# Reliability of a Global Gait Symmetry Index Based on Linear Joint Displacements
###### Abstract  
Gait symmetry is commonly used as an informal measure to assess functional gait recovery. As other outcome measures used for the evaluation of clinical change over time, gait symmetry indices must be reliable. However, studies assessing the reliability of symmetry indices are scarce and focused on discrete and local indices (i.e., peak joint angle, step length), which fail to assess overall gait symmetry. On the other hand, the repeatability of global symmetry indices (using multiple continuous waveforms) based on joint angles may be hampered by the sensitivity of these variables to marker placement imprecision. The aim of this study is to evaluate the test-retest intra-rater reliability and measurement error of an alternative global symmetry index. Two 3D gait analyses were performed on separate days (a week interval) on twenty-three healthy adults. Reliability and measurement error were assessed by calculating the intraclass correlation coefficient and the 95% limits of agreement, standard error of measurement and smallest detectable change, respectively. The new symmetry index presented acceptable results in terms of reliability (ICC = 0.71, 95% CI 0.33-0.88) and measurement error (95% LOA between \(-\)30.2% and 29.1%, SEM = 10.7% and SDC = 29.7%), thus being a more promising tool to assess overall gait symmetry. walling; asymmetry; test-retest reliability; healthy; kinematic; marker based Article  
Reliability of a Global Gait Symmetry Index Based on Joint Displacements  
[PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]. variable, such as step-length, a joint angle, ground reaction force, or other biomechanical variables. Assessing gait symmetry from spatiotemporal parameters alone is flawed, because step length may be symmetric even though overall gait is highly asymmetric [15]. Furthermore, when assessing symmetry in joint angles (or other biomechanical variables that vary throughout the gait cycle), these symmetry indices require the extraction of a single discrete value (i.e., peak angle, or angle at initial contact). Through this single value extraction, one may neglect the temporal information of gait waveforms [16] and may fail to capture important asymmetries that occur in other portions of the gait cycle [17]. Additionally, discrete values are prone to generating false positives [18]. The reliability of a global symmetry index called Global Gait Asymmetry (GGA) [19] was also previously analysed [14]. This index overcomes the limitations above, by including continuous gait data from various local parameters (the three components of the trunk, pelvis, hip, knee, and ankle joint angles). However, the GGA presented very poor reliability between sessions, possibly because of the cumulative sum (frame by frame and at various joints) of bilateral differences that are functionally irrelevant and/or caused by marker placement inconsistency. Errors in anatomical landmark position determination are known to affect the position and orientation of a segment's anatomical frame [20]. These errors are particularly detrimental to the precision of the longitudinal orientation component, especially on the femur and tibia. Furthermore, joint angles are highly sensitive to these variations in anatomical frame orientations and are less reliable for those components undergoing small variations. Specifically, while standing, transverse plane angles are generally the least precise, particularly at the knee. During motion, coronal and transverse plane angles are generally less reliable, and for the knee joint only, precision in these components depend on the level of flexion/extension [21]. Although one may expect that the exclusion of these less reliable joint angles from the symmetry index equation might improve the index's reliability, the sensitivity of the remaining joint angles to errors in the anatomical frame orientation may still be detrimental to the reliability of a symmetry index.