# Test-Retest Reliability of a 6 DoF Marker Set for Gait Analysis in Cerebral Palsy Children  
[PERSON]  
1 CIPER, Faculdade de Mortridade Humana, Universidade de Lisboa, Estrada da Costa, Cruz-Quebrada-Dafumdo, 1499-02 Oezias, Portugal, [EMAIL_ADDRESS] {T.J.}  
2  
[PERSON]  
1 CIPER, Faculdade de Mortridade Humana, Universidade de Lisboa, Estrada da Costa, Cruz-Quebrada-Dafumdo, 1499-02 Oezias, Portugal, [EMAIL_ADDRESS] {T.J.}  
2  
[PERSON]  
1 CIPER, Faculdade de Mortridade Humana, Universidade de Lisboa, Estrada da Costa, Cruz-Quebrada-Dafumdo, 1499-02 Oezias, Portugal, [EMAIL_ADDRESS] {T.J.}  
2  
[PERSON]  
1 CIPER, Faculdade de Mortridade Humana, Universidade de Lisboa, Estrada da Costa, Cruz-Quebrada-Dafumdo, 1499-02 Oezias, Portugal, [EMAIL_ADDRESS] {T.J.}  
1 CIPER, Faculdade de Mortridade Humana, Universidade de Lisboa, Estrada da Costa, Cruz-Quebrada-Dafumdo, 1499-02 Oezias, Portugal, jeles@frm. CP children are intensively studied in gait analysis, but unlike other populations with gait abnormalities [14] there are no specific biomechanical models to their gait characteristics. It is known that there are significant differences among the techniques, but the gait laboratories still opt to use their typical protocols, regardless of the population. It is essential to understand the possible errors associated with the different techniques of marker sets and underlying anatomical models [15] to reproduce the clinical gait measurements with confidence [16]. Significant differences exist in biomechanical models used in different laboratories. These include measured variables, degrees of freedom assigned to the joints, anatomical reference frames, and joint rotation conventions [17]. The conventional gait model (CGM) is a very widely used biomechanical model to calculate kinematic and kinetic variables in gait analysis [16]. It has been extensively validated but there are still some issues regarding its reliability, mainly due to its unconstrained segment dimensions which makes it more exposed to sources of errors [18]. The six-degree-of-freedom (6 DoF) models are the most common alternative to the CGM that, despite needing more extensive validation [18], assumes that the segments are rigid and do not constrain the joints motions [19]. Several 6 DoF modeling techniques were used in the assessment of repeatability in participants with motor and physical characteristics limiting the normal gait [14, 20, 21]. These 6 DoF models address the known limitations of the CGM, but unlike the latter it still needs to be better researched. However, some results have indicated some of those claims (e.g., the segments have a fixed length and soft tissue artifact is reduced). Soft tissue artifact between markers is certainly eliminated by using rigid clusters, but a different form of soft tissue artifact will affect the orientation and position of the whole cluster in relation to the bones [22]. In children in particular, the amount of soft tissue surrounding the limb segments is not the major reason for some oscillations, but the smaller distance between clusters and anatomical markers which do not minimize the magnitude of this type of error. According to a systematic review of [PERSON] et al. [11] about the repeatability studies of kinematic models, the majority of the included studies used the CGM or some variant of it. In previous test-retest reliability studies performed in CP children, the biomechanical models were based in CGM [23] and similar models such as the Helen-Hayes [24] and the Vicon Clinical Manager [25]. One study that used a 6 DoF variant (the Cleveland clinic marker set) [26] did not compare kinetic data and the authors assessed repeatability using a coefficient of multiple correlation (CMC) which has recently been determined not to be suitable as a tool for assessing reliability in gait measurements [27]. The lack of evidence regarding the reliability of 6 DoF models in subjects with abnormal gait patterns, particularly in kinetic variables, was the motivation to develop this research.