Table 1.

Pre-defined protocol for treadmill-based C-Mill therapy

Setting	Groups of two participants for 90 min; participants will alternately train and rest.
Frequency	Twice weekly treadmill training program with specific emphasis on walking adaptability.
Therapy	In the first week, a combination of obstacle avoidance (avoiding visual obstacles projected on the treadmill), practice of accurate foot placement on a step-to-step basis (walking to a regular or irregular sequence of visual stepping targets), and a functional and interactive walking adaptability game (game with the theme ‘beach’ or ‘forest’) will be performed. In weeks 2–5, the combination of obstacle avoidance, accurate foot placement on a step-to-step basis and the functional and interactive walking adaptability game will be complemented by walking speed adaptations (acceleration and deceleration evoked by a moving walking area).
	Participants will start in week 1 at a comfortable walking speed; this speed will be gradually increased during the 5 week period. The weekly increase of the walking speed will be 10 %, provided that the therapy remains safe and is tolerated by the participant. Besides the walking speed, the difficulty of C-Mill exercises will be gradually increased, as tolerated by the participant.
Therapist	C-Mill therapy will be provided by a single therapist, an expert in C-Mill therapy. The therapists involved in the C-Mill therapy were all trained with regard to operating the C-Mill and to the specific guidelines of the intervention before the study started. Most therapists were already experienced C-Mill users before the study started. The therapists regularly meet the research assistant to ensure adherence to the protocol (Additional file 1).