Fractal vs Isochronous Cueing in Athletes After ACL Reconstruction

Overview

Anterior cruciate ligament (ACL) injuries are among the most prevalent and functionally limiting knee injuries in sports, particularly those that involve pivoting movements. Despite advancements in surgical reconstruction and physical rehabilitation, many athletes continue to exhibit persistent motor control deficits and increased gait variability, both of which are closely linked to a heightened risk of re-injury and long-term joint degeneration. These deficits arise from biomechanical impairments and disrupt proprioceptive input that requires cortical reorganization, contributing to maladaptive neuroplasticity. However, conventional rehabilitation strategies often overlook this neural dimension. Recent findings emphasize the importance of fostering motor variability and promoting neuroplasticity through external focus strategies, including sensorimotor synchronization. While isochronous cues, an invariant stimulus, are commonly used, they do not reflect the natural fluctuations of healthy gait and may reduce its complexity. Fractal-based cues, in contrast, introduce structured variability resembling the natural dynamics of locomotion and have been shown to restore gait complexity in clinical populations. However, no study has yet explored their acute effects on gait variability and corticospinal function following ACL reconstruction (ACLR). This crossover randomized controlled trial aims to compare the acute effects of a single session of treadmill walking synchronized to either fractal or isochronous-based visual cues on gait variability and corticospinal measures in athletes with ACLR. The investigators hypothesize that fractal-based cueing will acutely restore gait variability and enhance corticospinal excitability, evidenced by increased corticospinal excitability and intracortical facilitation, and reduced short-interval intracortical inhibition, thus promoting adaptive neuroplasticity. Conversely, isochronous cueing is expected to maintain or decrease gait complexity without improving corticospinal measures. This study may provide insights that could be highly valuable as a way to promote neuroplasticity and optimize gait rehabilitation after ACLR, also allowing an objective quantification and aiming to restore variability to levels close to those observed in healthy individuals, thus contributing to reducing the re-injury rate.

This study will test a laboratory-based intervention using gait trials with sensorimotor synchronization, a common approach in gait rehabilitation aimed at restoring or acutely modifying gait variability. The intervention, called SyncGait, consists of treadmill walking synchronized with visual cues. Following a crossover design, each participant will complete two sessions, one with fractal cueing (FRC) and one with isochronous cueing (ISO), with the order randomized. Each cued trial will last 12 minutes and will be preceded and followed by uncued walking trials. The visual cues will be presented as a moving bar on a screen positioned in front of the treadmill. For the FRC condition, the cues will be individualized to each participant's stride time variability, generated through a fractal algorithm and validated using detrended fluctuation analysis. For the ISO condition, cues will match each participant's average stride time without variability. Participants will be instructed to synchronize heel strikes of the reconstructed limb with the top of the moving bar. The treadmill (Bertec Inc., USA) will record gait data at 1000 Hz. Each session will be conducted individually, supervised by at least two trained team members. A sample size of 36 participants was determined to ensure adequate power (90%) to detect a moderate effect size (ηp² = 0.25) at a 5% significance level, accounting for an anticipated 20% dropout rate. To encourage participation and retention, participants will receive a personalized evaluation report and a free isokinetic assessment within 12 months. Data collection will include both gait and corticospinal outcomes. Gait variability will be analyzed from treadmill force data, focusing on stride intervals and synchronization performance. Corticospinal outcomes will be assessed using transcranial magnetic stimulation (TMS) with a BiStim2 stimulator (Magstim®, UK). Electromyography (Delsys Trigno, AD Instruments, NZ) will be recorded from quadriceps and hamstrings, while knee strength will be assessed with an isokinetic dynamometer (Humac Norm, USA). Standardized electrode placement and contraction protocols will ensure reproducibility. During testing, single- and paired-pulse TMS protocols will be applied at 10% of maximal voluntary contraction. Ten stimuli per condition will be collected, providing reliable estimates of excitability and inhibition. Visual feedback will help participants maintain the required contraction level throughout assessments. Statistical analysis will begin with normality testing (Shapiro-Wilk). Linear mixed models (Time × Cue condition) will be used for primary and secondary outcomes. Post-hoc comparisons will use Bonferroni corrections. Synchronization accuracy will be compared with paired t-tests. Statistical significance will be set at p \< 0.05, and analyses will be performed using Jamovi software. This protocol is designed to rigorously test whether fractal cueing can restore healthy gait variability patterns and enhance corticospinal excitability in athletes with ACL reconstruction, compared with conventional isochronous cueing.