L3 and S2, respectively. Reliability indices for the wireless IMU system data during standing were assessed using an intra-class correlation coefficient (ICC) and the 95% confidence interval (CI) (Table 2).

Table 2
Reliability indices for IMU data during standing ( $N=$ 30)

Variables	ICC	95% CI
T1	0.985	0.973 $\sim$ 0.993
T7	0.988	0.979 $\sim$ 0.994
T12	0.995	0.990 $\sim$ 0.997
L3	0.996	0.993 $\sim$ 0.998
S2	0.949	0.907 $\sim$ 0.974

ICC intra-class correlation coefficient.

Table 3

Comparison of global and regional trunk angles during standing posture among the groups ( $N=$ 30)

Variables	Sway back group ( $n=$ 15)		Normal thoracic group ( $n=$ 15)		Effect size	$p$
Lumbar lordosis ( ${}^{\circ}$ )
Global	28.0	$\pm$ 8.6	28.1	$\pm$ 9.9	$-$ 0.003	0.993
Upper lumbar	9.5	$\pm$ 6.2	16.4	$\pm$ 9.2	$-$ 0.902	0.024 ${}^{\ast}$
Lower lumbar	18.8	$\pm$ 5.6	12.3	$\pm$ 7.4	1.035	0.011 ${}^{\ast}$
Thoracic kyphosis ( ${}^{\circ}$ )
Global	$-$ 35.3	$\pm$ 7.5	35.3	$\pm$ 6.0	$-$ 0.010	0.986
Upper thoracic	$-$ 27.4	$\pm$ 6.6	26.2	$\pm$ 5.2	0.217	0.570
Lower thoracic	8.0	$\pm$ 4.3	9.2	$\pm$ 6.0	$-$ 0.230	0.550
L3 ( ${}^{\circ}$ )	93.3	$\pm$ 6.7	87.4	$\pm$ 7.1	1.185	0.004 ${}^{\ast}$
S2 ( ${}^{\circ}$ )	77.3	$\pm$ 7.9	76.9	$\pm$ 7.5	0.052	0.892

All values are mean $\pm$ standard deviation. Sway angle, among the acromion, greater trochanter, and lateral malleolus. Sway group, a sway angle $<$ 170 ${}^{\circ}$ (among the acromion, greater trochanter, and lateral malleolus angle); Normal thoracic group, 170 ${}^{\circ}$ $<$ sway angle $\leqslant$ 180 ${}^{\circ}$ . Lumbar lordosis global (positive angle), T12 relative S2 angle; Upper lumbar lordosis (negative angle), T12 relative L3; Lower lumbar lordosis, L3 relative S2; Thoracic kyphosis global, T1 relative T12; Upper thoracic, T1 relative T7; Lower thoracic, T7 relative T12. $P<$ 0.05.

Figure 2.

Inertial measurement unit attachment (experimental condition).

2.4 Procedures

Before the task, the participant was asked to stand in a comfortable posture, looking ahead, with the feet were placed on the footboard with foot shapes drawn on it. The examiner used double-sided tape to attach three single markers to the participant’s right side on the acromion, greater trochanter, and right lateral ankle malleolus to measure sway angle and took a picture of the subject’s right side by the BodyStyle posture analysis system. After 2 minutes, each participant was asked to stand in a comfortable posture, looking ahead, with the feet shoulder-width apart. The examiner marked the participant’s T1, T7, T12, L3, and S2 spinous processes and the midpoints of transmitters mounted on a plastic frame and then attached the marked points on the five mounted transmitters to the T1, T7, T12, L3, and S2 spinous processes using Transpore medical tape (3M, Seoul, South Korea) (Fig. 2). Finally, the examiner began acquiring data. We measured the standing posture three times, for 5 seconds each. It took about 15 minutes to measure the sway angle and global and regional angle. After the sway angle measurements, participants were categorized into groups with and without sway (normal thoracic group) using a sway angle less than 170 ${}^{\circ}$ as the inclusion criterion for the sway back group [9].

2.5 Statistical analyses

Data were processed using SPSS ver. 22.0 for Windows (SPSS, Chicago, IL, USA). Differences in global and regional angles of the thoracic and lumbar spine were analyzed using independent $t$ -tests for comparisons between two groups. The effect sizes (Cohen’s d values) of the test differences were obtained by calculating the differences between the mean values of the sway and normal thoracic groups and dividing these by the pooled standard deviations [22]. The level of statistical significance was set at $p<$ 0.05. Reliability indices for the wireless IMU system data during standing were assessed using an intra-class correlation coefficient (ICC) and the 95% confidence interval (CI).

3. Results

Table 3 summarizes the descriptive statistics pertaining to the comparison of the global and regional thoracic and lumbar angles while standing between the groups. ULx (T12–L3) was significantly lower in the sway back group (9.5 ${}^{\circ}$ $\pm$ 6.2 ${}^{\circ}$ ) than in the normal thoracic group (16.4 ${}^{\circ}$ $\pm$ 9.2 ${}^{\circ}$ , effect size $=-$ 0.902, $p=$ 0.024) whereas LLx (L3–S2) was significantly higher in the sway back group (18.8 ${}^{\circ}$ $\pm$ 5.6 ${}^{\circ}$ ) than the normal thoracic group (12.3 ${}^{\circ}$ $\pm$ 7.4 ${}^{\circ}$ , effect size $=$ 1.035, $p=$ 0.011). The L3 level was significantly higher in the sway back group (93.3 ${}^{\circ}$ $\pm$ 6.7 ${}^{\circ}$ ) than in the normal thoracic group (87.4 ${}^{\circ}$ $\pm$ 7.1 ${}^{\circ}$ , effect size $=$ 1.185, $p=$ 0.004). There were no differences in GLL, GTK, UTx, LTx, and S2 between groups ( $p>$ 0.05).

4. Discussion

This study assessed global and regional sagittal spinal alignment of the lumbar and thoracic spine in swayback and normal thoracic groups while standing using IMUs. Regionally, the swayback group tended to have a significantly flatter ULx angle and increased LLx angle compared to the normal thoracic group, but there were no differences in GLL, GTK, or regional thoracic angles between the two groups.

Swayback is a poor habitual standing posture described as the posterior displacement of the trunk relative to the pelvis. In this posture, the body’s line of gravity moves forward in the sagittal plane to pass anteriorly to the acromion and posterior to the hip due to a backward leaning trunk posture [11]. In this posture, the human body compensates for the reverse posture of the backward trunk with a higher UTx angle or forward head posture to maintain stability by keeping the center of gravity over the base of support. Therefore, the more the trunk moves backward relative to the pelvis along with a posterior shift in body weight, the more the compensatory posture increases, similar to the change in posture during pregnancy [23, 24]. However, our results did not show significant differences in GTK, UTx, LTx, or GLL angles, although the regional lumbar angles differed significantly between the two groups. Our results are similar to those of Simorgh et al. [14] who reported that postural alignment in the sagittal plane of young females did not differentiate the amount of kyphosis and pelvic anterior tilt between swayback and normal groups. We believe that, in our study, the GTK, UTx, and LTx angles did not influence postural compensation because the subjects were young, asymptomatic, healthy, and did not have large swayback angles.

In this study, there were no significant differences in GLL angles between the two groups, but there was a difference in the regional angle. The normal thoracic group had a balanced lumbar lordosis arc between the ULx and LLx angles, whereas the swayback group tended to have a flatter ULx angle and increased LLx angle. Generally, the posterior displacement of the trunk relative to the pelvis associated with swayback posture makes it look like the abdomen is protruding. In this characteristic swayback posture, the apex of the lumbar spine is displaced backward compared to the normal thoracic group, which reduces lordosis of the upper lumbar spine. In our study, the value at the L3 level of the swayback group was significantly closer to vertical than in the normal thoracic group. Thus, participants with swayback posture had an increased LLx angle compared to the normal thoracic group. Although we selected pain-free subjects, our results show potential causes of lower back pain. Lumbar lordosis provides an efficient upright posture and decreases the load imposed on the spinal column. However, hypo- and hyper-lumbar lordosis are forms of spinal sagittal imbalance and an increased or decreased sagittal lumbar profile is associated with lower back pain [8, 25, 26]. Thus, our findings showed postural malalignments associated with increased sway angle, such as flattened ULx and increased LLx angles, a potentially increased load on the ULx and a risk for lower back pain in the LLx. Our quantified results are supported by Wiles [23], who suggested that swayback posture begins in the lower lumbar region, with a general increase in the LLx curve and corresponding decrease in the ULx. In addition, some previous studies have reported that people with swayback posture demonstrate a decrease in the thickness of the transverse abdominus and the cross-sectional area of the lumbar multifidus [9, 12, 13]. Because the multifidus and transverse abdominal muscles are inner core muscles that are important for the control of abdominal pressure and stabilization of the trunk, decreased activation of these muscles increases the risk for lower back pain. Therefore, our results provide a quantifiable criterion for the adverse health aspects associated with swayback posture. Contrary to our results, however, Simorgh et al. [14] reported that lumbar lordosis is increased in subjects with swayback posture compared to healthy controls. However, Simorgh et al. [14] measured only the GLL angle, while we measured the GLL and regional angles, which clearly revealed differences between the ULx and LLx. Another possible reason for the incongruent results is that they classified swayback posture using grid paper and a plumb line. When the plumb line passed more than 3 cm posterior to the midpoint of the anterior superior iliac spine and posterior superior iliac spine axis, it was classified as swayback posture. We tried new methods to classify swayback posture, however, it was not possible to classify swayback using a plumb line alone because the criteria were not clear. definition of a plumb line requires that the cervical spine is in a precisely neutral position. Therefore, in many previous studies, the pelvis, seventh cervical vertebra, trunk, and lower extremities were used as landmarks, and based on the angle between them and when the pelvis was displaced in front of the trunk, it was classified as swayback posture. Therefore, differences in these measurement methods may give different results. Dolphens et al. [27] classified sagittal posture into neutral global, swayback, and leaning-forward types and developed a swayback posture classification system based on lumbopelvic characteristics, such as flat, long lordosis, and lordotic types. We believe that sway angle and compensatory movement affect global and regional lumbar lordosis in individuals with swayback posture, which causes flattened or increased global and regional lumbar lordosis angles. Therefore, we examined the characteristics of sagittal alignment on global and regional thoracic and lumbar angles in subjects with small sway angles.

This study had several limitations. First, our participants were all asymptomatic healthy young adults. Therefore, our findings may not be generalizable to the general population. Second, we did not examine sway angle subtypes; therefore, we cannot suggest a relationship between sway angle and compensation of global and regional angles. Further studies should evaluate the effect of sway angle on global and regional spine angles and movement patterns during diverse daily tasks such as forward bending and gait. Furthermore, studies should be performed to identify the best predictors of swayback angle.

5. Conclusion

Although there was no difference in the global lumbar angle between the two groups, the regional lumbar angles differed: the sway back group tended to have a flat upper lumbar angle and an increased lower lumbar angle. This study shows that it is useful to assess individuals with sway-back posture using global and regional angles using IMUs. In addition, the results of this study provide health professionals with information that will improve assessments of spinal sagittal alignment based on IMUs.

Ethical approval

Ethical approval was granted by the Ethics Committee for Human Investigations at INJE University (Approval number: 2020-07-010).

Informed consent

Informed consent was obtained from all participants.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. NRF-2020R1F1A1049191) and the Ministry of Education (No. 2019R1A6A3A01096384).

Author contributions

Concept development: SS; Design: SS; Supervision: WY; Data collection and processing: SS and DA; Analysis and interpretation: SS; Literature search: SS and DA; Writing: SS; Critical review: SS and WY.

Footnotes

Acknowledgments

Not applicable.

Conflict of interest

None to report.

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Assessing the effects of sway-back posture on global and regional spinal sagittal angles using inertial measurement units

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Materials and methods

2.1 Subjects

Table 1 General characteristics of the participants ( N = 30)

Table 2 Reliability indices for IMU data during standing ( N = 30)

2.5 Statistical analyses

3. Results

4. Discussion

5. Conclusion

Ethical approval

Informed consent

Funding

Author contributions

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
General characteristics of the participants ( $N=$ 30)

Table 2
Reliability indices for IMU data during standing ( $N=$ 30)