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child’s parent were present during testing. All temporal and spatial gait parameters were calculated utilizing the software of the Gaitrite system using pre-programmed definitions, calcula- tions and data from the three cycles. Parameters analysed included walking velocity, step length and step length variability, step time and step time variability [16]. Any partial footfalls that did not have a clearly defined beginning and ending or were in contact with the edge of the mat were edited out. Gait variability was expressed as the coefficient of variation (CV) which is SD/mean �100. Procedure After a rest period the children were asked to walk at their regular pace on the Gaitrite, three cycles for each one of the trials: regular walking followed by performing the two different cognitive tasks—the first was to memorize and recall three random numbers one digit each and the second was to identify simple auditory sounds. In a pre-test period children with post-severe TBI were assessed for their ability to memorize and recall three numbers consecutively. It was noticed that the 582 M. Katz-Leurer et al. children could perform the assignment correctly and sequentially only with one digit numbers. So for this study, the children were asked to recall three random numbers of one digit each. In addition, in a pre-test period children with post-severe TBI were asked to identify different sounds. The sounds chosen were clear and simple as commonly experienced by children (such as dog barking, doorbell ringing) and included a total of 15 sounds that the children could identify without mistakes. After three walking cycles with no concurrent task, the children were asked to remember the numbers presented to them and to repeat the numbers over and over in exactly the same order. All the children could perform the task with no mistakes. Then the children were asked to walk while repeating the numbers. Correct and incorrect repetitions of num- bers were counted and the incorrect repetitions out of the total number were recorded. In the second cognitive task the sounds were first projected through a speaker at the far end of the walkway. All of the sounds were less than 2 seconds in duration and the children were asked to identify each as quickly as possible by naming the sound out loud. Then the children were asked to walk while listening and identifying each sound as quickly as possible. The number of sounds that each child identified incorrectly out of the total of 15 possible sounds was recorded. Statistical analysis A preliminary paired t-test was used to assess differences in gait parameters between legs. No significant differences were noted; therefore, the mean value of both legs is presented. The normality of the parameters’ distribution was verified using the Kolmagirove Smirnove test. The values of gait velocity, step time, step length and step time and length variability were summarized using means and standard deviations. To assess differences between groups in the fulfilling of the cognitive assignment—number of cycles achieved in each task, a t-test was performed. To evaluate any different effect of the task on performance quality, a mixed ANOVA model was performed. The between-subject factor is the group factor (TBI vs TD), the within-subject factor is the two cognitive tasks and the interaction term (group� task) and the dependent variable was the percentage error in each assignment. For assessing any differences between the three walking conditions without and with concurrent tasks, a mixed ANOVA model was applied when the dependent variable was the gait parameter. A separate model was built for each parameter. The between-subject factor is the group factor (TBI vs TD), within subject factor (without and with the first and the second concur- rent tasks) and the interaction term (group� task). A p-value< 0.05 was considered statistically signif- icant. All statistical analyses were performed using SPSS v. 17 software packages. Results Cognitive tasks–percentage error Both groups exhibited significantly higher mistakes in the sound recognition assignment. The mean percentage errors between the two cognitive tasks was significantly different between groups; by mean, children post-severe TBI have 31.9% more mistakes compared to the controls (F(1: 28)¼29.6, p<0.01); 23%�5% more mistakes at the memorization assignment (mean of 24.5% among children post-TBI vs 1.5% among the controls) and 39%�6% more at the sound recognition assign- ment as compared to the TD group (mean of 46.3% among children post-TBI vs 6.7% among the con- trols) there was a significant group*assignment interaction effect (F(1: 28)¼ 9.45, p< 0.01), as chil- dren post-TBI exhibited larger differences between assignments (TBI; 20%�16%, TD 5%�7%) (Table I). Baseline differences between groups Gait parameters during all three gait conditions are presented in Table II. In the regular walking test, only the step length was significantly different between groups; children with post-severe TBI walked with a smaller step length as compared with TD controls (mean of 59.1� 10.1 cm vs 66.1�6.1 cm, respectively, t28¼ 2.34. p¼ 0.02). Gait parameters during cognitive assignments In the repeated model, significant differences were noted between groups in all assessed gait parameters. Children post-TBI exhibit significant slower gait velocity, significant shorter step length and signifi- cantly larger step length and step time variability. Table I. Number of cycles and percentage of errors in task performance by groups. Task Numbers Sounds TBI TD TBI TD No. of cycles 11.0� 4.3 16.6� 2.6 17.6� 2.6 14.7�2.0 % of error 24.5� 22.9 1.5� 3.2 46.3� 22.9 6.7�5.3 Values are mean�SD. Effect of concurrent cognitive tasks on gait 583 There were significant differences in gait param- eters between assignments in both groups. Gait velocity Gait velocity was significantly different between trials (F2;50¼38.1, p< 0.01). After bonferroni adjustment for multiple compression it was noted that walking velocity was significantly faster at usual walking and it decreased by a mean of 23.8� 3.9 cm s�1 at the number assignment (mem- orization) and by 37.0�5.1 cm s�1 at the sound (auditory) assignment. In addition, a significant difference was noted between dual task conditions; during the sound assignment children walked signif- icantly slower as compared to the walking velocity during the number assignment (by a mean of 13.1� 3.7 cm s�1, p< 0.01). Additionally, there was a significant group-assignment interaction effect; gait velocity decreased for both groups in the dual task conditions, but significantly more among children post-TBI. In the number assign- ment, gait velocity decreased by 36.5� 6.9 cm s�1 among children post-TBI as compared to 8.3� 3.6 cm s�1 among TD children (p< 0.01). In the sound assignment walking velocity decreased by 48.7� 7.9 cm s�1 among children post-TBI as compared to 25.3� 6.5 cm s�1 among TD children (p¼0.03). Step time Step time was significantly different between trials (F2;50¼ 23.42, p<0.01). Mean step time was sig- nificantly increased in the dual task conditions as compared to baseline; it increased by a mean of 0.10� 0.02 seconds in the number assignment and by 0.10� 0.01 seconds in the sound assignment. No significant difference was noted between dual task conditions. A significant group-assignment interac- tion effect was noted also for the step time param- eters. Step time increased during dual task conditions in both groups but significantly more among children post TBI. In the number assign- ment, step time increased among children post-TBI by 0.17�0.15 seconds as compared to 0.03� 0.04 seconds among TD children (p< 0.01). In the sound assignment, step time increased by 0.16� 0.10 seconds among children