Virtual environments can make repetitive motor rehabilitation exercises more motivating and thereby more effective. We hypothesize that participation-dependent multimodal stimuli increase the patientpsilas activity as expressed through force exertion during robot-aided treadmill training. In a single case study with one patient (12 years old), we were able to show that active participation increased in the presence of visual stimuli and decreased in their absence. For a feasibility study, we included four children with cerebral palsy in order to assess the user acceptance of four different virtual environment scenarios including a soccer scenario, a traffic situation, obstacle crossing and wading through deep snow. Using questionnaires, we found that only the soccer scenario provided sufficient interactive elements to engage the patients.