Feasibility of a Mindfulness and Physical Activity Promotion Intervention After Anterior Cruciate Ligament Reconstruction

Rationale. Anterior cruciate ligament reconstruction (ACLR) is among the most common orthopaedic procedures performed in the United States, occurring most frequently in adolescents and young adults. Although surgery restores mechanical knee stability, more than 60% of patients fail to meet physical activity guidelines by 9-12 months after surgery, and patients with ACLR are markedly less likely to meet aerobic activity recommendations than uninjured peers. This shortfall is not explained by physical impairment alone: injury-related fear (fear of reinjury or of pain with movement) is one of the most consistent barriers to physical activity reengagement after ACLR, and prior work from this research group found that injury-related fear explained a substantial proportion of the variance in weekly moderate-to-vigorous physical activity (MVPA) in this population. Because ACL injury and reconstruction typically occur during adolescence, when lifelong activity habits are established, unresolved fear and inactivity during this period carry downstream consequences for reinjury risk, early knee osteoarthritis, cardiometabolic health, and psychological well-being. Standard post-surgical rehabilitation rarely addresses fear directly or supports sustained activity reengagement after medical clearance, and access to behavioral health support is often limited for this population.

Mind in Motion is a mobile-delivered behavioral intervention developed by the study team that integrates (1) mindfulness meditation delivered through the commercially available Headspace application and (2) personalized physical activity promotion driven by daily MVPA goals generated from Oura Ring wearable data. Preliminary work by this group demonstrated that mobile mindfulness delivery is feasible in the post-ACLR population and that wearable-based, intrapersonal goal-setting increased daily step counts in this population. This study extends that preliminary work to formally evaluate feasibility, engagement, acceptability, and within-person mechanism in a well-characterized sample of young people 1-5 years post-ACLR, prior to a planned multisite randomized controlled trial.

Design. This is a single-arm, single-site feasibility study. All enrolled participants receive the Mind in Motion intervention; there is no control or placebo arm and no randomization or stratification. A single-arm design was selected because the primary goal is to characterize feasibility, engagement, and within-person mechanism rather than to test between-group efficacy, consistent with established guidance for feasibility/pilot studies. The study is not powered for hypothesis testing.

Objectives.

• Aim 1 (primary): Determine the feasibility, engagement, and acceptability of the 6-week Mind in Motion intervention. The study team hypothesizes that participants will complete at least 80% of prescribed mindfulness sessions, wear the Oura Ring on at least 80% of intervention days, and rate the intervention as acceptable, with study procedures well tolerated.
• Aim 2 (secondary): Characterize the within-person, day-level relationship between mindfulness engagement, injury-related fear, and physical activity using intensive longitudinal ecological momentary assessment (EMA) data collected across the 6-week intervention. The study team hypothesizes that days following a completed mindfulness session will show lower anticipatory fear of movement and higher MVPA than other days, after accounting for the prior day's activity, time trend, and the daily activity-goal stimulus.

Population and eligibility. Eligible participants are 15-30 years old, are 1-5 years from a primary, unilateral ACLR, are medically cleared for physical activity, and show elevated injury-related fear at baseline (Tampa Scale of Kinesiophobia-11 score ≥ 17) together with a self-reported reduction in physical activity since surgery (confirmed via the ACL-RETURN survey during prescreening). Participants must be willing and able to wear an Oura Ring daily for 6 weeks, complete Headspace-delivered mindfulness sessions through the study's mobile application, and use a compatible smartphone; they must be able to read and understand English. Exclusions include ACLR performed less than 1 year or more than 5 years earlier; concomitant ligamentous/articular cartilage procedures or other lower-extremity conditions currently limiting activity; any medical condition restricting physical activity; a diagnosis of arthritis; suspected or confirmed pregnancy; and inability to provide informed consent/assent independently (e.g., cognitive impairment, incarceration). There are no restrictions on concurrent medications or treatments.

Enrollment. The study team plans to enroll up to 24 participants at the University of Virginia, anticipating approximately 20% screen failure and 20% dropout, in order to reach a target of 20 study completers.

Procedures and schedule.

• Prescreening: Interested individuals complete telephone/electronic prescreening (age, time since ACLR, medical status, smartphone access, English proficiency), followed by electronic administration of the TSK-11 and ACL-RETURN survey to confirm eligibility.
• Visit 1 (Week 1, \~60-75 minutes, in person): Informed consent/parental permission and assent (for minors); demographics and medical history; height/weight; a patient-reported outcome battery (TSK-11, ACL-Return to Sport after Injury scale [ACL-RSI], Reinjury Anxiety Inventory [RIAI], Athletic Identity Measurement Scale [AIMS], ACL-RETURN, International Knee Documentation Committee [IKDC] form, Knee Injury and Osteoarthritis Outcome Score [KOOS]); and fitting of a thigh-worn activPAL accelerometer for a 7-day pre-intervention MVPA measurement.
• Visit 2 (Week 2, \~25 minutes, in person): Return of the activPAL accelerometer; orientation to the Mind in Motion application and fitting/pairing of the Oura Ring; orientation to the twice-daily EMA protocol.
• Intervention period (Weeks 2-7, remote, 6 weeks): Five 10-minute Headspace mindfulness sessions per week (sequential Basics, Sports Rehabilitation, and Sport Recovery courses); daily Oura Ring wear, with each day's MVPA goal generated from the 60th percentile of the participant's preceding 9 valid days of Oura-measured MVPA; and twice-daily brief EMA items (morning: anticipated fear, confidence, mood; evening: fear experienced during activity, soreness, perceived exertion), yielding approximately 84 EMA observations per participant.
• Mid-intervention assessment (Week 4, remote): Repeat of the patient-reported outcome battery via REDCap.
• Visit 3 (Week 7, \~45-60 minutes, in person): Return of a second 7-day activPAL accelerometer (worn during the final intervention week) with data download; repeat patient-reported outcome battery; Acceptability of Intervention Measure (AIM); a brief audio-recorded semi-structured exit interview about intervention experience; and return/unpairing of the Oura Ring.
• Week 11 retention assessment (remote): Repeat of the TSK-11, ACL-RSI, RIAI, and AIMS to characterize durability of any observed changes.
• A limited chart review (demographics, injury and surgical characteristics) may be conducted for participants who underwent ACLR at UVA, solely to confirm eligibility-relevant clinical history.

Outcomes.

• Primary (feasibility/engagement/acceptability): recruitment and retention rates; percentage of prescribed Headspace sessions completed; percentage of intervention days with valid Oura Ring wear; AIM acceptability scores. These will be summarized descriptively (means, percentages, confidence intervals), with no inferential testing planned for these endpoints. Pre-specified go/no-go benchmarks include retention ≥85%, Headspace completion ≥80%, Oura wear ≥80% of days, mean AIM ≥4.0, and TSK-11 change of at least 4 points (a previously published minimal detectable change).
• Secondary (within-person change): pre-to-post change in TSK-11, ACL-RSI, RIAI, AIMS, and activPAL-measured MVPA, characterized descriptively and with Hedges' g effect sizes and bootstrapped 95% confidence intervals.
• Secondary (mechanistic): a within-person fixed-effects regression testing whether days following a completed Headspace session are associated with lower anticipatory fear and higher MVPA, controlling for the prior day's MVPA, time trend, and the daily activity-goal stimulus. Analytic code will be pre-registered on the Open Science Framework prior to unblinding; all analyses will be conducted in R.
• Exit interview data will be analyzed using an inductive, team-based thematic coding approach (familiarization, code generation, iterative theme development, consensus review), with themes interpreted within a biopsychosocial framework.

Risk profile. This study has been categorized as minimal risk. All procedures are non-invasive; Headspace and the Oura Ring are commercially available consumer products, and the activPAL accelerometer is a validated research instrument. The main anticipated physical risk is mild lower-extremity soreness from incremental increases in daily activity, mitigated by an intrapersonal goal-setting approach anchored to each participant's own recent activity (rather than a fixed external target). The main anticipated non-physical risk is a minor breach of privacy or confidentiality, addressed through the study's data security infrastructure.

Statistical/analytic collaboration. The study protocol was developed in consultation with a biostatistician (Department of Statistics, University of Virginia).