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post-TBI as compared to an increase of 0.05� 0.03 seconds among TD children (p<0.01). Step length In addition, step length was significantly different between trials (F2;50¼ 13.9, p<0.01). Step length was significantly shorter at the sound assignment. It was shorter by a mean of 9.48� 2.2 cm as compared to the baseline value and by 5.9� 1.1 cm as com- pared to number assignment. Step length and step time variability Step length and step time variability were signifi- cantly higher among children post-TBI across the assignments, with no significant difference between the assignments (step length variability, mean between groups difference 12.2� 2.2, F1;25¼ 8.3, p< 0.01, step time variability mean between groups difference 7.9� 2.6, F1;25¼ 9.2, p< 0.01). Discussion The present study examined the effects of concur- rent cognitive tasks on gait performance in children post-TBI in comparison to TD controls. There was a negative effect on walking performance when the children did a concurrent task and this effect was task-dependant. During the auditory assignment, the performance error rate was significantly higher as compared to the memorization assignment and the effect on walking was more detrimental: walking speed decreased and step time increased significantly in the auditory assignment in both groups but with a Table II. Gait parameters in children post-TBI and TD controls in usual walking and in the two dual task conditions. Usual walking Dual task- numbers Dual task- sounds ^p-value #p-valueTBI TD *p-value TBI TD TBI TD Velocity 1.2�0.3 1.3� 0.1 0.15 0.8� 0.2 1.3� 0.2 0.7� 0.1 1.1� 0.2 <0.01 <0.01 Step length 59.1�10.1 66.1� 6.1 0.02 51.2� 11.1 65.4� 5.7 43.9� 11.7 58.7� 8.0 <0.01 0.58 Step length variability 9.3�9.1 8.2� 10.2 0.27 13.4� 14.6 5.4� 3.0 24.0� 17.0 8.4� 5.7 0.12 0.53 Step time 0.45�0.07 0.48� 0.05 0.55 0.60� 0.11 0.51� 0.07 0.61� 0.09 0.53� 0.06 <0.01 <0.01 Step time variability 11.5�15.4 6.8� 7.6 0.23 13.6� 10.1 6.8� 4.8 16.2� 7.8 8.6� 9.2 0.15 0.80 *p-value between groups at usual walking (t-test). ^p-value within subjects (usual walking, numbers, sounds) (Repeated measure ANOVA). #p-value interaction effect group and manipulation (Repeated measure ANOVA). 584 M. Katz-Leurer et al. significantly prominent effect among children post-TBI. The interference effects of concurrent cognitive tasks on locomotor performance has been assessed and described before among children as well as among adults [12, 17, 18]. It has been suggested that the outcome of dividing attention between two concurrent tasks depends on the type and level of difficulty of the tasks and on the priority given to each task [3, 4]. Since attention resources are limited, two tasks will compete for resources if they are carried out at the same time. They may interfere with each other when the total amount of resources demanded exceeds those available. In the present study, the auditory assignment caused greater interference than the memorization assignment. It might be that memorizing a set of three numbers is a simple memory task which requires short-term learning and recall as compared to the sound recognition assignment which involves identification, recall of existing knowledge and naming capabilities. The auditory assignment is clearly more demanding and requires more attention resources [4, 5, 18]. In previous studies, the negative effect on locomotor speed while performing second- ary tasks involving processing of sensory stimuli was higher than for those involving other types of cognitive processing. Specifically, it has been noted that an auditory identification task produced greatest interference effects on gait than other concurring tasks . One may claim that the lower effect of the concurrent cognitive task on gait characteristics in TD children is due to the fact that the cognitive task was too simple for them. It has been suggested before that it is essential to adjust the task difficulties to the individual’s capacity . However, it has also been noted that even with a very light overall load on the cognitive system a difference between patients with brain damage and healthy controls could be detected . In this study the memorization and the auditory assignments were adjusted to the capabilities of children post-severe TBI. By doing so one may reduce any significant baseline differ- ences between groups, but it is argued that any reduced ability in performance is the result of the demand to combine performances. One of the most frequent cognitive limitations among children post-TBI is an attention distur- bance. Performing dual tasks demands splitting attention simultaneously between two tasks and each assignment might be too difficult to perform in those whose attention ability is reduced. It has been well established that when combining cognitive and walking tasks, the walking requires attention  and dual tasking is sensitive enough to distin- guish between patients post-concussion from control subjects [22, 23]. The interactions between concur- rent tasks cannot be automatically generalized and should be assessed in different conditions and modalities. This may imply that the attention needed for motor and cognitive activities are not necessarily always competitive and interruptive. Step variability increases when step velocity decreases . Therefore, the dual-task related increase in step length variability shown in this study could also be explained by the dual-task related decrease in step velocity. However, the results of this study show that the decrease in step velocity while performing the memorization task did not modify step variability. During the auditory assignment the mean step time was the same as for the memorization assignment, but step variability increased significantly in both groups, more so in the children post-TBI. This suggests that the increase in step variability was task-specific, in contrast to step time. These findings have clinical implications for the physiotherapist. Firstly, therapists use a lot of verbal instructions and feedback during the actual treat- ment. Therefore, a child post-severe TBI who is practicing a new and higher level of motor perfor- mance will benefit more from instructions or feed- back given before or after performing the task. On the other hand, if the child needs further refinement of an achieved motor skill and the therapist indeed wants to challenge the child, the therapist can provide concurrent feedback or add a secondary cognitive task. Another clinical implication is related to the finding that step variability increases during dual task performance. It has been noted before that step variability is associated with balance performance among children post-TBI , a higher step variability being associated with poor balance. Possible, the therapist should work on improving balance as much as possible before progressing to higher motor skills. However, Silsupadol et al.  described two adults patients who received balance training under dual task conditions. Benefits were maintained over 3 months, whereas they were not evident in a patient who trained under a single task balance training programme. The question of whether training under dual task conditions is more effective in improving gait after the patient has progressed in the single task performance needs further investigation. The therapist might instruct the family of a child post-TBI to avoid cognitive tasks in the early stages of motor training. Dual tasking and adding cognitive activities while walking should, however, be intro- duced as an effort to improve the child’s ability to perform dual tasks in a safe and functional manner. Effect of concurrent cognitive tasks on gait 585 Declaration of Interest: The authors report no conflicts of interest.