Abstract
[Purpose] This study examined the relationship between the angle of trunk lateral tilt and lateral trunk shift in the standing position and the angle of trunk rotation during right and left hand-behind-back motion. [Participants and Methods] We included twenty healthy, right-handed men. Hand-behind-back motion in the standing position was recorded using a three-dimensional motion-capture system. Correlations between the angle of trunk lateral tilt and lateral trunk shift in standing position and the angle of trunk rotation during hand-behind-back motion were analyzed. [Results] A significant correlation was observed between the angle of trunk lateral tilt in the standing position and the angle of trunk rotation during right and left hand-behind-back motion. Greater rightward trunk tilt in standing was associated with greater leftward trunk rotation during right hand-behind-back motion and greater rightward trunk rotation during left hand-behind-back motion. A significant correlation was observed between the lateral trunk shift in standing and the angle of trunk rotation during right hand-behind-back motion. [Conclusion] The angle of trunk lateral tilt and lateral trunk shift in the standing position were associated with the angle of trunk rotation during right and left hand-behind-back motion. These findings may be useful when assessing hand-behind-back motion in physical therapy.
Key words: Hand-behind-back motion, Trunk lateral tilt, Trunk rotation
INTRODUCTION
The hand-behind-back (HBB) motion, in which the hand reaches behind the body, is a composite motion involving internal rotation and extension of the glenohumeral joint, as well as flexion of the elbow joint1). The finger-vertebral distance (FVD), defined as the distance between the tip of the thumb and the seventh cervical spinous process (C7), is commonly used as an indicator of HBB motion. Previous research reported that the FVD was shorter on the left side than on the right2), suggesting laterality in HBB motion. We hypothesized that factors other than glenohumeral internal rotation range of motion contribute to this laterality in FVD. During HBB motion, internal rotation of the glenohumeral joint is generally completed by the time the thumb touches the sacroiliac joint3). Furthermore, FVD demonstrated only a weak-to-moderate correlation with glenohumeral internal rotation range of motion4). A previous study showed that scapulothoracic joint internal rotation and contralateral trunk rotation were observed during HBB motion5). However, no right-left differences were observed in these measures, and the factors underlying HBB laterality remain unclear.
The scapula lies on the thorax and contributes to upper-arm movement6). Additionally, scapular motion is influenced by posture. Posterior scapular tilt was reduced during upper limb anterior elevation in a slouched posture7). Standing posture also affects HBB motion. Spinal alignment in the sagittal plane affects the range of motion during HBB motion following reverse total shoulder arthroplasty8).
Greater weight-bearing on the left lower limb than on the right during standing9, 10) indicates that standing posture is asymmetrical. As the trunk accounts for nearly half of the total body mass11), lateral shift of the trunk during standing may contribute to this asymmetry; however, this remains unclear. Trunk rotation while standing is accompanied by lateral shift of the trunk toward the contralateral side12) and lateral tilt toward the same side13, 14). We hypothesized that lateral shift and lateral tilt of the trunk in the standing position influence trunk rotation during HBB motion, although this relationship has not been elucidated. Clarifying these relationships may improve understanding of the motor components involved in HBB motion and may be useful in physical therapy assessments aimed at improving this motion.
Therefore, the purpose of this study was to investigate the relationships between the angle of lateral trunk tilt and lateral trunk shift in the standing position and the angle of trunk rotation during right and left HBB motion. We hypothesized that, in standing, the trunk would shift laterally to the left and tilt laterally to the right relative to the pelvis. Furthermore, we hypothesized that greater leftward trunk shift and rightward trunk tilt while standing would be associated with smaller leftward trunk rotation during right HBB motion and greater rightward trunk rotation during left HBB motion.
PARTICIPANTS AND METHODS
A total of 20 healthy male participants with right-hand dominance were recruited. The demographic characteristics of the participants were as follows: age, 27.6 ± 4.3 (standard deviation) y; height, 170.9 ± 4.3 cm; and weight, 62.3 ± 5.5 kg. This cross-sectional study was conducted between February and October 2023. The sample size was determined with reference to a previous study15). As reliable estimates of the expected effect size were unavailable, the sample size was determined with reference to previous studies that used similar participants and kinematic measurements. Participants were excluded if they had symptoms involving the glenohumeral joint or trunk, a history of surgery involving the glenohumeral joint or other upper limb joints, or neurological disorders. Participants with sports experience predominantly involving the right upper limb were also excluded. It has been reported that baseball16), volleyball17), and tennis18) players exhibit lateral differences in the range of motion for internal rotation of the glenohumeral joint. Written informed consent was obtained from all participants before participation. The study protocol was approved by the Showa University Research Ethics Review Board (No. 22-163-B).
Prior to measurement, the same examiner attached 14-mm-diameter infrared reflective markers to the participants’ spine, sternum, and pelvis, as described in our previous study5). Participants performed right and left HBB motion in standing, with five repetitions on each side in random order. Because of minimize excessive anteroposterior trunk inclination, participants were instructed to keep their gaze forward and begin from an arm-at-side posture with both arms relaxed at their sides. They then moved the thumb to a point one finger width lateral to the posterior superior iliac spine (PSIS) and continued the movement as far as possible along the direction lateral to the spinous processes. The movement from the arm-at-side posture to the maximum position was performed in 2 s (120 beats/min) using a metronome. Measurements were obtained using eight infrared cameras three-dimensional motion capture system (VICON NEXUS2; Vicon Motion System Ltd., Oxford, UK), and marker position data were acquired at a sampling frequency of 100 Hz. A low-pass Butterworth filter with a cut-off frequency of 6 Hz was applied to remove noise from the marker position data.
Lateral trunk shift both in the standing position and during HBB motion was calculated as the frontal-axis displacement of the midpoint between the eighth thoracic spinous process (Th8) and the xiphoid process (XP), whereas lateral pelvic shift was defined as the frontal-axis displacement of the midpoint between the left and right PSIS. Lateral trunk shift relative to the pelvis was calculated by subtracting lateral pelvic shift from lateral trunk shift. The relative trunk position was calculated with lateral pelvic shift set to 0.
Using the same method as in our previous study5), we defined the local coordinate systems (segments) of the upper-trunk and pelvis utilizing VICON Body Builder (version 3.6.4; Vicon Motion System Ltd.). The upper-trunk segment was defined using four landmarks: XP, jugular notch, C7, and Th8. The pelvic segment was defined using the bilateral anterior superior iliac spines (ASIS) and PSIS. The angle between the upper-trunk and pelvic segments was defined as the trunk angle in the HBB motion and calculated using Euler angles. Rotation around the vertical axis was defined as the angle of trunk rotation, with rightward rotation denoted as positive. Rotation around the sagittal axis was defined as the angle of trunk lateral tilt, with rightward tilt denoted as positive. The trunk angles in the standing position were defined as the upper-trunk segment relative to the global coordinate system. The pelvic angle was defined as the angle of the pelvic segment relative to the global coordinate system. Angles were calculated using an X-Y-Z rotation order, where X represented the frontal axis, Y the sagittal axis, and Z the vertical axis. Rotation around the sagittal axis was calculated as the angle of pelvic lateral tilt, with a right tilt denoted as positive.
The arm-at-side posture and the position in which the thumb reached the Th8 level during HBB motion were selected for analysis (Fig. 1). The average of the five trials for each variable was used for analysis. Changes in each trunk angle during the HBB motion were calculated by subtracting the value in the arm-at-side from that in the Th8 position.
Fig. 1.
Measurement positions. (A) arm-at-side posture, (B) eighth thoracic vertebra (Th8) position in the right hand-behind-back (HBB) motion, (C) Th8 position in the left HBB motion.
Shapiro–Wilk test was used to assess normality for all calculated variables. As lateral pelvic shift was set to 0, a one-sample t-test was used to determine whether lateral trunk shift relative to the pelvis differed significantly from 0 in the arm-at-side posture. Correlations were examined between (1) the angle of trunk lateral tilt and (2) lateral trunk shift in the standing position and the angle of trunk rotation in the Th8 position during HBB motion. Pearson’s product-moment correlation coefficient was used for normally distributed data, whereas Spearman’s rank correlation coefficient was used for non-normally distributed data. JMP Pro 17.0.0 (SAS Institute Japan Co. Ltd., Tokyo, Japan) was used for statistical analyses, and the significance level was set at 5%.
RESULTS
Table 1 presents the lateral trunk shift, angle of trunk lateral tilt, and angle of trunk rotation in the standing position and the HBB motion.
Table 1. Trunk lateral shift and trunk angles in standing and HBB motion.
| Standing | Right HBB | Left HBB | |
| Trunk lateral shift (mm) | −5.9 ± 16.2 | −2.3 ± 16.8 | 5.5 ± 18.3 |
| Angle of trunk lateral tilt (°) | 16.8 ± 4.7 | −1.5 ± 1.9 | 0.9 ± 2.0 |
| Angle of trunk rotation (°) | 0.02 ± 1.5 | −1.5 ± 1.1 | 1.2 ± 1.8 |
Values are mean ± standard deviation. HBB: hand-behind-back.
In the standing position, the trunk was shifted to the left relative to the pelvis, and the angle of trunk lateral tilt demonstrated a significantly greater rightward tilt than the angle of pelvic lateral tilt (both p<0.001; Table 2).
Table 2. Lateral shift and lateral tilt angle of trunk and pelvis in standing.
| Trunk | Pelvis | |
| Lateral shift (mm) | −5.9 ± 16.2* | 0.0 ± 0.0 |
| Angle of lateral tilt (°) | 16.8 ± 4.7* | 0.2 ± 2.1 |
Values are mean ± standard deviation. *p<0.001 (vs. pelvis).
A significant correlation was observed between the angle of trunk lateral tilt in the standing position and the angle of trunk rotation during right and left HBB motion. Greater rightward trunk tilt in the standing position was associated with greater leftward trunk rotation during right HBB motion (r=−0.55, p=0.013; Table 3) and greater rightward trunk rotation during left HBB motion (r=0.46, p=0.041; Table 3). A significant correlation was also observed between the lateral trunk shift in the standing position and the angle of trunk rotation during right HBB motion. Greater leftward trunk displacement in standing position was associated with smaller leftward trunk rotation during right HBB motion (r=−0.47, p=0.035; Table 3). No significant correlation was observed between lateral trunk shift in standing position and the angle of trunk rotation during left HBB motion (Table 3).
Table 3. The correlation between angle of trunk lateral tilt and lateral trunk shift in standing, and angle of trunk rotation during HBB motion.
| Angle of trunk lateral tilt | Lateral trunk shift | ||
| Right HBB | Angle of trunk rotation | −0.55* | −0.47* |
| Left HBB | Angle of trunk rotation | 0.46* | 0.00 |
Values are correlation coefficients. *p<0.05. HBB: hand-behind-back.
DISCUSSION
This study investigated the relationship between the angle of trunk lateral tilt and lateral trunk shift in standing, and angle of trunk rotation during right-left HBB motion. As hypothesized, the trunk was shifted to the left and tilted to the right relative to the pelvis in standing. As hypothesized, greater right lateral tilt of the trunk in standing was associated with greater right rotation of the trunk during left HBB motion. Nonetheless, contrary to this hypothesis, greater rightward tilt of the trunk during standing was associated with greater leftward rotation of the trunk during right HBB motion. In addition, greater left trunk shift in standing was associated with smaller left rotation of the trunk during right HBB motion. Contrary to this hypothesis, no significant correlation was observed between lateral trunk shift in standing and angle of trunk rotation during left HBB motion.
In the standing position, the trunk was laterally shifted to the left and tilted to the right relative to the pelvis, indicating an asymmetrical standing posture. We speculate that this asymmetry may facilitate use of the right lower limb as the swing leg during movements such as walking by positioning the trunk over the left lower limb, which serves as the support leg. The trunk accounts for 50% of total body mass11). During walking, the right lower limb provided a locomotor function, whereas the left lower limb served a supporting function19). A rightward tilt of the trunk in the standing position is thought to be a postural control mechanism that maintains balance in response to leftward lateral trunk shift. We believe this response prevented the center of gravity from shifting outward as the trunk shifted laterally to the left in the standing position. The trunk tilted toward the contralateral side in response to lateral pelvic displacement in the standing position20).
A correlation was observed between the angle of trunk lateral tilt in the standing position and the angle of trunk rotation during HBB motion, suggesting that standing posture might be involved in trunk movement during HBB motion. Greater rightward lateral trunk tilt in standing was associated with greater rightward trunk rotation during left HBB motion. This finding is consistent with the concept of spinal coupling motion13), in which thoracic rotation is accompanied by ipsilateral thoracic lateral flexion. In contrast, greater rightward trunk tilt in standing was also associated with greater leftward trunk rotation during right HBB motion. These results contradicted the hypothesis that the greater the rightward tilt of the trunk in the standing position, the smaller the leftward rotation of the trunk during HBB motion. From the perspective of spinal coupling motion, the combination of right lateral flexion of the trunk and left rotation of the trunk is disadvantageous. We speculate that participants with a significant rightward tilt of the trunk in the standing position might find it difficult to rotate the trunk during right HBB motion. Because no laterality was observed in the angle of trunk rotation during HBB motion5), participants may have compensated by performing trunk rotation in an unfavorable posture. It is possible that leftward trunk rotation during right HBB motion was accompanied by leftward trunk lateral tilt in participants with greater rightward trunk tilt during standing. This aspect requires further consideration. Thus, the correlation between the angle of trunk lateral tilt in the standing position and the angle of trunk rotation during HBB motion differed between the bilateral sides. We believe that this may be related to the fact that the FVD during the HBB motion shows side-to-side differences.
A correlation was observed between lateral trunk shift in the standing position and the angle of trunk rotation during right HBB motion. These findings suggest that lateral trunk shift in standing might contribute to angle of trunk rotation during right HBB motion. Previous research demonstrated that the trunk rotates leftward during right HBB motion5). Because trunk rotation in standing is accompanied by contralateral lateral trunk shift12), leftward trunk rotation is accompanied by rightward trunk shift. Therefore, when the trunk is already shifted leftward in standing, additional leftward trunk rotation during right HBB motion may become difficult. In contrast, no significant correlation was observed between lateral trunk shift in standing and the angle of trunk rotation during left HBB motion. We consider that the change in the angle of trunk rotation during left HBB motion was not large enough to be associated with lateral trunk shift. Furthermore, the relationship between changes in the angle of trunk rotation and lateral trunk shift varied among participants. We speculate that, in some participants, greater trunk lateral shift resulted in greater contralateral trunk rotation, whereas in others, contralateral trunk rotation was greater regardless of trunk lateral shift. Furthermore, the relationship between lateral trunk shift in standing position and the angle of trunk rotation during HBB motion differed between the left and right hands. We believe that lateral trunk shift in standing position could also potentially reflect the lateral difference in FVD during the right and left HBB motion. Although we focused on standing trunk lateral displacement in this study, we cannot exclude the possibility that trunk lateral displacement during HBB also contributes to trunk rotation during the movement. This aspect was beyond the scope of the present study and warrants further investigation. As this study was based on a cross-sectional design, the direction of causality could not be determined. Asymmetry in standing posture could potentially influence trunk motion during HBB. In contrast, habitual movement patterns, including HBB, might contribute to asymmetry in standing posture. Further longitudinal studies would be required to clarify these relationships. Moreover, upper-trunk sagittal inclination might be involved in scapulothoracic motion during HBB. In our previous study, upper-trunk anteroposterior tilt was significantly associated with scapular internal rotation during HBB21). However, in this study, we focused on the relationship between frontal-plane standing trunk posture and trunk rotation during HBB rather than scapulothoracic kinematics. Future studies should investigate these factors simultaneously.
Several limitations of this study should be acknowledged. First, only healthy right-handed male participants in their 20s and 30s were included, thereby potentially limiting the generalization of the results to other populations. Second, the manner in which the HBB movement was performed was constrained. Other procedures for HBB movement might lead to different results. Third, angle calculation was performed using a rigid-body model, which may not fully reflect the actual angles. Consequently, angle measurements may vary depending on the measurement system used.
In conclusion, this study revealed that the trunk was laterally shifted to the left and tilted to the right relative to the pelvis in the standing position, indicating an asymmetrical standing posture. Furthermore, the angle of trunk lateral tilt and lateral trunk shift in standing were associated with the angle of trunk rotation during HBB motion. These findings suggest that standing trunk posture may influence trunk rotation during HBB motion. Therefore, we speculate that, when conducting a physical therapy assessment aimed at improving HBB motion, focusing on the relationship between the angle of trunk lateral tilt and lateral trunk shift in standing position is useful.
Conference presentation
Part of this study was presented at the 2025 Japanese Society of Musculoskeletal Physical Therapy Congress (https://cms.jspt.or.jp/upload/branch/jsmspt/obj/files/13gakkai/13th_shoroku.pdf).
Funding
This study did not receive any specific grants from funding agencies.
Conflict of interest
The authors declare no conflicts of interest.
Acknowledgments
The authors would like to express their sincere gratitude to all the participants who contributed to this study.
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