# Test-Retest Reliability of a 6 DoF Marker Set for Gait Analysis in Cerebral Palsy Children
## 4 Discussion  
The purpose of this study was to evaluate the inter-session reliability and measurement error of a 3D gait analysis protocol in a group of CP children, in order to better understand the causes of intrinsic and extrinsic variation. Knowing this variability is crucial to improve clinical analysis that supports decision-making in the rehabilitation process. [PERSON] et al. [17] have found that when comparing five protocols on the same gait cycles, the main cause for the variability of outcomes between variables was the biomechanical model used and its definitions, regardless of the number of raters or even different laboratories. These different biomechanical models make it more difficult to compare results between reliability studies, as they present different sources of variability [17]. Repeated testing of a single subject allows for a clinical usefulness of the data, since it provides some understanding into the extent of variation of the measured outcomes that can be expected due to the pathology and those that are truly a consequence of a therapeutic intervention [15]. Despite extreme caution and compliance with the protocol instructions regarding the marker placement procedure, some inconsistency is still unavoidable [16], while possible sources of error can occur due to subjects' natural oscillations or skin motion [13] or movement between the skin markers and the underlying bones [50; 51]. This source of error is totally disruptive for the joints with a limited range of motion, such as knee abduction-adduction, internal-external rotation, and linear displacements [52; 53]. CP children can demonstrate different gait patterns in each leg. This occurs not only in unilateral spastic CP, where each legs presents different kinematic values [23], but also in some bilateral spastic CP children with an asymmetrical gait pattern, combining at least two different types of gait pattern [48]. A previous study by [PERSON] et al. [26] used the 6 DoF Cleveland marker set with unilateral CP children and presented similar results at both normal and hemiplegic limbs, where the highest repeatability was at the sagittal plane (CMC values of 0.96-0.99) and lower in the transverse and frontal planes (CMC \(\geq\) 0.7). In this study, the CP children presented different gait patterns (Table 1): five had bilateral spasticity, two had unilateral spasticity with their right limb affected and one was affected in the left limb, which contributed to some degree of variation of the data. The overall ICC results of kinematic and kinetic variables were lower on the right side, which can indicate that--to some degree--the instability of the affected lower limbs could influence the propagation of the STA. [PERSON] and co-investigators reported that the soft tissue motion can originate additional movement, resulting in an overestimation in kinematic peak values of the segments by as much as 100% [54]. This is in accordance with our research, where a larger variation was noted in the transverse and frontal planes of the knee (Table 4). In the 6 DoF models it is assumed that the limbs' segments are independent and do not share a fixed joint centre, which often originates non-physiological translations between the proximal and distal bones at some joints [22]. However, in pathological gait, care should be taken because non physiological movements may occur. Typically true equinus gait patterns constrain CP children to stand with the ankle in a neutral position [48]. However, according to [PERSON] et al. [55] when passive dorsiflexion is detected in the clinical examination, it is possible for some subjects to walk with their feet flat on the ground upon request. This variability in walking pattern during development is considered typical. Nevertheless, when unable to perform heel contact, some biomechanical compensation is detected, mainly in the coordination of movement at the hip, knee andankle joints.