Walking fatigability is a motor impairment happening in approximately 43.5% of people with multiple sclerosis (pwMS). Walking fatigability is defined as a decrease in 10% of distance walked, comparing the first minute with the last minute of the 6-minute walking test (6MWT), which one may call distance walking fatigability. More recently, our research group has found that walking fatigability manifests also in terms of gait quality, with different gait characteristics affected from patient to patient, suggesting that different gait profiles might be observed for walking fatigability. One possible explanation for the walking fatigability motor impairment is the lower capacity or neural reserve of pwMS to sustain optimal levels of cortical activity to either maintain performance (i.e., distance walked) or gait quality. The study proposes two research questions. Both part A and B are observational studies. The study includes 70 pwMS and 30 healthy controls. The study consists of 3 tests. The clinical assessment session will be composed of responses to different questionnaires, while parts A and B comprehend walking overground (part A) or on fixed-speed treadmill (part B). Part A Session 1 (1 hour and 30minutes, including equipment preparation and rest time). First, anthropometric characteristics and measurements of force, spasticity, cognition, walking speed, and performance foot tap test will be performed. For cortical activity measurement of the prefrontal cortex, supplementary motor area, premotor cortex, and primary motor cortex, a functional Nears-infrared Spectroscopy (fNIRS) will be positioned in the participant\'s head. Wearable sensors will be positioned in a pre-defined body position for the measurement of gait characteristics. Both gait characteristics and cortical brain activation will be measured concomitantly during fast and comfortable speed short walking (10 trials of 15 seconds with 25-30 seconds standing still resting periods between trials, in a block design) before and after and during a prolonged walking (6-minute walking test (6MWT)) protocols. A resting period of 20 minutes will be provided to the participants before the beginning of the 6MWT. Part B. Participants who participated in Part A will be invited to participate in Part B. This study also consists of a single visit that is 1 hour and 30 minutes long (preparation and assessment included). The participants will be asked to walk on an immersive treadmill in two fixed-speed conditions (relative to 80% and 50% of the Timed 25-foot Walking test) while infrared cameras will record the reflective markers position. Forty minutes of rest will be provided to the participants between the fixed-speed conditions.