The aim of this study was to compare the effect of different supportive strategies during RAGT on the degree of active participation in children. This work investigated differences in therapy conditions on a single day and showed active participation during a short time period of two minutes. Within this period, we showed that VR has the same immediate effect on motor output as therapist instructions in subjects with neurological gait disorders. Most importantly, the study revealed that both children with and without neurological disorders achieved significantly higher motor output during all supportive conditions as compared to walking without any motivational assistance. In other words, active participation was increased either by verbal encouragement given by a physical therapist (THER), by a VR soccer scenario or by the combination of both (VR + THER). It is not yet known whether such enhanced active performance can also be maintained over longer time periods or during a whole training session and whether this leads to a more effective rehabilitation process for patients. Furthermore, as there was no significant difference between the motor output measures and the three baseline measurements (walking without motivational assistance), it might be inferred that, with regard to the degree of active participation, the walking at the beginning of a therapeutic session is comparable to that at the end and shows no general fatigue effect. Although fatigue was not systematically verified during the training session, it might be interesting to include a fatigue score in further research.
It has been proposed that active training is more effective than passive training for motor learning and cortical reorganization . Important findings in stroke patients suggest that simply moving or passively exercising the impaired limb does not lead to maximum recovery. Furthermore, it has become apparent that new motor skills, enriched, highly functional and task-oriented practice environments and primarily motivating tasks which increase engagement are necessary for motor re-learning and recovery after stroke . Although children with CP might be substantially different in motor learning than those having experienced stroke or spinal cord injury, in cases in which the patients did have an intact and normally functioning nervous system prior to injury, it has been shown that activity, task-specificity and goal-orientedness are also crucial aspects in treatment of children with CP [24, 25].
Therefore, in RAGT it is essential that patients participate actively instead of just letting themselves "be walked". The patient's performance during the RAGT is difficult to estimate due to the loss of physical contact between therapist and patient [17, 18]. With the advanced biofeedback facility integrated in the Lokomat system used for the present study, we were able to record force interaction between the patient and the Lokomat and, on the basis of this data, to estimate the subject's performance. Although Lünenburger et al.  could demonstrate that biofeedback values are useful for evaluating and assessing the walking performance of subjects during Lokomat training, only the values for swing-phase correlated highly with the instructed activity, whereas the correlation of the stance-phase was less and sometimes even inversely correlated. Therefore, we recorded the four biofeedback values (bilateral hip and knee joints) during all conditions for the swing-phase only.
As outlined in the introduction, patient motivation plays a crucial role in determining therapy outcome, especially in the field of pediatric rehabilitation. The RAGT sessions, which consist of standardized monotonous walking for 30-45 minutes, are usually rather boring for children and can even be inconvenient. Hence, pediatric rehabilitation centers using RAGT try to boost patient motivation by showing DVDs or playing music. Such strategies, however, may distract children from the actual therapy, causing them to become completely passive in the Lokomat. VR techniques make it the possible to directly interlink the patients' motor performances during the gait training with actions in a computer-game-like virtual world. VR games adequately adapted to children's needs provide motivation and yet keep the focus on the actual gait training. Furthermore, the VR soccer scenario used is adaptable to children's individual skill levels and adjusts interactive elements to maximize motivation. In the current VR soccer implementation, the therapist could manipulate the opponent's speed offset and walking speed according to the skills of the participants. Assuming that a constant competitive situation could serve as a motivational factor, we included two different opponents in the present VR, one represents the first line of defense, over which the participant must kick the ball. The second approaches from behind and is able to take over the soccer ball from the avatar when he is in front.
In this study, we investigated the effect of adopting a VR scenario during RAGT based on the individual's level of active participation and compared this to a regular training session involving therapist encouragement and motivation. It should be pointed out, however, that the social interaction between a therapist and participant undoubtedly plays a crucial role, especially for patients. Thus, the use of VR during rehabilitation therapy should not replace the physical therapist, but rather provide an additional means of enhancing training efficiency.
Children with neurological disorders as well as healthy controls achieved higher active participation levels not only with therapist encouragement but also with a VR soccer scenario during RAGT. Based on our clinical experience, the measurements gathered indicate that higher motivation and focused attention during RAGT have a positive influence on children's motor output, which in turn might lead to enhanced motor learning. Further research is required in this area.
Given that the four supportive conditions varied in patients and healthy control children, we will compare and discuss these conditions for the two groups separately. Besides the fact that the mean motor output for patients revealed significant differences under all conditions involving motivational assistance compared with the normal walking condition, we also found significant differences between VR and VR combined with therapist instruction. All other comparisons of the supportive conditions exposed no significant differences.
It should be noted that the therapist's behavior during the two minutes of the "therapist-only" condition of the present study is not likely to be representative of normal behavior during a standard RAGT session of 30-45 minutes. In fact, motivating children during an entire training session is a very difficult and exhausting task and requires a great amount of engagement, creativity or even imagination. The use of a VR environment in RAGT, on the other hand, has the potential to constantly enhance and adapt training motivation and therefore increase active participation and training outcome. Moreover, VR may also be viewed as an additional medium used by the therapist to convey motivation and encouragement, e.g. by cheering when the patients' performance was particularly good or by encouraging the patient when something special must be achieved in the VR environment. This idea is in accordance with the fact that the combined condition VR+THER was significantly better than VR alone.
While mean active participation during baseline condition was similar for both groups, healthy control children achieved higher mean biofeedback values than patients for the condition therapist and the condition VR, but the difference was not significant. Furthermore, in healthy control children, there were significant differences between comparison therapist's instruction and VR values.
One explanation for the difference between patients and healthy children may be found in the safety system of the Lokomat. The device has built-in force monitoring which stops the robotic drives if participants provide too much force input. These technical limitations influenced the measurements, primarily those of the healthy children because healthy children have more power than patients and therefore occasionally activated the safety mechanism. Hence, some conditions may be slightly underestimated in terms of motor output values. On several occasions, the force exerted under VR and VR plus therapist's instruction conditions triggered the Lokomat's safety mechanism. This led to frustration, which in turn caused the healthy children to reduce their force and therefore produce lower motor output values than would otherwise have been possible during the affected conditions. This may explain decreased results during VR and VR+THER conditions in healthy control subjects.
In order to gain knowledge about the patient's perspective regarding the motivational properties of the soccer scenario used during RAGT, participants were asked to complete a self-designed motivation questionnaire. Overall the answers submitted indicated that all participants had fun during RAGT, were highly motivated and had done their best.
We are aware of potential shortcomings in our study, one of which might be the choice of the tested schedule order. Although, attempts were made to alter the order of the conditions, the VR alone condition was always placed in the middle of the session. As a result, subjects always had some practice with the Lokomat system before participating in the VR condition, which might have improved their performance. Secondly, the patient group may be biased due to previous experiences with training on the Lokomat and also with VR scenarios. However, the positive results obtained with the VR soccer condition seem to indicate the motivational aspect of VR games. Other limitations of this study are the small sample size of the groups as well as the heterogeneous abilities of the patients. Therefore, it may be difficult to make generalizations regarding the benefit of using VR as a motivational tool in RAGT with other patient populations.
VR in rehabilitation has become a promising and useful adjunct to traditional therapy by providing objective quantification of the training process as well as safe environments which motivate children to exercise [16, 26]. The VR scenario presented has the potential to achieve higher motor outputs in children with neurological disorders as well as in healthy controls. Our observations support the idea that VR might be a promising supplement for RAGT in pediatric rehabilitation. However, further research and development is necessary in order to optimize such VR systems as a motivational tool and to investigate their clinical effectiveness in the rehabilitation process. Follow-up studies are needed in order to determine if the increase in active participation caused by patient cooperative strategies like VR leads to better clinical outcome. In addition, emphasis should be placed on the development of engaging and immersive game designs which allow for human gait variability and performance levels. These variables must be optimized in order to keep children attentive during consecutive training sessions of 30-40 minutes.
In summary, the VR scenario used here has an immediate effect on motor output (biofeedback values) similar to one resulting from verbal instructions by a therapist. Therefore, VR represents a valuable tool to keep patients and healthy control children participating actively during RAGT.