# Reliability of a Global Gait Symmetry Index Based on Linear Joint Displacements
## 4 Discussion  
This study assessed the reliability of a newly developed global gait symmetry index, the LGGA. This index was developed with the intention to provide the clinicians with a tool that can complement the clinical gait analysis with a measure of overall gait symmetry (similarly to how the Gait Profile Score [23] is used as a measure of overall gait quality but focused on symmetry). Thus, it is intended to aid the clinician in assessing the patient progress in terms of gait symmetry during injury recovery and/or in response to an intervention. A strength of this study is its careful design. The time between visits was approximately equal and close to 7 days for all participants, with the second visit being performed at the same time of day as the first. Furthermore, participants maintained their healthy condition in both visits, were allowed a familiarisation period before each data collection, and did not know when the data were being recorded. These characteristics assured identical testing conditions and trustworthy data. Additionally, the present data set has been shown to be highly reliable [26]. On the other hand, the current results are limited to a single examiner and therefore should not be extrapolated to studies performed by multiple examiners. As hypothesised, the LGGA presented higher reliability and lower measurement error than its predecessor, the GGA. More specifically, in terms of reliability, the ICC of 0.71 obtained for the LGGA is much higher than the 0.30 previously obtained for the GGA [14]. Additionally, the corresponding 95% confidence intervals obtained for the LGGA are also much narrower, indicating that this ICC value was estimated with greater precision. Nonetheless, these confidence intervals are still quite wide. The ICC result obtained was also well above the ICC values (0.01 to 0.35) obtained for a step time symmetry index in healthy individuals [13]. An ICC of 0.71 means that 71% of the variability observed in LGGA scores are attributed to real differences in gait symmetry between participants while the remaining 29% of this variability are attributed to measurement error [36]. This value is just above the 0.70 recommended for research purposes, but below the recommendations for clinical purposes (>0.90) [37]. However, these recommendations are not absolute standards, leaving space for the judgement of the researcher or clinician within the context of the specific score [38]. Previous research [9] has shown that the symmetry ratio presented higher ICC values (between 0.93 and 0.98), in a sample of stroke patients, than the LGGA. However, the ability of the symmetry ratio to discriminate stroke survivors from healthy individuals was not very good [2], despite obtaining ICC values above the recommendations for clinical purposes. The poor to moderate discriminative ability of the symmetry ratio in the face of high ICC values may be because spatio-temporal symmetry does not provide a fair assessment of overall gait symmetry [15]. Thus, by quantifying the bilateral differences in joint positions throughout the entire gait cycle, it is possible that the LGGA may be more sensitive to real differences in symmetry among individuals (i.e., more reliable), despite its greater proportion of error variance (lower ICC value). Of course, this can only be ascertained with future research to verify the ability of the LGGA to discriminate individuals with clinically relevant differences in gait symmetry. In terms of measurement error, the 95% LOA of the GGA were close to plus and minus 60%, the SEM was 22.4% and the SDC was 62.1% [14]. The corresponding results obtained for the LGGA in the present study, with the same data set, were cut in half, as shown in Table 2. This drastic reduction in measurement error, in comparison to the GGA, can be explained by the lower variability in LGGA differences between visits (i.e., these differences were more consistent among participants), as seen by the standard deviation of the differences in Table 1 of both manuscripts.