# Absolute Reliability and Concurrent Validity of the Modified
## 3 Results
### Validity
In contrast, the right trunk rotations of the two raters in our study were 45.4\({}^{\circ}\)\(\pm\) 8.2\({}^{\circ}\) and 45.3\({}^{\circ}\)\(\pm\) 8.5\({}^{\circ}\), and those of the left trunk rotations were 43.8\({}^{\circ}\)\(\pm\) 9.1\({}^{\circ}\) and 46.0\({}^{\circ}\)\(\pm\) 8.6\({}^{\circ}\), which were smaller than previously reported values [23]. The difference between the two sets of findings may be due to two main reasons: age and the test protocol [10]. Healthy swimming athletes who are from 10 to 18 years of age (mean age: 13.52 \(\pm\) 1.57 years) have excellent trunk rotation, which might be explained by the flexible shoulder joint caused by swimming, which is an open kinetic chain exercise, and their mean right trunk rotation angle range was found to be from 63.5\({}^{\circ}\) to 68.4\({}^{\circ}\), which was bigger than our findings [2]. [PERSON] et al. [24] also reported that age could affect the coordination pattern of the trunk and pelvis in the transverse plane, which could explain the smaller trunk rotation angles recorded in our study. In addition, the angle differences may have been caused by the fact that [PERSON] et al. 's study did not restrict pelvic movement during trunk rotation, as supported by [PERSON] et al. 's study [10], who study reported that trunk rotation movement patterns would differ due to age differences and shoulder-pelvic patterns. In our study, the inter-rater reliability of the MGP for trunk rotation (0.989-0.990) was higher than that reported by using other types of goniometers (0.736-0.853), such as bubble inclinometers, dual inclinometers, and smartphone clinometers [25]. Thus, the MGP could be a more reliable instrument for use by trained or even unskilled practitioners. A good-to-excellent within-day intra-rater reliability of the MGP in measuring trunk rotation at the sitting position was observed in this study. Our results were consistent with those of [PERSON] et al. [26], who showed that the intra-rater reliabilities for the right and left sides were 0.970 and 0.980, respectively. Therefore, the MGP has excellent reproducibility compared to previously reported goniometer measurements. Furthermore, two within-day measurements using the MGP showed a very strong correlation (r > 0.90); this finding is supported by the work of [PERSON] et al. [6]. [PERSON] et al. [2] reported that the intra-rater reliabilities of goniometric measurements for right and left trunk rotations in the lumbar lock rotation position were 0.910 (95% CI, 0.780-0.960) and 0.960 (0.890-0.980), respectively, which were similar to our findings. In conclusion, the seated trunk range of motion was shown to be relatively consistent within a day when measured using the MGP. This is the first time the MGP was compared against a more precise measurement instrument, 2D video analysis, in quantifying transverse trunk rotation. In a previous study, 2D video analysis was shown strongly correlate with 3D motion capture, which was considered to be the relative standard instrument for measuring trunk rotation (r = 0.810-0.993) [27,28]. Therefore, 2D video analysis is a valid tool for measuring left and right trunk rotations in the transverse plane. The MGP showed a strong correlation with 2D video analysis for evaluating trunk rotations in the transverse plane (0.981-0.986) and is considered a valid instrument. The mean difference between the MGP and 2D video analysis on the right side was 0.706\({}^{\circ}\) (95% CI: \(-\)3.485\({}^{\circ}\), 2.073\({}^{\circ}\)), and that on the left side was 1.171\({}^{\circ}\) (95% CI: 4.726\({}^{\circ}\), 2.384\({}^{\circ}\)). The right trunk rotation angle was smaller than the left trunk rotation angle, indicating that some of the participants in this cohort may have trunk tightness. Previous studies have reported that trunk tightness reduces the trunk rotation range of motion [29,30]. In addition, the findings of this study were similar to those reported by [PERSON] et al.