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.
Undergone unilateral ACL reconstruction less than 2 years ago;
Have medical release for the full load on the injury limb for at least 2 weeks;
Be independently pain-free walking;
Being sports athletes according to an athlete description: Training regularly to improve performance, actively participating in competitions or formally registering in a sports federation or association;
Participants with previous surgery on either knee;
Those with more than 3 months between ACL injury and surgery;
More than 2 weeks between surgery and the start of physical therapy;
Had another musculoskeletal injury in the lower limb within the past 6 months;
Crossover
Fractal Cueing
DeviceWalking synchronized to a visual fractal metronome.
Isochronous Cueing
DeviceWalking synchronized to a visual Isochronous metronome.
Gait Variability Measured by Fractal Scaling Exponent of inter-stride intervals
The fractal-scaling exponent, α, of inter-stride intervals (α-ISIs) will be determine by calculating the time between two consecutive heel strikes of the same foot. First, the investigators will identify the heel strike events of the dominant foot. To improve the identification of this event, the signals will be filtered using a 2nd order, zero lag low-pass Butterworth filter with a cutoff frequency of 20 Hz. Then, Detrended Fluctuation Analysis (DFA) will be used to determine the fractal-scaling, an index of complexity. The DFA is a modified random-walk analysis that makes use of a long-range correlated time series. The long-range correlation can be mapped to self-similar calculations through simple integration.
Corticospinal Excitability Measured by Motor Evoked Potential Amplitude
Conducted using Spike2 software (version 10; Cambridge Electronic Design, Cambridge, United Kingdom) and subsequently exported to, MATLAB R2018a (The MathWorks, Natick, Massachusetts, United States) for processing and analysis. Corticospinal Excitability will be quantified as the peak-to-peak amplitude of MEPs elicited by a single-pulse TMS stimulus, with values averaged across 10 trials per participant.
Cortical Silent Period (SP) Duration
Silent period will be calculated as the time from MEP onset to the resumption of voluntary EMG activity, expressed as SP = resumption of voluntary EMG time - MEP onset (58), and averaged across all 10 stimuli.
Intracortical Facilitation (ICF)
ICF will be determined using the same calculation method but with an interstimulus interval of 12 ms (ICF = conditioned MEP / control MEP).
Magnitude of variability
Calculate the coefficient of variation of inter-stride intervals, representing the magnitude of variability.
Synchronization Accuracy
Asynchronies (ASYNC), i.e., the time difference between the heel strikes and the cues, expressed in milliseconds. This will be used to ensure that differences at the primary outcome are not the result of different synchronization performances. A negative value of ASYNC indicates that the heel strike occurred before the cue. The ASYNC provides information regarding the strategies used and the performance of the synching processes, serving as a control parameter to reliably interpret our results in terms of gait complex.
Demographic characterization of the sample by Lysholm Knee Score (LKS).
The LKS show good internal consistency (Cronbach's α = 0.802) and high test-retest reliability (ICC = 0.859). The LKS consists of 8 items with 3 to 6 closed-response options, assessing knee function aspects such as pain, swelling, locking, and instability. The scale is completed in ≈5 minutes, with higher scores indicating better knee function.
Demographic characterization of the sample by Tegner Activity Scale (TAS)
The TAS show good internal consistency (Cronbach's α = 0.802) and high test-retest reliability (ICC = 0.972). The TAS comprises a single item with 11 activity levels (0-10), reflecting increasing physical demand. The scale is completed in ≈2 minutes, with higher scores indicating greater physical activity levels (TAS).
Demographic characterization of the sample by Tampa Scale of Kinesiophobia (TSK)
The TSK consists of 13 items rated on a 4-point Likert scale (1 = strongly disagree to 4 = strongly agree), with a total score ranging from 13 to 52, where higher scores indicate greater kinesiophobia. It required 4 to 6 minutes to complete the instrument and demonstrated good internal consistency (α = 0.82) and test-retest reliability (ICC = 0.99), confirming its reliability for assessing kinesiophobia.
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