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. 2026 Mar 1;38(3):89–95. doi: 10.1589/jpts.38.89

Influence of thoracic asymmetry on trunk movement patterns during sit-to-stand motion

Tetsuya Mamizu 1,2, Fujiyasu Kakizaki 3,*, Tatsuya Sano 1,4, Naruyoshi Komuro 5, Akira Hirosawa 5, Yutaro Natsuyama 1, Shinichi Kawata 1, Tomiko Yakura 1, Hidenobu Miyaso 1, Zhong-Lian Li 1, Masahiro Itoh 1
PMCID: PMC12981954  PMID: 41835171

Abstract

[Purpose] To investigate the relationship between thoracic asymmetry in the seated position, sagittal head position, and trunk movement patterns during the sit-to-stand motion. [Participants and Methods] Twenty-three healthy adults participated. Using a three-dimensional motion analysis system, we calculated the anteroposterior diameter ratios (right/left) of the upper and lower thorax at rest and measured the sagittal displacement of the head relative to the thorax. During the sit-to-stand task, we analyzed trunk movement relative to the pelvis and hip joint flexion-extension angles. [Results] Of the 23 participants, 20 exhibited leftward thoracic deviation relative to the pelvis. In these cases, trunk flexion preceded hip flexion, reaching maximum flexion before the hip joint. Significant leftward trunk rotation also occurred during the transition from flexion to extension, compared with that in the initial position. A correlation was observed between upper thoracic asymmetry and trunk flexion and between forward head displacement and trunk flexion. [Conclusion] Thoracic asymmetry and sagittal head position were associated with trunk movement patterns during sit-to-stand. These factors may be important considerations for physical therapy interventions aimed at improving sit-to-stand performance.

Key words: Thoracic asymmetry, Sagittal head position, Trunk movement pattern

INTRODUCTION

The sit-to-stand (STS) motion is one of the most frequently performed daily activities, such as standing up from the toilet or moving between rooms. It is a complex task that involves transitioning from a wide base of support (the buttocks and thighs) to a narrower one (the feet). Successful execution requires coordinated activity of the lower limbs while maintaining extension from the head to the pelvis.

For individuals with difficulty performing STS, acquiring this movement pattern is an important goal in physical therapy. STS performance is known to differ in individuals with low back pain, among different age groups, and according to seat height and surface characteristics1,2,3). Tateoka et al.4) reported that in older adults, lower limb muscle strength and fall risk are closely associated with STS performance. Smith et al.5) found that older individuals with decreased knee extensor strength tend to compensate by increasing activity of the hip extensors and ankle plantar flexors. Similarly, van der Kruk et al.6) reported that reduced knee extensor strength in older individuals is associated with greater forward trunk lean and longer movement duration. Conversely, insufficient forward trunk lean results in decreased forward movement of the center of mass (COM), impairing postural stability and increasing fall risk, particularly in older adults with a history of falling7).

When lower limb muscle strength and coordination decrease, efficient transfer of movement may be disrupted, leading to compensatory trunk motion and asymmetrical COM strategies. Once such asymmetrical movement patterns become habitual, they may reduce movement variability and potentially cause secondary impairments. Lomaglio and Eng8) reported that, in individuals with stroke, greater symmetry between limbs was associated with shorter STS performance times and improved efficiency. In addition, Abujaber et al.9) investigated individuals before and after total hip arthroplasty and demonstrated that improving weight-bearing symmetry enhanced ground reaction forces and joint kinematics. These findings suggest that, for individuals with pathological conditions, acquiring symmetrical movement patterns is essential for improving functional performance.

Recent studies have highlighted the involvement of the trunk, reporting that even healthy adults may exhibit leftward thoracic deviation10) and associated asymmetry in thoracic shape11, 12). Other studies have reported asymmetry in the thickness of the quadratus lumborum muscle, which attaches to the thorax13), indicating the influence of muscle groups associated with thoracic morphology. Furthermore, associations between thoracic shape and head position have also been reported14). These findings suggest that the forward propulsion task during the STS movement may be influenced by thoracic morphology, leading to characteristic movement patterns between the head and pelvis.

STS is generally divided into flexion and extension phases15). Thoracic asymmetry and sagittal head position may exert characteristic effects on trunk movement in each phase and during transitions. Understanding these relationships may allow clinicians to assess STS more comprehensively and develop more efficient movement acquisition strategies during rehabilitation interventions.

Therefore, in this study, we aimed to clarify the relationship between thoracic asymmetry, sagittal head position, and trunk movement patterns during STS. Using three-dimensional motion analysis, we analyzed thoracic movement relative to the pelvis, while also considering thoracic morphology and head position.

PARTICIPANTS AND METHODS

Twenty-three healthy adult males participated in this study (mean age: 26.4 ± 2.8 years, height: 172.8 ± 5.6 cm, weight: 64.6 ± 8.2 kg, body mass index: 21.7 ± 2.9 kg/m2). None had a history of orthopedic disorders involving the spine or thorax. Prior to data collection, all participants were informed of the study’s purpose and procedures and provided written informed consent. This study was approved by the Ethics Review Committee of the Tokyo Medical University Graduate School (Approval No. T2020-0085).

A three-dimensional motion analysis system (Vicon Nexus 2; Vicon Motion Systems Ltd., Oxford, UK) equipped with 11 infrared cameras was used to examine the relationship between thoracic shape asymmetry, sagittal head position, and trunk movement patterns during STS. The sampling rate was set at 100 Hz. Infrared reflective markers (9.5 mm diameter) were attached according to the Plug-in Gait full-body model.

For thoracic shape assessments, markers were placed with participants seated, following previous studies16, 17). At the upper thoracic vertebra level, markers were attached to the midpoints of the right and left second costosternal joints (A), and a marker was also affixed to the corresponding dorsal spinous process (B). At the lower thoracic level, markers were placed on the xiphoid process (C) and the corresponding posterior spinous process (D). Using a laser line aligned horizontally from the midpoints of the second costosternal joints and the xiphoid process, we attached three equally spaced markers bilaterally at each level (totally 12 markers, A1-6, D1-6; Fig. 1). Each marker was set at 13% of the distance between the two acromions. Thoracic anteroposterior diameter ratios (right/left) were calculated at rest during end-expiration. We calculated the upper and lower thoracic ratios as (BA1 + BA2 + BA3) / (BA4 + BA5 + BA6) and (CD1 + CD2 + CD3) / (CD4 + CD5 + CD6), respectively (Fig. 1). A left-right ratio value of 1 indicates symmetry in the anterior-posterior diameter between the left and right sides, while a value greater than 1 indicates that the right side is larger than the left.

Fig. 1.

Fig. 1.

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Positioning of thoracic markers.

The upper thoracic cage was defined at the level of the second thoracic rib joint. The lower thoracic cage was defined at the level of the xiphoid process.

To quantify the lateral thoracic deviation relative to the pelvis, based on previous studies10, 11, 16, 17), we used the difference in coordinates between the midpoint of the anterior and posterior superior iliac spines and the midpoint of the xiphoid process and its posterior projection. Positive values indicated rightward thoracic deviation, whereas negative values indicated leftward deviation (Fig. 2).

Fig. 2.

Fig. 2.

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Marker placement for thoracic lateral deviation.

ASIS: anterior superior iliac spine; PSIS: posterior superior iliac spine.

The movement task was STS from a chair. The participant’s upper limbs were relaxed and placed at their sides, and the chair height was adjusted to match the length of their lower limbs. The midpoint of the thigh was aligned with the front edge of the chair (Fig. 3). The subject was instructed to gaze forward along the midline and perform STS at a comfortable speed of their choice. Nuzik et al.15) described the STS task as consisting of two phases: flexion and extension. The transition between these phases was defined as the instant at which hip flexion reached its maximum (Fig. 4). STS movement was defined from the onset of trunk forward tilt to the moment of maximum hip extension. Total movement duration was normalized to 100% and calculated in 10% increments. The average timing of maximum hip flexion was calculated, and the time interval within that interval was defined as the flexion-extension transition interval.

Fig. 3.

Fig. 3.

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Seating arrangement.

The lower leg length and seat height were aligned. The midpoint of the thigh was aligned with the front edge of the seat.

Fig. 4.

Fig. 4.

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Sit-to-stand motion.

The flexion phase extends from the start of trunk flexion to maximum hip flexion. The extension phase extends from maximum hip flexion to full hip extension.

Using the Plug-in Gait full-body model, we calculated anterior head displacement as the distance between the COM of the head and that of the trunk at rest in the seating position. During the STS task, the thoracic flexion/extension and rotation angles relative to the pelvis, and the hip joint flexion/extension angle, were calculated. All kinematic data were normalized to 100% of the STS duration, and the mean of three trials was used for analysis. Mean values were calculated for each 10% movement interval. The analysis used the initial zone and the flexion-extension transition zone.

For statistical analysis, thoracic shape at rest was determined using 95% confidence intervals. The calculated parameters were evaluated for normality using the Shapiro–Wilk test. Thoracic rotation angles (relative to the pelvis) were compared between the start of the flexion phase and the transition zone using a paired t-test. Pearson’s product-moment correlation coefficient was used to assess the relationships between thoracic shape ratios at rest and thoracic flexion/rotation angles, and between anterior head displacement and thoracic angle changes. All analyses were performed using SPSS Statistics version 24 (IBM Japan, Tokyo, Japan), and the significance level was set at 5%.

RESULTS

The anteroposterior diameters of the upper and lower thorax at rest are presented in Table 1. Among the 23 participants, 20 exhibited leftward thoracic deviation while three showed rightward deviation. In most participants, the upper thorax was larger on the left side, and the lower thorax was larger on the right side. Since there were only three participants who showed rightward deviation and detailed analysis is not possible, the following discussion will be limited to the 20 participants who showed leftward thoracic deviation.

Table 1. Thoracic lateral deviation and anteroposterior diameter ratio of the thorax.

All participants (n=23) Left thoracic deviation (n=20) Right thoracic deviation (n=3)
Thoracic lateral deviation (mm) −6.1 ± 7.3 (−9.1 to −3.1) −7.9 ± 5.7 (−10.4 to −5.4) 6.0 ± 5.7 (−0.4 to 12.4)
Upper thoracic shape (Right/Left) 0.98 ± 0.01 (0.98 to 0.99) 0.98 ± 0.02 (0.98 to 0.99) 0.98 ± 0.01 (0.97 to 1.00)
Lower thoracic shape (Right/Left) 1.02 ± 0.02 (1.01 to 1.03) 1.02 ± 0.01 (1.02 to 1.03) 1.00 ± 0.01 (0.99 to 1.01)
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Values are mean ± SD (95% confidence interval).

In participants with left lateral deviation, the timing of peak hip flexion during the STS movement occurred at 42.7 ± 3.23% of the total movement time, with the 41–50% interval being the transition zone. The time series changes of each parameter are shown in Fig. 5.

Fig. 5.

Fig. 5.

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Time series changes in each parameter.

The left axis shows the hip joint and spinal flexion angle. Positive values ​​indicate flexion. The right axis shows the spinal rotation angle. Positive values indicates left rotation. Participants with left thoracic deviation, n=20.

Regarding thoracic rotation relative to the pelvis, the thorax showed significant leftward rotation during the transition zone (41–50%) compared with that in the initial position (1–10%) (Table 2).

Table 2. Comparison of thoracic rotation angle relative to pelvis.

Initial position (1–10%) 0.69 ± 2.03*
Flexion-extension transition phase (41–50%) 1.49 ± 2.45
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The data are from 20 participants with left thoracic deviation. Values are mean ± SD.

*Significantly different (p<0.05).

Regarding the relationship between thoracic shape and trunk movement (flexion phase), a correlation was observed between upper thoracic asymmetry and the magnitude of trunk flexion angle change; participants with greater upper thoracic asymmetry exhibited greater trunk flexion angles. No significant association was observed in the lower thorax (Table 3).

Table 3. Relationship between thoracic cage shape at rest and thoracic cage angle change during standing up (flexion phase).

Upper thoracic shape (Right/Left) Lower thoracic shape (Right/Left)
Change in thoracic flexion angle relative to the pelvis −0.457* 0.06
Change in thoracic rotation angle relative to the pelvis 0.172 0.387
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The data are from 20 participants with left thoracic deviation. *Significantly different (p<0.05).

Furthermore, no significant correlation was observed between thoracic shape (upper or lower) and the amount of trunk rotation during the STS movement (flexion phase) (Table 3). A correlation was found between anterior head displacement at rest and the magnitude of trunk flexion relative to the pelvis during STS (flexion phase). In contrast, no relationship was observed between head displacement and trunk rotation angle (Table 4).

Table 4. Relationship between forward displacement of the head at rest and change in thoracic flexion angle (flexion phase).

Forward displacement of the head at rest
Change in thoracic flexion angle relative to the pelvis 0.633**
Change in thoracic rotation angle relative to the pelvis 0.203
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The data are from 20 participants with left thoracic deviation. **Significantly different (p<0.01).

DISCUSSION

In this study, most participants exhibited leftward thoracic deviation relative to the pelvis, accompanied by opposing asymmetries in the upper and lower thoracic regions. Kakizaki10) similarly reported that, even among healthy adults, the thorax often deviates leftward relative to the pelvis. Hirayama et al.12) further noted that, in such cases, the upper thorax typically shows right anterior rotation on the right and posterior rotation on the left, whereas the lower thorax exhibits the opposite pattern. Our findings were consistent with this thoracic morphology, as reflected by a lower upper thoracic ratio (<1) and a higher lower thoracic ratio (>1) in participants with leftward thoracic deviation.

Among participants with left lateral deviation, trunk flexion preceded hip flexion, as shown in Fig. 5. Then, trunk rotation to the left occurred simultaneously with maximum hip flexion. The amount of trunk rotation to the left varied among individuals, and while small on average, it was present in many participants. This indicates the presence of asymmetry even in healthy individuals. The characteristics of movement during the initial and transitional stages of standing-up are considered as follows.

Gotoh et al.18) reported that during STS, the sartorius and rectus femoris muscles are activated first, followed by antigravity activity in the hamstrings, gluteus maximus, and trunk muscles. The flexion-extension transitional period in this study marks a switchover between trunk flexor and extensor muscles, and we believe that trunk muscle activity may be involved in rotational movement during this period. Previous studies13, 19) have reported that in patients with thoracic asymmetry, there are left-right differences in the quadratus lumborum and latissimus dorsi muscles. Sano et al.20) also reported that the cross-sectional area of ​​the right psoas major muscle is larger in people with left thoracic deviation. When the trunk muscles are activated bilaterally, they move in the sagittal plane, but when activated predominantly on one side, lateral bending and rotation occur, suggesting that the shape of the thorax and the trunk muscles are closely related.

Clinically, repeated small mechanical stresses applied locally in cases such as osteoarthritis and lumbar disorders can reportedly lead to subsequent illnesses. Maintaining slight, controlled unidirectional movement may play a role in both preventing impairment and supporting functional improvement.

Regarding the relationship between thoracic asymmetry and trunk movement, greater upper thoracic asymmetry was associated with increased trunk flexion relative to the pelvis. Because the ribs articulate with the thoracic spine via the costovertebral joints, asymmetry in the thoracic shape may induce rotational displacement of the thoracic vertebrae, leading to asymmetrical loading of the facet joints. This could restrict extension and facilitate flexion. Panjabi et al.21) reported that thoracic coupling motions are influenced by the facet joint condition and spinal posture, supporting our findings that thoracic morphology affect spinal kinematics.

Furthermore, we observed a correlation between anterior head displacement at rest and increased trunk flexion. In a previous study14), a relationship between thoracic shape and head position was reported. In the present study, an asymmetrical thoracic shape was observed, suggesting that the muscles spanning from the thorax to the head may also exhibit asymmetrical activity. Lin et al.22) reported that the length of the cervical muscles changes in a forward head posture (FHP). An asymmetrical thoracic shape may alter the tension of the muscles on either side, making it difficult to maintain the head in an appropriate position, and potentially contributing to FHP even at rest. Abd-Elshafy et al.23) found that children with FHP showed reduced back muscle endurance, while Nejati et al.24) reported associations between neck and thoracic postures during work and neck pain in office workers. These findings suggest that an FHP increases stress on the posterior structures and may contribute to trunk flexion. As the STS task requires forward propulsion, anterior head positioning in the initial phase may promote increased trunk flexion relative to the pelvis.

This study has some limitations. The number of participants with rightward thoracic deviation was small, and only healthy adult males were included. Additionally, muscle activity during the task was not monitored. Future research should involve participants with rightward deviation and incorporate detailed electromyographic analyses.

In conclusion, this study showed that trunk movement patterns during STS are associated with thoracic shape and head posture. When STS is impaired, individuals adopt compensatory strategies to accomplish the task. Efficient STS requires smooth transfer of force from the pelvis to the lower limbs, initiated by the head and upper body. Our findings highlight the importance of evaluating and treating alignment above the pelvis, including thoracic and head positioning, to optimize STS performance.

Conflict of interest

The authors declare that there is no conflict of interest regarding the publication of this article.

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