This study aimed to investigate: 1) the effect of optic flow speed manipulation on active participation during robot-assisted treadmill walking (RATW), 2) the influence of the type of virtual environment, and 3) the level of motion sickness and enjoyment. Twenty-eight healthy older adults were randomized in two groups: "stimulus rich" Park group (50% male, 61± 6 year) and "stimulus poor" Hallway group (43% male, 62± 5 year). Subjects walked in the Lokomat with immersive virtual reality (VR) with a matched, slow and fast optic flow speed, each lasting 7 minutes. Active participation was measured by continuously assessing the human-machine interaction torques at the hip and knee joints and muscle activity of the Vastus Medialis and Biceps Femoris. Motion sickness and enjoyment were assessed with the Simulator Sickness Questionnaire (SSQ) and Physical Activity Enjoyment Scale (PACES) respectively. In both groups optic flow speed manipulation in both directions led to a decrease in bilateral hip interaction torques towards flexion at the end of the stance phase compared to matched speed. In the Hallway group, walking with slow optic flow elicited 32% more muscle activity of the Vastus Medialis. There were no significant differences between both groups for the SSQ and PACES. Optic flow speed manipulation appears to have only a small effect on the active participation of healthy people during RATW. The type of virtual environment did not affect their activity, motion sickness or enjoyment. However, the addition of immersive VR during RATW was well tolerated and enjoyable. Further research with patients is necessary.