Table 2.

Definitions for the annotation of FOG and the tasks during the FOG-provoking protocol.

Definitions video annotations
Freezing of gait (FOG)
Akinetic FOG	Onset	The moment the intention to move is first observeda and the participant is unable to do so, showing ‘clear sticking of the feet’ without considerable trembling movements in the legs.
	Termination	The moment of initial toe-off after the FOG when the participant is again able to perform at least two effective alternating steps with both legs showing no FOG-related features.
Trembling FOG	Onset	The moment when the foot of the participant is suddenly no longer producing an effective step forward and is displaying trembling in the legs, despite the participant's intention to continue walking, or the moment the intention to move is first observed and the participant is unable to do so, showing clear trembling in the legs.
	Termination	The moment of initial toe-off after the FOG when the participant is again able to perform at least two effective alternating steps with both legs showing no FOG-related features.
Festination	Onset	The first moment of toe-off when an abnormal and high-pace oscillatory stepping behavior is observed without considerable FOG-related features.
	Termination	The moment of initial toe-off when the participant is again able to perform at least two effective alternating steps with both legs showing no FOG- or festination related features.
Movement interruptionsb	Onset	The moment a movement interruption is first observed - when the foot of the participant is not or is suddenly no longer producing an effective step forward (despite the task instruction to do so), without a definite indication to consider it either as a FOG subtype or as voluntary stopping.
	Termination	The moment of initial toe-off after the movement interruption when the participant is again able to perform at least two effective alternating steps with both legs showing no FOG-related features or the first moment a definite indication is observed to annotate it as FOG or voluntary stopping.
Task duration
As outcomec	Onset	The moment the researcher gives the ‘Go’ signal to start the task.
	Termination	The moment the participant has finished the task.
For calculation %TFc	Onset	The moment the first intention to start the task is observed.
	Termination	The moment the participant has finished the task.

FOG: freezing of gait. %TF: percentage time frozen.

^aAn all-encompassing definition for the ‘first intention to move’ cannot be provided as this varies between situations and is therefore left for the interpretation of the expert rater. Examples are movements observed in the upper body, arms and/or clothing indicating an attempt to initiate the intended movement, multiple APA's during gait initiation, etc.).

^bNot included in the calculation of %TF.

^cThe start of the Task is defined differently for task duration as an outcome on its own and for calculation of the %TF, as the duration for the patient to respond to the ‘Go’ signal can be indicative for impairment but would add noise to the calculation of %TF.