Completed no study activities for 16 weeks after pre-assessment, then returned for post-assessment

Based on high intensity cardiovascular resistance training (HICRT), this intervention includes a combination of aerobic and strength training. Sessions are conducted in a group of ≤20 participants per class, with ≤5 participants per trainer. This intervention includes 28 70-min sessions (32.67 hrs) completed over 16 weeks: 3/wk weeks 1-4, 2/wk weeks 5-8, 1/wk weeks 9-16. At the first and last session, participants complete the Army Physical Readiness Test. All other sessions consist of: Warm-up; Walk/Run; 2 HICRT circuits with 3 sets of 3-4 exercises; 1-2 min jump rope and 4-min power series between HICRT segments; Drills & Skills; and Cool-down/stretch.

The active control condition consists of 2 tasks (change detection and visual search) with 3 variants each. This intervention includes 48 70-min sessions (56 hrs) completed over 16 weeks: 3/wk. The 6 tasks are played in random order for 10 min each session.

Active HD-tDCS is administered during the first 30 minutes of each Mind Frontiers session. Therefore, the schedule is 20 30-min sessions (10 hrs) completed over 12 weeks: 1/wk weeks 5-8, 2/wk weeks 9-16. The HD-tDCS devices are Soterix Medical 2x2 models, using 4 electrodes (2 anodal and 2 cathodal). Electrodes are held in place with custom, 3D-printed electrode holders inserted into EEG caps, using EEG gel. Participants receive 2.0 mA current for 30 min, split over the 2 anodal electrodes, each drawing ≤1.0 mA. Stimulation is ramped up and ramped down over 30 sec at the beginning and end of the 30-min stimulation period.

Sham HD-tDCS is administered during the first 30 minutes of each Mind Frontiers session. Therefore, the schedule is 20 30-min sessions (10 hrs) completed over 12 weeks: 1/wk weeks 5-8, 2/wk weeks 9-16. The HD-tDCS devices are Soterix Medical 2x2 models, using 4 electrodes (2 anodal and 2 cathodal). Electrodes are held in place with custom, 3D-printed electrode holders inserted into EEG caps, using EEG gel. Participants do not receive sustained stimulation; it is ramped up and ramped down over 30 sec at the beginning and end of the 30-min stimulation period to elicit the sensation of simulation and make the sham condition more convincing.

Mind Frontiers cognitive training software (Aptima Inc.) is a Wild West-themed Android application played on Nexus 10 tablets. This intervention includes 20 70-min sessions (23.33 hrs) completed over 12 weeks: 1/wk weeks 5-8, 2/wk weeks 9-16. The 6 mini-games, played in random order for 10 min each session, are Ante Up (Tower of London, executive functioning); Irrigator (visuospatial reasoning); Pen 'Em Up (dual task switching, executive function); Supply Run (Keep Track, working memory updating); Riding Shotgun (visuospatial working memory); and Sentry Duty (dual n-back, working memory). There is also a meta-game in which players build a town using resources earned from the mini-games, played for 5 min each session.

This test is a measure of Gf. Participants are shown a series of arithmetic number series and need to select the next number in the series from 5 possible answers at the bottom of the screen. The participant is given 5 practice problems at the beginning of instruction, and must complete as many problems as possible in 5 min (10 total).

This is a test of working memory. Participants must remember a series of arrows presented (16 possible arrows varying in length and direction), while maintaining accuracy on an interleaved letter rotation task (determining if a letter is facing the correct direction after rotation).

This is a test of working memory. Participants must remember a series of blocks presented (from a 4x4 grid) while maintaining accuracy on an interleaved symmetry judgment task.

This is a test of working memory. Participants must remember a series of words presented (18 possible 4-letter words) while maintaining accuracy on an interleaved task where they judge sensible or nonsensical sentences.

The Garavan task is a serial attention task that measures executive function, specifically task-switching ability. Participants are presented with a series of large and small squares, 10 series/trial in total. Subjects must keep a mental count of how many large and small squares have been presented. For each square, the participant rapidly updates this mental count and then presses the spacebar to see the next square. At the end of the sequence (length is random), a response screen appears and participants report their final count of large and small squares. On a given trial, 11-15 updates are made. Dependent measures are response time for consecutive trials of the same type (no-switch RT) and response time for consecutive trials that differ (switch RT). A cost RT metric can be computed by taking the difference between Switch and No-Switch RT as a measure of set shifting.

The Stroop task is a measure of executive function, specifically inhibition. In this task, subjects are asked to indicate the color of each word stimulus. The stimulus may be congruent (the word meaning and text color match), or incongruent (the word meaning and text color differ).

The Keep Track task measures executive function, specifically updating items held in short-term memory. In the task, a series of word stimuli are presented, each belonging to a category. Subjects must remember only the last item presented from each category.

This is a relational memory task. Participants are shown a sequence of 12 words (all 4-letter nouns/verbs), 750 ms for each word. Afterwards, the participant has 30 sec to enter as many words as they can remember from the trial. They do 1 practice trial, followed by 5 test trials. Approximately 20-30 min later, participants are given 90 sec to recall as many words as they can from all 5 previous trials.

This is a relational memory task. Participants are shown a sequence of 12 images (all objects, no "action" images), 750 ms for each image. Afterwards, the participant has 30 sec to enter words to describe as many images as they recall from the trial (1 word/image). Each image has multiple words that could be accepted as a correct response (i.e., if shown an image of a chimpanzee, answers "chimpanzee" and "monkey" would both be acceptable). They do 1 practice trial of this task, followed by 5 test trials. Approximately 20-30 minutes later, the participant is given 90 sec to recall as many images as they can from all the previous trials.

This is a relational memory task. In Paired Associates, a list of 6 unrelated word pairs are presented. At recall, only the first word from each pair is presented. Subjects must enter the second word in the pair. This procedure is repeated with a second list of 6 word pairs. Approximately 20-30 minutes later, the first words from all 12 word pairs are presented. Subjects must enter the second words from each pair.

Aerobic fitness, defined as maximal oxygen consumption (VO2 max), is assessed using an indirect computerized calorimetry system during a modified Balke treadmill test. During the test, participants' heart rates are monitored using a Polar heart rate monitor, and ratings of perceived exertion are assessed every 2 min. During the test, speed remains constant while incline increases 2-3% every 2 min. Average O2 consumption and respiratory exchange ratio are sampled every 15 sec using a mouthpiece. Relative VO2 max is reported relative to body weight, calculated as ml O2/kg/min. Maximum effort is defined using at least 2 of the following: (1) age-defined maximum heart rate norms (HR > 85% of predicted max HR), (2) respiratory exchange ratio (CO2/O2) >1.1, (3) subjective rate of perceived exertion >17 (of 20), and (4) leveling of VO2 despite increasing aerobic demand. VO2 max percentile is also be reported based on age norms.

BMI is calculated from measures of height and weight

High-resolution structural MRI scan of brain anatomy using MPRAGE sequence, acquired in sagittal orientation (resolution: 0.9 mm, 192 slices, TR 1900 ms, TE 2.32 ms).

Using functional MRI to measure blood-oxygen-level-dependent signal while the subject is at rest and not performing a task

Functional MRI that measures brain activity (based on hemodynamic response and blood-oxygenation-level dependent signal) during an experimental task.

Completed during fMRI scanning. In the task, an image with 4 objects is presented on a grid of 9 possible locations. Participants are instructed to memorize the spatial locations of all 4 objects and mentally rotate the grid 90⁰. Participants are then presented with a probe image containing the same 4 objects rotated 90⁰. Participants must indicate whether or not any of the objects changed location in the rotated probe image. There are three possible responses: either all objects match; 1 object moves to a new location; or 2 objects swap locations. This task is a measure of relational memory.

Completed during fMRI scanning. Participants are presented with a continuous stream of stimuli (either faces or words), and for every stimulus, the participant must indicate (yes/no) whether or not the current stimulus is the same as the one presented three trials back. This task is a measure of working memory.

Diffusion tensor imaging (DTI) is a magnetic resonance imaging method that is sensitive to the diffusion of water molecules. Diffusion of water in the brain depends on the tissue type, structural architecture, and presence/absence of barriers within imaging voxels. Using this raw imaging information, diffusion-based properties can be estimated including: 1. fractional anisotropy (FA), which is a measure of whether or not the 3-dimensional diffusion is largely directional (e.g., within an axon; thus, anisotropic diffusion), or if the diffusion is equal in all directions (e.g., within CSF; thus isotropic diffusion); 2. radial diffusivity (RD), which is a measure of how diffusion in the plane transverse to the principal direction of diffusion (i.e., within a cross-section of an axon, rather than down the length of an axon).

Technique to measure cell metabolites within a single super voxel of the brain (20 mm3), such as N-acetyl aspartate (NAA)

During magnetic resonance imaging scans, using the arterial spin labeling technique to acquire multi-slice cerebral blood flow maps

Saliva collected from passive drooling, to be analyzed for biomarkers related to fluid intelligence

Height measured to calculate BMI estimate

Weight measured to calculate BMI estimate

Questionnaire that requests age, birth date, sex, race/ethnicity, occupation.

Personality assessment with five dimensions: openness, conscientiousness, extraversion, agreeableness, and neuroticism

This questionnaire assesses the personality trait of grit, a characteristic associated with passion and adherence to long-term goals, along with strong motivation to achieve goals despite potential obstacles.

The PSQI assesses sleep habits on eight dimensions over the past month with 11 open-ended and multiple choice questions. Relevant dimensions include sleep duration, disturbance, latency, day dysfunction, efficiency, quality, use of medications to sleep, and an overall binary score of sleep habits.

This survey asks about level of physical activity during leisure time in the past month, from which longer-term exercise habits can be extrapolated. By collecting height, weight, and resting heart rate with this measure, a VO2 max estimation can be reliably calculated.

This survey asks about the frequency (never, rarely, sometimes, often) of 20 statements regarding feelings of loneliness and social connection.

This survey asks 45 Likert-type questions regarding the value an individual places upon cognition in daily life.

The ITIS asks 8 Likert-type questions regarding personal beliefs about the relative fixedness/malleability of intelligence.

The Physical Activity Enjoyment Scale (PACES) measures individuals' enjoyment with their current activity level.

This survey was customized for each arm to assess: participant feedback on the various interventions they experienced; strategies developed and used in cognitive training and active control training; expectancy effects of video game-training, physical fitness training, and mindfulness meditation training; and the contribution of external (outside-study) physical activity/exercise and video game play.

The Preference and Tolerance of the Intensity of Exercise Questionnaire (PRETIE-Q) measures individuals' preferences for physical activity type and duration. For instance, does one prefer high-intensity activity for a brief period of time, or low-intensity activity for a longer duration? How does one respond to physical challenge and exhaustion?

Game feedback surveys were developed in-house and administered to evaluate participant responses to cognitive training. These surveys were administered at Sessions 1, 24, and 48 for the 48-session intervention, or at Sessions 1 and 20 for the 20-session intervention. These surveys evaluated enjoyment, engagement, motivation, demand, and frustration with each mini-game.

Game feedback surveys were developed in-house and administered to evaluate participant responses to active control training. These surveys evaluated enjoyment, engagement, motivation, demand, and frustration with each task.

The APRT is five exercises: (1) 60-yard shuttle run, (2) standing long jump, (3) one-minute of rowers, (4) one-minute of push-ups, and (5) a 1.5 mile run

Participants' heart rates were monitored using Polar heart rate monitor E600 during all sessions of the fitness intervention