Participants alternate between 30 min of walking on half of the days and 30 min of gamified inhibitory control on the other days.

In the Letter-Number task, working memory will be assessed. Participants are presented with a mixed order of letters and numbers, which they must remember and sort them numerically and alphabetically. For example, the sequence 'H8T3K5' would be sorted into '358' and 'HKT'. The mixed orders of letter and numbers ranges from 2 to 15. After a short tutorial, the participant practices the task and must get three out of the five trials correct to proceed to the test. The Letter Number Task is split into two parts: The first is a method of limits to find the participants threshold, whereas the second provides additional trials around that threshold. Characters that could be confused with other letters or numbers are excluded (0, I, O, U, X). Moreover, a given sequence does not include any of the characters in the previous trial and it does not contain consecutive numbers and letters.

In the N-Back task, working memory will be assessed. Participants are presented with a consecutive stream of pictures, and the objective is to tap on the pictures that match those presented N items earlier. Higher levels of N levels increase WM load and make the task more difficult. All participants complete 1-back (Figure 4), 2-back, and 3-back (in that order). Before each level starts, there is a practice session consisting of 10 trials during which performance feedback is provided. The test phase for each N-level consists of 30 trials with 9 targets. Progression to 4-back (and beyond) is allowed if the participant made no more than 2 errors on the previous level. Stimuli are presented for 2500 ms (participant can respond during this period) with an ISI of 500 ms.

The Cancellation Task is a timed, tablet-based test of selective attention and inhibitory control, akin to D2 (Brickenkamp & Zilmer, 1998). There are two versions that can be used interchangeably: Cancellation Letters and Cancellation Pictures. Cancellation - Letters resembles the D2 in that it consists of similar stimuli: characters "d" and "p" with one to four dashes, arranged either individually or in pairs above or below the letters. For Letters, the participant must scan the items from left to right and select all "d"s with two dashes (targets). In Cancellation - Pictures, letters are replaced with pictures of dogs and monkeys, some of which are rotated along the vertical axis or are presented upside down. For Pictures, the participant must select the upright dog (tail on the left) and the upside-down monkey (tail on right) separately in single blocks and together in a mixed block.

The Flanker Task tests selective attention and inhibitory control. Participants are presented with a row of arrows and are asked whether the centrally presented arrow is pointing either to the left or to the right. In the congruent trials, the non-target arrows point in the same direction as the target arrow and in the incongruent trials they point in the opposite direction. The stimuli are presented in a pseudo-random order in which neither the trial type nor the trial direction can repeat more than three times in a row. Each condition is presented 12 times resulting in 48 total trials. The row of arrows is presented for 500ms with a 500ms break. The location of the row is randomly jittered so that it appears in one of 9 possible locations to prevent participants from fixating their gaze on the location of the central arrow. The user has 15 seconds to choose a response from the moment a trial is presented. Participants should respond as quickly and accurately as possible.

The Rule Switch Task is a tablet-based measure of inhibitory control modeled after standard rule switching tasks such as the NIH Examiner (Kramer et al, 2014). In this task, participants must match the stimulus at the top of the screen (either a red or blue circle or square) by selecting one of the buttons (i.e., a red square or blue circle). Participants are instructed to classify the shape by one of two rules, either shape or color. There are 24 color trials and 24 shape trials, half of one shape and half of the other, for a total of 48 trials. There are three variables: shape, color and trial type. The three variables are pseudo-randomly intermixed so that any subset of 5 consecutive trials cannot have one of those traits in common across all 5 trials (no more than 4 trials in a row can share any of the three variable traits in common). There is also a switch rule in that there need to be a minimum of two trials of the same type before switching to a different trial type.

Assessed by semi-structured interviews and will include: participants' satisfaction with the app and/or walking regime; perceived benefits of cognitive training and/or walking; and barriers to engaging with the app and/or walking regime.