###### Abstract  
Gait analysis is clinically relevant in persons with multiple sclerosis (PwMS) and consists of several joint angular displacement-time relationships and spatiotemporal parameters. However, it lacks representation by means of diagrams in which knee-angle/hip-angle and knee-angle/ankle-angle variations are plotted against each other at the same points of time. Three-dimensional kinematic analysis was performed on 20 subjects (10 PwMS/10 healthy controls, HCs), and the knee-angle/hip-angle and knee-angle/ankle-angle diagrams of both lower limbs were determined in the sagittal plane while walking on a motorized treadmill. The area (a quantifier of range of motion) and the perimeter (a quantifier of coordination) of angle-angle diagram loops were calculated. PwMS showed reduced knee-angle/ankle-angle loops compared to HCs (\(p<0.05\)), whereas the hip-angle/ankle-angle loops between the PwMS and HCs was not significant (\(p>0.05\)). Similarly, the activation of leg muscles showed significant differences between PwMS and HCs (\(p\) ranged from 0.05 to 0.001). The results indicate that the proposed knee-angle/hip-angle diagram is feasible and could be applied as a reliable tool in future studies aimed at assessing the acute and long-term effects of specific exercise programmes and/or pharmacological treatment in PwMS. gait analysis; walking; hip-angle/knee-angle; knee-angle/ankle angle; EMG activity; neurodegenerative disease +
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usually consist of relationships between joint angular variations and time, which gives us information that is not easy to interpret in clinical assessments [6; 7; 8]. Conversely, angle-angle diagrams plot the angles at adjacent joints against each other rather than plotting angles versus time. For example, in the sagittal plane of motion, the hip-angle/knee-angle diagram displays the range of hip flexion-hyperextension along the \(X\)-axis and the range of knee flexion along the \(Y\)-axis. Therefore, the area delimited in this diagram depends on hip and knee angles, which correspond to the total conjoint range of angular motion performed by the two joints during one complete gait cycle [9]. When changes occur in both joints, a corresponding perimeter variation is determined in the angle-angle diagram. If any joint rotation is uncoordinated, the perimeter will increase, whereas the conjoint range of motion (area) may remain approximately constant. Although perimeter is a function of both angles, when the variation involves one joint more than the other, a change in the perimeter may define a pattern describing a robot-like gait in which only one joint rotates and the other joint is fixed. In other words, the perimeter may reflect the coordination between two joints as seen in the gait [9]. Therefore, it might also be interesting to analyse gait from a quantitative perspective through three joints (hip, knee and ankle) that define two angle-angle diagrams in the lower limb [9; 10], which is an approach that is still not applied for gait analysis in persons with multiple sclerosis (PwMS). The hip-angle/knee-angle and the knee-angle/ankle-angle loops can provide quantitative information such as range of motion (ROM) and coordination by calculating the areas and perimeters, respectively. It is also possible to obtain qualitative information observing the \"shape\" of the diagrams. We hypothesized that the main characteristics of the hip-angle/knee-angle loops and the knee-angle/ankle-angle loops (i.e., area and perimeter) may represent a new approach to observing gait pattern alterations in people affected by multiple sclerosis, providing additional information about the status/progress of the disease and for monitoring pharmacological treatment and/or kinesiological intervention. In addition, gait measures can be synchronized with surface electromyography (sEMG) of the lower limb muscles to describe the activation in proximal and distal leg muscles during walking [11].