Study aim: Investigating effects of mental fatigue on cognitive-motor interference during walking in people with multiple sclerosis and healthy Design: A randomized, controlled, double-blind, parallel-group study with a pretest–posttest design in 30 healthy and 30 individuals with MS. Computer-generated block randomization will be used. Settings and conduct: This study will be conducted at the Rehabilitation Research Center located in the Faculty of Rehabilitation Sciences, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran. After the healthy and MS participants were classified into two fatigue and control groups, walking performance will be assessed on a treadmill at a preferred speed in the single walking and walking with concurrent consecutive subtraction of 7 numbers from a randomly selected number. Gait assessment will be performed using Vicon.Gait assessment will be conducted before and after fatigue induction and watching the documentary film. This study is double-blind and participants and outcome assessor will not be aware of type of grouping (mental fatigue induction or control). Participants/Inclusion and exclusion criteria: Inclusion criteria: MMSE score >24, EDSS score <6 Exclusion criteria: Recurrence of the disease in the past 1 month, Visual involvement Intervention groups: Participants in healthy and MS group will be randomly assigned to two groups of mental fatigue and control. Mental fatigue protocol: In this study, mental fatigue will be induced using a modified version of 100% incongruent visual Stroop task(color-word). Control: In this condition, participants will choose their favorite film from 3 suggested documentaries. Main outcome variables: Dual task cost of double support phase; step width; cadence; stride length; stride time; swing phase; dual task cost of the coefficient of variation for double support phase; step width; stride length; stride time; swing phase