Table 4. Agreed definitions of terms related to real-world walking.

Term	Aspect	Definition
Walking	Physiological	Human walking is a method of locomotion and is defined as initiating and maintaining a forward displacement of the centre of mass in an intended direction involving the use of the two legs, which provide both support and propulsion. The feet are repetitively and reciprocally lifted and set down whereby at least one foot is in contact with the ground at all times [44, 45]. Walking with walking aids is included in this definition. A step is the interval between the initial contacts of the ipsi- and contralateral foot [44] and corresponds to the forward displacement of the foot together with a forward displacement of the trunk [46]. A stride is the interval between two successive initial contacts of the same foot. As such, a stride is equivalent to the gait cycle and every stride contains two steps [44].
	Relation to walking bouts	Walking is made up of walking bouts and is equivalent to taking steps/stepping forward (thus stepping in place does not constitute walking) and is defined as starting from initial contact for the initial step until ending with full floor contact of the foot making the last step [46].
Purposeful	Characteristics	Purposeful walking includes an intentional component of the movement (e.g., getting to the bathroom, catching the bus, going to the grocery store, going for a walk in the park, etc.). Purposeful walking may constitute certain characteristics (e.g., more constant walking velocity, lower variability of gait characteristics, straighter direction of locomotion than non-purposeful walking, specific context, etc.). Those gait characteristics are quantified based on discrete walking bouts.
Real-world	Characteristics	Real-world relates to the context in which walking takes place—that is free-living, unsupervised, uncontrolled and non-standardised. As such, it is unscripted as there are no instructions to the subject who does not need to interact with the wearable device(s). Real-world actions occur in non-simulated everyday situations in unconstrained environments with minimal consciousness of being tested. It is equivalent to actions at home or in the community over continuous periods of time [28]. Synonymous terms are (environment of) daily living, or relating to daily-life. Home environment is used synonymously to real-world and daily-life without a separation of indoor and outdoor environment [11]. Real-world is distinct from laboratory-based [47], supervised (= fully controlled and observed), and semi-controlled (walking‘freely’ but with supervision) tests. It also is different from scripted/instructed walking, which can take place in the home or lab (such as walking tests like the 4x10m test, 6-minute walk test (6MWT) and timed up and go (TUG)).
	Clinical environment	Free walking in hospitals is part of the real-world definition, but standardized supervised tests in a hospital are not. This excludes instructed actions, e.g., by medical professionals.
	Standardized measurement	Home-based tests, which are semi-standardized measurements performed in the home environment in a controlled or semi-controlled environment (such as short walk tests), are thus not regarded as being part of real-world. Home-based tests can nevertheless be an alternative to clinical tests and might be easier to conduct operationally and analyse than continuous monitoring (assuming standardized instructions).
Walking bout	Characteristics	A walking bout (WB) is a walking sequence containing at least two consecutive strides of both feet (e.g., R-L-R-L-R-L or L-R-L-R-L-R). Start and end of a walking bout are determined by a resting period or any other activity (non-walking period). The initial step of a WB follows a non-walking period and the final step precedes the next non-walking period.
Walking speed	Physical definition	Walking speed (WS) is the distance covered by the whole body within a certain time interval / per unit time of walking. It is measured in meters per second and is the magnitude of the velocity vector (velocity includes direction and magnitude of walking) [45].
	Granularity	Walking speed can be estimated at different granularities: Instantaneous WS varies from one instant to another during the walking cycle [45] Step-wise WS is the ratio between step distance (length) and step time [28] Stride-wise WS [33] Averaged over WBs Averaged over other time intervals (hourly, daily, weekly) based on multiple WBs The granularity by which the WS is assessed should be related to clinical parameters for each population separately.
	Relation to walking bouts	Walking speed will be assessed with regard to walking bouts. Thus, the minimal length of one walking bout required to assess average walking speed is based on a sequence of 2 consecutive strides (e.g., R-L-R-L-R-L or L-R-L-R-L-R).
Turning	Characteristics	The process of turning consists of decelerating the forward motion, rotating the body as a whole, and stepping out toward the new direction [48]. Thus, turning includes a change of walking direction and change in angular orientation including a rotational movement of the body around the longitudinal axis. Turning, curvilinear walking, and straight walking involve different neuromotor strategies and need to be discriminated.