Methods: In this mixed model design study, spatiotemporal parameters were measured in 22 patients with ACLD (25.95±4.69 y) and 22 control subjects (24.32±3.37 y) while they were walking with different levels of gait velocity (high velocity, self-selected velocity, low velocity) in isolation or concurrently with auditory Stroop task. Coefficient of Variation (CV) was used to calculate variability of step length, step time and step width as dependent variables using custommade MATLAB code. Mixed model of analysis of variance and post hoc analyses were used for data analysis. 

Results: The results showed that interactions of group by gait velocity due to cognitive difficulty were not significant for all mentioned variables (P>0.05). Group Interactions due to cognitive task difficulty were significant only in CV of step width (P=0.05). Interactions of motor task difficulty by cognitive difficulty were significant for CV of step length, step time, and step width in all participants (P<0.05). The main effects of gait velocity in all dependent variables were significant (P<0.05). 

Conclusion: Results showed that step width variability is a more sensitive measure for detecting interaction of group due to cognitive task difficulty compared to variability of step length and step time. Future studies could test this hypothesis in ACL deficient subjects using different measures.