Abstract
[Purpose] The purpose of this study was to compare the trunk displacement and ground reaction force during hand-behind-back (HBB) movements between the right and left hands. [Participants and Methods] Twenty healthy right-handed men participated in this study. The measurement task involved performing HBB movements while standing, using a three-dimensional motion capture system and two force plates. Changes in trunk displacement and ground reaction force were measured to evaluate differences between the right and left hands at the examined level. [Results] Anterior trunk displacement was observed when the thumb reached the pelvis. The ground reaction force on the side of the HBB movement increased during this phase and decreased as the thumb touched the eighth thoracic vertebra. No significant differences were observed between right and left hands. [Conclusion] Anterior trunk displacement and changes in ground reaction force occurred in both right and left hands during HBB movement, with no differences observed between the right and left sides. During physical therapy evaluation and treatment, focusing on changes in trunk displacement and loading on the lower extremity on the side of the HBB movement may be useful.
Keywords: Hand-behind-back movement, Trunk displacement, Ground reaction force
INTRODUCTION
The scapula is located on the thorax1), and thus the glenohumeral and scapulothoracic joints moments are influenced by trunk motion. The motion of an object comprises both translational and rotational components, and the body’s motion is often divided into the translational and rotational motion of the center of mass of each body part2). The translational movement of the trunk during anterior elevation of the upper extremity has been studied, and greater anterior displacement of the trunk results in more upward rotation of the scapula during anterior elevation of the upper extremity3).
While it is evident that the glenohumeral and scapulothoracic joints engage during the hand-behind-back (HBB) movement4)—defined as reaching the hands to the lumbar and back regions behind the body—there are few reports addressing trunk movement in this context. In a sitting posture, with restricted anterior displacement of the trunk, it was more difficult for the participants to perform the HBB movement5). However, the anterior displacement of the trunk during right-left HBB movements has not yet been investigated and remains unclear. Thus, we hypothesize that anterior trunk displacement affects HBB movement. In a previous study, we observed that greater anterior displacement of the upper end of the sternum, located at the top of the trunk, allowed participants to reach their thumbs higher during right HBB movements6).
The height at which the thumb reaches the back during the HBB movement is reportedly higher on the left than on the right7). A right-left difference has been observed in the HBB movement; however, these factors have not been thoroughly considered. The standing posture is asymmetrical, with a greater load on the left lower extremity than on the right8, 9). Since the trunk accounts for approximately 50% of body mass10), we predict that changes in the lateral displacement of the trunk—located above the lower limbs—occur in the standing posture due to this asymmetrical lower limb loading. Furthermore, we predict that these changes will influence body movement during the HBB movement and contribute to the observed right-left differences. However, changes in the trunk shift and lower limb loading during the HBB movement have not been reported. We believe it would be useful to investigate these factors to enhance our understanding of the mechanics related to the HBB movement.
Therefore, the purpose of this study was to compare anteroposterior and lateral trunk displacement, as well as ground reaction force (GRF), between the right and left hands during the HBB movement. We hypothesized that anterior and lateral trunk displacement, and an increase in the GRF of the lower limb on the side engaged in the HBB movement, would occur during the HBB movement. Furthermore, we posited that there would be right-left differences in these changes, with the right HBB movement being greater than that of the left.
PARTICIPANTS AND METHODS
This study was a cross-sectional design conducted between February and October 2023. The participants were the same as those in a previous study (right-handed, healthy males in their 20s)11). Participants with complaints related to the upper extremities or trunk, those with a history of surgery, and those engaged in sports that heavily utilize the right upper extremity were excluded from the study. This exclusion was because baseball, volleyball, and tennis players have right-left differences in the range of motion of shoulder joint internal rotation12,13,14). Before conducting the study, informed consent was obtained from all participants, and measurements were performed after obtaining their written consent. This study was approved by the Showa University Research Ethics Review Board (No. 22-103-B)11).
The same system and task employed in the previous study11) were used for measurements. A three-dimensional motion capture system comprising eight infrared cameras (VICON NEXUS2; Vicon Motion System Ltd., Oxford, UK) and two force plates (AMTI Ltd., Watertown, MA, USA) was used for data collection. The sampling frequency for the infrared cameras was set at 100 Hz to capture marker coordinate data while the force plates operated at a frequency of 1,000 Hz to acquire GRF data.
The same examiner, a physical therapist, measured the passive range of motion (ROM) for internal rotation and extension at the shoulder joint, as well as flexion at the elbow joint, using a goniometer before the measurements. Following a previous study11), infrared reflective markers with a diameter of 14 mm were affixed to the thorax, spine, and pelvis by the same examiner. Participants maintained a standing posture with both the right and left lower extremities positioned on the force plates; this position served as the starting point. They were instructed to gaze forward with their thumbs pointed anteriorly. This position was used as the starting position, and the participants performed the right and left HBB movements to the maximum level within 2 seconds (Fig. 1). Measurements were taken five times on each side, with the order of measurement randomized.
Fig. 1.
Measurement task. (A) the starting position, (B) right hand-behind-back (HBB) movement, (C) left HBB movement.
The anteroposterior displacement of the trunk (displacement along the sagittal plane at the midpoint of the line connecting the eighth thoracic vertebra and the xiphoid process) and the lateral displacement of the trunk (displacement along the frontal plane) were calculated from the position coordinates of the measured infrared reflective markers (Fig. 2). The data collected from the force plates were normalized by dividing the body weight to calculate the vertical component of the GRF on the side performing the HBB movement. Higher values indicated greater displacement toward the anterior in the sagittal plane and greater displacement on the side performing the HBB movement in the frontal plane, as well as increased GRF. A Butterworth Filter was applied to remove noise from the marker data and force plate data acquisition, using cutoff frequencies of 6 Hz and 15 Hz, respectively. As in the previous study11), the phases where the thumb reached the posterior superior iliac spine (PSIS) level, the third lumbar vertebra (L3) level, the eighth thoracic vertebra (Th8) level, and the maximum (Max) level were extracted from the measured movements and designated as the examined levels. The average of the five measurements was used as the analytical value. The amount of change was calculated by subtracting the starting position value from each of the extracted thumb reach levels.
Fig. 2.
Calculation of trunk displacements. Index of trunk displacement was the midpoint of the line connecting the eighth thoracic vertebra (Th8) and the xiphoid process (XP). Displacement in the sagittal axis for anteroposterior displacement and in the frontal axis for lateral displacement were calculated.
The normality of all data was confirmed using the Shapiro–Wilk test. A two-way repeated measures analysis of variance was used for statistics to compare the anteroposterior and lateral displacements of the upper trunk and the GRF between the examined levels and the right and left sides was also assessed. Paired t-tests were performed on the shoulder and elbow joints for passive ROM, and if a significant right-left difference was observed, it was included as a covariate in the analysis of variance. The interaction between the examined levels and the right and left was also assessed. When a main effect was observed between the examined levels, the Steel–Dwass test was used as a post-hoc analysis, and the combined right-left values were compared between the examined levels. All statistical analyses were conducted using JMP Pro17.0.0. (SAS Institute Japan Co. Ltd., Tokyo, Japan) with a critical rate of less than 5% considered significant.
RESULTS
No significant interaction was observed between the examined levels and the right and left sides for the anteroposterior displacement of the trunk (p=0.935), lateral displacement of the trunk (p=0.052), and vertical component of the GRF (p=0.076). There were no right-left differences in the changes between the examined levels (Table 1).
Table 1. Changes in trunk displacement and vertical component of ground reaction force.
| PSIS level | L3 level | Th8 level | Max level | ||
| Anteroposterior displacement of trunk | Right | 7.3 ± 4.8 | 8.2 ± 6.2 | 9.7 ± 6.6 | 10.9 ± 7.1 |
| (mm) | Left | 5.8 ± 4.4 | 6.6 ± 6.0 | 9.6 ± 6.6 | 10.1 ± 6.4 |
| Combined | 6.6 ± 4.6* | 7.4 ± 6.0* | 9.7 ± 6.5* | 10.5 ± 6.7* | |
| Lateral displacement of trunk | Right | 1.5 ± 2.3 | 2.1 ± 2.8 | 3.6 ± 4.2 | 3.2 ± 4.7 |
| (mm) | Left | −0.1 ± 2.7 | −0.3 ± 3.4 | −0.04 ± 4.2 | −0.9 ± 5.1 |
| Combined | 0.7 ± 2.6 | 0.9 ± 3.3 | 1.8 ± 4.5 | 1.2 ± 5.3 | |
| Ground reaction force | Right | 0.07 ± 0.06 | 0.05 ± 0.1 | 0.01 ± 0.1 | 0.02 ± 0.1 |
| (N/kg) | Left | 0.03 ± 0.09 | −0.002 ± 0.1 | −0.03 ± 0.1 | −0.02 ± 0.1 |
| Combined | 0.05 ± 0.1* | 0.02 ± 0.1 | −0.01 ± 0.1** | −0.001 ± 0.1 |
“Combined” indicates the combined values of the right and left sides; *p<0.05 (vs. starting position), **p<0.05 (vs. PSIS level). PSIS: posterior superior iliac spine; L3: third lumbar vertebra; Th8: eighth thoracic vertebra; Max: maximum.
A significant main effect was observed at the examined levels. The change in the combined right and left values of the anteroposterior displacement of the upper trunk at the PSIS level was significantly greater than that at the starting position (p<0.0001; Table 1). In contrast, the change in the combined right and left values for lateral displacement of the upper trunk was not significantly different between the examined levels. Furthermore, the change in the combined right and left values for the vertical component of the GRF was significantly greater at the PSIS level compared with the starting position (p<0.0001; Table 1) and significantly smaller at the Th8 level than at the PSIS level (p=0.007; Table 1).
DISCUSSION
This study was conducted to compare the anteroposterior and lateral displacement of the trunk, and the vertical component of the GRF during HBB movement between the right and left hands. As hypothesized, an increase in the anterior displacement of the trunk and the vertical component of the GRF on the side of the HBB movement was observed during the activity. However, contrary to our hypothesis, no significant change was observed in the lateral displacement of the trunk, and no significant difference was observed in the anteroposterior and lateral displacements of the trunk, nor the vertical component of the GRF between the right and left hands.
The anteroposterior displacement of the trunk at the PSIS level was significantly larger than that at the starting position, indicating that anterior displacement of the trunk occurs when the thumb reaches the PSIS during the HBB movement. A previous study demonstrated that HBB movement was restricted when the trunk was not moved forward5). We speculate that the anterior displacement of the trunk observed in this study is a necessary kinematic component of the HBB movement. As the thumb reaches the PSIS, glenohumeral joint extension occurs,4) causing the upper limb to displace backward. In response, we consider that the center of gravity is displaced backward, prompting the trunk to move forward to counteract this displacement. Notably, the lateral displacement of the trunk did not change significantly among the analyzed positions. In a previous study, upper trunk rotation contralateral to the side of the HBB movement occurred when the thumb reached the PSIS level during the HBB movement11). We hypothesized that lateral displacement of the trunk toward the side of the HBB movement would occur because trunk rotation is typically accompanied by lateral translation of the thorax to the opposite side15). However, in this study, because the standard deviation of the lateral displacement of the trunk was large, we considered that the lateral displacement of the trunk did not vary significantly among the participants, with some participants exhibiting substantial lateral trunk displacement while others showed minimal displacement. We speculate that the initial position of the trunk in the standing posture may have varied, with some participants displaced to the left and others to the right. However, further studies are required to clarify this.
The vertical component of the GRF on the HBB side at the PSIS level was significantly greater than that at the starting position, indicating that the load on the lower limb of the HBB movement side was greater when the thumb reached the PSIS level. We speculate that the lateral displacement of the pelvis allows the PSIS to approach the thumb at this PSIS, resulting in a greater load on the lower extremity on the side of the HBB movement. Furthermore, the vertical component of the GRF on the HBB side at the Th8 level was significantly smaller than that at the PSIS. To maintain a stable standing posture while allowing the thumb to reach the Th8 position, the GRF returned to a level comparable to that in the starting position.
No significant differences were observed in trunk displacement or the vertical component of the GRF between the right and left hands. The load on the left lower limb while standing was greater than that on the right lower limb in the right-handed participants8). We hypothesized that there would be minimal changes in load to the left lower extremity during left HBB movement, while the load on the right lower extremity would be greater during right HBB movement. However, no significant right-left difference was observed. We speculate that this discrepancy may stem from the diverse right-left difference in the load on the lower limb during the standing posture for each participant. In addition, the changes in GRF during the HBB movement also varied among participants; some exhibited large changes in GRF when the thumb reached the PSIS level while others had small changes in GRF. In addition, the participants showed variations in the lateral tilt and rotation angles of the trunk in the static standing posture; this variation caused some participants to have a considerable leftward trunk displacement, whereas others experienced a minor displacement. Trunk rotation is accompanied by lateral displacement15) and flexion16).
This study has some limitations. First, the participants were exclusively right-handed, young, healthy males. Second, the method of HBB movement was specified to require the thumb to move laterally by one finger breadth from the spinal column. Finally, it is important to acknowledge that the applied markers may shift between the bone and the skin.
In conclusion, we revealed in this study that the anterior displacement of the trunk and the vertical component of the GRF on the side of the HBB movement changed, with no significant right-left differences observed during the HBB movement. Therefore, physical therapy evaluations and treatment focusing on these changes may help understand the characteristics of the HBB movement in participants and in approaching them to improve the HBB movement. The study results suggest that not only upper limb motion but also anterior trunk displacement and increased load on the ipsilateral lower limbs are involved during the HBB movement; this finding is clinically significant as it provides new insights into the motor functions associated with the HBB movement. Future research should investigate the relationship between the trunk rotation angle, lateral tilt angle, and lateral displacement in the standing posture and the trunk movement during the HBB movement.
Conference presentation
A part of this study was presented at the Japanese Society of Musculoskeletal Physical Therapy Congress 2024.
Funding
This study did not receive any specific grant from funding agencies.
Conflicts of interest
We have no conflicts of interest to disclose.
Acknowledgments
We would like to express our gratitude to all of our research participants for their cooperation.
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