Effectiveness of transcutaneous spinal direct current stimulation combined with exercise training in patients with anterior cruciate ligament reconstruction: study protocol for a randomized controlled trial.

Effectiveness of transcutaneous spinal direct current stimulation combined with exercise training in patients with anterior cruciate ligament reconstruction: study protocol for a randomized controlled trial - PMC Skip to main content An official website of the United States government Here's how you know Here's how you know Search Log in Primary site navigation Search Log in Search PMC Full-Text Archive Search in PMC Journal List User Guide As a library, NLM provides access to scientific literature. Inclusion in an NLM database does not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of Health. Learn more: PMC Disclaimer | PMC Copyright Notice Trials . 2026 May 26;27:407. doi: 10.1186/s13063-026-09810-4 Effectiveness of transcutaneous spinal direct current stimulation combined with exercise training in patients with anterior cruciate ligament reconstruction: study protocol for a randomized controlled trial Jiang Ma 1 , Lin Chen 1 , Shiyi Tong 2 , Guosen Li 2 , Shaopeng Wu 1 , Qinghua Luo 1 , Xin Yang 1 , Hongbing Li 1 , Junjie Xiong 1 , Chuansheng Hong 1 , Chunyu Duan 1 , Dejie Fu 3 , Peng Tang 1, ✉, # , Jingming Hou 1, ✉, # Author information Article notes Copyright and License information 1 Department of Rehabilitation Medicine, Southwest Hospital, Third Military Medical University (Army Medical University), Chongqing, 400038 China 2 Department of Rehabilitation of Traditional Chinese Medicine, Southwest Hospital, Third Military Medical University (Army Medical University), Chongqing, 400020 China 3 Centre for Joint Surgery, Southwest Hospital, Third Military Medical University (Army Medical University), Chongqing, 400038 China ✉ Corresponding author. # Contributed equally. Received 2025 Nov 10; Accepted 2026 May 18; Collection date 2026. © The Author(s) 2026 Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/ . PMC Copyright notice PMCID: PMC13224716  PMID: 42186052 Abstract Background Current standardized rehabilitation protocols for anterior cruciate ligament reconstruction (ACLR) often fail to fully resolve functional deficits arising from initial injury and subsequent surgery. These lingering issues are tied to neural impairments such as reduced corticospinal excitability and weakened motor cortex activation, which disrupt sensorimotor signal transmission, impair motor control, and hinder recovery even after post-operative rehabilitation. Growing evidence suggests that advancing post-ACLR care should synchronize the normalization of neural pathways with peripheral musculoskeletal recovery. Transcutaneous spinal direct current stimulation (tsDCS), an emerging non-invasive neuromodulatory technique, can modulate neural function and enhance motor performance. This study aims to investigate the safety and efficacy of interventions that combine simultaneous exercise with tsDCS in the rehabilitation of patients following ACLR. Methods This study will be a double-blind, randomized controlled trial with a two-arm parallel-group design and a 1:1 allocation ratio. All participants get routine rehabilitation. Patients will remain supine during stimulation, with a total duration of 20 min and a constant current of 2.0 mA. The experimental group receives tsDCS plus exercise training, with cathode at T10 spinous process and anode at uninjured deltoid. During tsDCS, they perform straight leg raises with the reconstructed limb. The control group gets sham stimulation plus exercise training. The primary outcome is the Knee Injury and Osteoarthritis Outcome Score at 18 months post-surgery. This study explores the effect of tsDCS intervention on early corticospinal excitability in post-ACLR patients, with indicators measured via transcranial magnetic stimulation. The secondary outcomes include self-reported measures such as pain, sports psychology, motor function and quality of life, as well as objective assessments such as range of motion, muscle strength, muscle endurance, gait, balance, proprioception, stability and cerebral function changes. Discussion This study addresses a critical gap in ACLR rehabilitation, as standardized protocols often overlook neural impairments linked to persistent functional deficits. The combined intervention of tsDCS and exercise training is expected to promote accelerated rehabilitation after ACLR. This study offers guidance for optimizing neuromodulatory strategies in orthopedic rehabilitation. Trial registration ChiCTR ChiCTR2400079530. Registered on 05 January 2024. https://www.chictr.org.cn/showproj.html?proj=216406 . Supplementary Information The online version contains supplementary material available at 10.1186/s13063-026-09810-4. Keywords: Rehabilitation, Anterior cruciate ligament reconstruction, Transcutaneous spinal direct current stimulation, Corticospinal excitability, KOOS, Functional near-infrared spectroscopy Introduction Background and rationale The anterior cruciate ligament (ACL) is a critical stabilizer of the knee joint that integrates biomechanical support and proprioceptive function. It maintains joint stability through three key mechanisms: processing proprioceptive feedback, activating protective neuromuscular reflexes, and regulating articular kinematics [ 1 ]. However, ACL injury is among the most common ligament injuries of the knee, and anterior cruciate ligament reconstruction (ACLR) is widely used in clinical practice to restore joint stability [ 2 , 3 ]. Even with standardized rehabilitation, many patients still exhibit persistent functional deficits postoperatively [ 2 , 3 ]. Studies indicate that only 67% of individuals return to pre-injury athletic activity levels [ 4 ]. The recurrent ACL injury rate can reach 23%, with the rate reaching 30% in adolescents [ 5 ], and 80% of elite athletes develop post-traumatic osteoarthritis following ACLR [ 6 ]. All of these outcomes severely impact their quality of life and athletic careers. Following ACLR, the sensorimotor system develops multi-level dysfunction across sensorimotor pathways, including afferent pathways, central integration, and efferent pathways [ 7 ]. Such dysfunction contributes to abnormalities in motor planning and execution, leading to functional deficits that often persist despite conventional rehabilitation [ 8 ]. The pathophysiological underpinning of these deficits lies in adaptive changes in neural excitability, specifically reduced corticospinal excitability in the reconstructed leg, as evidenced by elevated motor thresholds (MT) and decreased motor evoked potential (MEP) amplitudes. Notably, these deficits are more evident in both legs of ACLR patients than in those of healthy controls [ 9 , 10 ], which has also been confirmed by our previous pilot study [ 11 ]. This neuroplastic remodeling is directly associated with persistent neuromuscular impairments, such as quadriceps inhibition and compromised postural control, which ultimately limit functional recovery [ 10 ]. Accordingly, promoting post-operative neuroplasticity has emerged as a critical therapeutic target for optimizing rehabilitation outcomes. Recent evidence further highlights that interventions targeting sensorimotor pathways must simultaneously increase peripheral afferent efficiency, such as via proprioceptive re-education; central integrative capacity, such as through cognitive-motor dual-tasking; and descending efferent function, such as by optimizing motor neuron recruitment [ 8 ]. This integrated strategy is essential for fostering synergistic neuromusculoskeletal recovery. Against this backdrop, transcutaneous spinal direct current stimulation (tsDCS), an emerging non-invasive neuromodulatory technique, modulates spinal excitability via polarity-dependent current flow and has demonstrated therapeutic potential for rehabilitating motor dysfunction [ 12 ]. In humans specifically, anodal tsDCS suppresses responses mediated by spinal ascending pathways while moderately facilitating segmental reflex circuit function. In contrast, cathodal tsDCS tends to enhance responses of the ascending spinal pathways and suppress segmental reflex function, while also alleviating pain and improving motor function [ 12 ]. Its underlying mechanisms include modulation of synaptic transmission efficiency (e.g., enhanced motor neuron recruitment [ 13 ]), regulation of neurotransmitter dynamics (e.g., glutamate-GABA balance [ 14 , 15 ]), and potential remodeling of corticospinal pathway conduction [ 16 , 17 ]. Our pilot study [ 11 ] has confirmed that cathodal tsDCS has a favorable safety profile and can improve corticospinal excitability, providing a solid rationale for the present trial, thus presenting a novel neurorehabilitative approach for addressing post-ACLR recovery challenges. Objectives The primary hypothesis is that cathodal tsDCS combined with exercise training will yield greater long-term improvements in knee function at the 18-month follow-up, as measured by differences in the Knee Injury and Osteoarthritis Outcome Score (KOOS), than exercise training plus sham stimulation. It is further hypothesized that cathodal tsDCS during exercise training will improve short-term corticospinal excitability in post-ACLR patients in comparison to training combined with sham stimulation, with measurements obtained via TMS to elucidate early CNS electrophysiological effects. Additionally, it is hypothesized that gains in knee motor function correlate with increased excitability characterized by reduced MT and increased MEP, thereby supporting the “central modulation for peripheral improvement” pathway. Pre/post-intervention functional activity changes in key cerebral cortices will be evaluated via fNIRS, with measures including cortical activation and inter-regional connectivity. This evaluation aims to clarify the role of tsDCS in central cortical remodeling and refine the evidence for “spinal-cortical” neuromodulation. Multi-dimensional data will be integrated to analyze the overall impact of cathodal tsDCS combined with training on central-peripheral synergistic recovery. These findings will inform optimized ACLR neuromodulation rehabilitation protocols and the application of ERAS in sports medicine. Trial design This study will be a double-blind, randomized controlled trial (RCT) with a two-arm parallel-group design and a 1:1 allocation ratio, comparing two interventions in post-ACLR patients: cathodal tsDCS combined with exercise training versus sham tsDCS combined with exercise training. This protocol is reported according to the SPIRIT 2025 Statement: Updated Guideline for Protocols of Randomized Trials, which is available as an additional file (Additional file 1) [ 18 ]. Methods: participants, interventions, and outcomes Study setting Patients will be recruited during their admission at the Department of Rehabilitation Medicine of two participating hospitals in the urban area of Chongqing: Southwest Hospital, Third Military Medical University, and the 958th Army Hospital (Jiangbei District, Southwest Hospital, Third Military Medical University). All participants will be recruited from the Departments of Orthopedic Surgery and Sports Medicine at the two aforementioned hospitals. anterior cruciate ligament reconstruction (ACLR) will be performed by Professors DEJIE FU and LIN GUO, who have long specialized in ACLR, possess over 20 years of surgical experience, and each performs more than 200 such surgeries annually. Recruitment started in February 2024. The baseline and follow-up assessments will be completed at the Department of Rehabilitation Medicine of Southwest Hospital, Chongqing, China, by five blinded physiotherapists. The interventions in this study will be conducted by 8 licensed physical therapists, among whom 6 will be responsible for routine rehabilitation and the remaining 2 will be in charge of administering both active and sham tsDCS. All costs associated with the treatment and assessment procedures within the study will be covered by the project. Following discharge, the home-based rehabilitation phase is primarily self-managed by patients. Any inquiries or issues arising during this period will be addressed via online chat or video consultation. If necessary, patients may be required to visit the hospital rehabilitation center for in-person guidance and supervised training by physiotherapists. The results of routine clinical examinations and demographic data are presented in Table 1 . Table 1. Routine examinations and demographic information Routine examinations Demographics Knee X-ray Age; height; weight; gender; BMI Magnetic resonance imaging of the knee Education level; marital status; ethnicity; place of birth Ultrasound of reconstructed leg Meniscus injury (yes/no), if yes, location, severity, and treatment modality, will be detailed Complications; graft type (synthetic/autograft) ​The intervals between injury, surgery, and enrollment Reconstructed side (left/right); the cause of the injury Open in a new tab Patient and public involvement Patients and the public were not directly involved in the design, recruitment, or conduct of this trial. However, they will be involved in the dissemination of study results by helping to determine the content and format of information provided to participants and relevant clinical groups. Eligibility criteria Patients with ACLR will be included on the basis of the following predefined eligibility criteria. Inclusion criteria Males and females aged between 18 and 45 years fulfilling the following inclusion criteria: Subjects who underwent first-onset ACLR on one side, the post-operative period following reconstruction is approximately 1 week but does not exceed 4 weeks. There is no limitation on the types of reconstruction grafts used, such as bone-patellar tendon-bone, hamstring tendon and ligament augmentation and reconstruction systems. Common complications, such as meniscus injury, cartilage damage and bone contusion, can also occur. Severe multiligamentous knee injuries, including those requiring medial collateral ligament surgery or posterior cruciate ligament (PCL) reconstruction, were excluded. No brain, heart, lung, or other diseases affecting the activity level. Volunteers to participate in this study. Exclusion criteria Patients will be excluded if they fulfill any of the following criteria: Severe complications, such as knee collateral ligament rupture, fracture, or Achilles tendon tear. History of lower limb surgeries, such as ACL reconstruction, PCL reconstruction, anterior talofibular ligament repair, or post-fracture internal fixation surgery. Severe complicating systemic diseases, such as hypertension or diabetes, may occur. Patients for whom electrical stimulation and magnetic stimulation are contraindicated, such as those with pacemakers, pregnancy, and epilepsy. Inability to comply with the study protocol. Inability to provide informed consent. Intervention—conventional rehabilitation program Both the experimental group (tsDCS combined with exercise training) and the control group (sham tsDCS combined with exercise training) will receive standardized post-operative rehabilitation. However, specific post-operative rehabilitation methods have slight individual variations, depending on the graft type and the presence or absence of concurrent meniscal injury. The rehabilitation protocol will primarily cover the following components: Pain and swelling management Regaining range of motion (ROM) Enhancing neuromuscular control Restoring muscle strength Achieving normal gait Returning to functional activities and sport The initial rehabilitation protocol is developed on the basis of multiple clinical guidelines [ 19 – 22 ]. Twelve registered therapists, with an average of 6.3 years of musculoskeletal rehabilitation experience (range: 3–12 years), will attend specialized training, including sessions on rehabilitation protocol discussion, effusion change management, and handling muscle soreness reports, to ensure effective implementation of the training plan. A detailed final rehabilitation protocol will be formulated (Additional file 2). The overall rehabilitation protocol is centered on the principle of enhanced recovery: patients who undergo reconstruction with artificial ligament grafts will have a faster recovery process than those with autologous grafts [ 23 ], whereas patients with comorbidities will have a relatively delayed post-operative rehabilitation process. During the 4-week intervention period, the responsible therapist will instruct participants to adhere to the rehabilitation protocol as closely as possible. During the follow-up period, the responsible therapist will not only provide online responses to patients’ questions but also offer in-person rehabilitation guidance via outpatient visits. Intervention—transcranial direct current stimulation In this study, enrolled ACLR patients will receive tsDCS via a device (Model: 0112; Qiushi Academy, Zhejiang University). The device connects to a mobile phone via Bluetooth, enabling direct stimulation parameter adjustment via the phone interface. The stimulation protocol is identical to that used in our pilot study [ 11 ]. Patients will remain supine during stimulation (total duration: 20 min; constant current: 2.0 mA). Specifically, 4.5 cm × 4.5 cm rubber electrodes will be used, each wrapped in a 5 cm × 7 cm sponge pad (pre-soaked in normal saline for conductivity and skin adherence). The current density is 0.10 mA/cm 2 . Stimulation includes two modes: Active mode: Current ramps up linearly to 2.0 mA over 15 s, maintains 2.0 mA for 19 min for 30 s, and then ramps down to 0 mA over 15 s (without interruption). Sham mode: This mode is designed to match the initial/final skin sensations of the active mode without neurophysiological effects. It ramps up to 2.0 mA (15 s) and then down to 0 mA (15 s) at the start, has no current for 19 min 30 s, and repeats the 15 s up/15 s down cycle 15 s before the end. The brief 30-s current periods (start/end) enable impedance testing but no physiological changes. This mode is pre-programmed in Zhejiang University’s DC stimulator. The experimental group will undergo tsDCS combined with exercise training: the electrodes will be placed in saline-moistened sponges, with the cathode positioned over the T10 spinous process and the axis parallel to the spine, and the anode over the uninjured-side deltoid (see Fig. 1 ). During tsDCS, patients perform straight leg raises with the reconstructed limb. The intervention is administered once daily, 5 times per week, for a total of 4 weeks. Fig. 1. Open in a new tab The tsDCS electrode placement and intervention procedure. The cathode electrode was placed on the T10 spinous process, and the anode electrode was placed on the contralateral deltoid muscle. The bottom panel demonstrates the combined intervention: a participant performing a standardized straight leg raise exercise while receiving tsDCS The control group will receive sham stimulation combined with exercise training. Its treatment protocol is the same as that of the experimental group, but only involves 30-s current periods for the ramp-up and ramp-down phases at the start and end of stimulation, and no current is applied for the intervening 19 min and 30 s. This setup ensures consistent skin sensations without inducing neurophysiological changes, and follows the same intervention frequency and duration as the experimental group. Intervention—exercise training Both groups of patients will simultaneously perform quadriceps strength training during tsDCS. The quadriceps plays a critical role in knee joint movement; therefore, patients will be instructed to perform straight leg raise exercises during electrical stimulation. Specifically, 20 repetitions per set will be required in the first and second weeks, and 25 repetitions per set in the third and fourth weeks, with a 5-min interval between sets, totaling 4 sets per session. If a patient presents with knee extension lag, quadriceps isometric contraction training will be prioritized initially. The specific training intensity will vary among patients and will generally follow the principle of progression, with adjustments made on the basis of individual tolerance. Criteria for discontinuation Each participant retains the right to withdraw from the study at any time. Additionally, a participant’s study participation may be discontinued by the investigator at any time if deemed necessary, including for the following reasons: Failure to complete at least 75% of the tsDCS sessions (fewer than 15 sessions) Adverse effects arising from tsDCS (e.g., severe skin burns) Post-operative infection requiring anti-infective treatment Loss to follow-up Participants will remain in the study unless they choose to withdraw consent or are unable to continue due to clinical reasons. The reason for withdrawal will be documented in the study’s case report form (CRF) under the “Change in study status” section. For all other changes in study status (excluding formal withdrawal of consent), every effort will be made to continue follow-up with the participant to collect all planned study outcomes. Strategies to improve adherence to intervention The responsible therapist will record each participant’s tsDCS session and rehabilitation training completion daily. For participants who miss a session, the therapist will contact them via phone within 24 h to emphasize the synergy between tsDCS and training for ACLR recovery, and adjust the session schedule to fit the participant’s daily routine. During the 18-month follow-up, support for maintaining adherence focuses on three key areas: participants can consult the responsible therapist via messages anytime (with responses within 24 h) to address recovery or training doubts; free in-person rehabilitation guidance is provided at outpatient visits to adjust training on the basis of their recovery status; and the therapist also emphasizes the importance of regular rechecks to monitor ACLR recovery and reinforce adherence to long-term plans. Outcomes Primary outcome measure The primary outcome of this study is the Knee Injury and Osteoarthritis Outcome Score (KOOS 4 ) at 18 months post-surgery, which roughly aligns with the time point after randomization. The KOOS4 is the mean score for the self-reported KOOS 4 subscales of pain, symptoms, function in sports and recreational activities and quality of life [ 24 , 25 ]. Total scores range from 0 to 100, with higher scores indicating better knee health. The KOOS 4 is a validated patient-reported outcome measure (PROM) widely used in ACL research, including in a recent randomized controlled trial (RCT) involving patients with ACLR [ 26 ] and large-scale databases such as the National Ligament Registry [ 27 ]. Notably, the KOOS 4 is sensitive and specific for identifying functional deficits caused by knee instability. Eighteen months after randomization, most patients will have engaged in sufficient activity to allow functional knee stability to be assessed [ 26 ]. Secondary outcome measures The relevant secondary outcomes in this study include condition-specific and generic patient-reported outcomes, as well as objective measurements (Table 2 ). For these outcomes, the main analyses will focus on changes from baseline to post-intervention and at the 18-month follow-up; the results will be reported as between-group differences in the mean change for continuous outcomes or proportion differences for dichotomous outcomes, respectively, unless otherwise indicated. Table 2. Time schedule of enrollment, interventions, assessments, and visits for participants Open in a new tab KOOS 4 four subscales of knee injury and osteoarthritis outcome score, ACL-QoL anterior cruciate ligament-quality of life questionnaire Secondary self-reported outcomes Data from the five subscales of the Knee Injury and Osteoarthritis Outcome Score (KOOS), including pain, symptoms, activities of daily living, sports and recreation function, and quality of life, will be reported. Scores are derived from each subscale’s respective items, with each item scored on a 1–5 scale: 1 represents “no difficulty/symptom” and 5 represents “extreme difficulty/symptom.” These raw scores are then converted to a 0–100 scale where higher scores reflect poorer knee-related outcomes to enhance interpretability. The visual analog scale (VAS) is used to assess patients’ self-reported knee pain intensity. The VAS is a horizontal line without numbers or verbal descriptors at intermediate points, ranging from 0 to 100 mm (0: no pain; 100: worst imaginable pain). During the assessment, patients mark the line on the basis of their most severe knee pain in the past 24 h to determine the maximum pain score. Fear of movement is assessed using the Tampa Scale of Kinesiophobia-17, a 17-item assessment tool where each item is rated on a 4-point Likert scale: 1 denotes “strongly disagree” and 4 denotes “strongly agree.” Total scores range from 17 to 68, with higher scores indicating greater fear of movement [ 28 ]. This scale has established reliability and validity in musculoskeletal pain populations [ 29 ]. To assess pain during movement in more patients, the Victorian Institute of Sport Assessment-Patella (VISA-P) is utilized. It is a widely accepted tool for measuring symptoms and pain severity in patients with chronic knee patellar tendinopathy [ 30 ]. The Chinese version is reliable and valid, with psychometric properties similar to those of the original VISA-P [ 31 ]. Although VISA-P is primarily intended for patients with patellar tendinopathy, pain in post-ACLR patients often occurs during movement, and these patients frequently have patellar tendon issues, which also broadens the assessment tools available for post-ACLR pain. We will assess knee-related quality of life, with the ACL-Quality of Life Questionnaire [ 32 ]. Return to activity and sports participation levels are evaluated using the modified Tegner scale [ 33 ]. Activity levels assessed via this scale are scored on a scale ranging from 1 to 10, where 1 represents low activity levels and 10 indicates professional-level sports participation. The Lysholm score is a tool for assessing knee ligament injuries with high reliability and validity [ 34 ]. It focuses on daily symptoms, mobility, stability, and key indicators (pain, swelling, stair climbing, squatting, lameness, support reliance, joint locking, instability). A score of up to 100 is used to classify knee function into four levels: excellent, very good, fair, and poor. Secondary objective outcomes Corticospinal excitability is assessed via monopulse transcranial magnetic stimulation (TMS) targeting the quadriceps motor cortex (MagNeuro R270, MagVenture, Denmark), with two key metrics measured: the resting motor threshold (MT, which is more reliable according to previous studies [ 35 ]) and the vastus medialis motor-evoked potential (MEP) amplitude [ 36 , 37 ]. The TMS assessment protocol is identical to that described in our previously published study [ 11 ]. Resting MT is defined as the lowest TMS intensity evoking MEPs ≥ 50 μV in 5 of 10 consecutive trials during complete muscle relaxation, and the Vastus Medialis’s motor hotspot is localized to the primary motor cortex ~ 1–2 cm posterior to the central vertex (Cz). For the MEP amplitude, five responses are recorded at 120% of the resting MT (the mean peak-to-peak amplitude is calculated afterward). Lower resting MT and/or higher MEP amplitude indicates greater corticospinal excitability. Surface electromyography will be used to collect signals from the affected leg. The participants will be positioned in standardized postures: for the quadriceps, seated with the knee flexed at 60°; for the hamstrings, prone with the knee flexed at 90°; and for the gluteus medius, side-lying with the hip in a neutral position. Electrodes will be placed at specified anatomical locations in accordance with the Surface Electromyography for the Non-Invasive Assessment of Muscles (SENIAM) guidelines [ 38 ], targeting the quadriceps, including the rectus femoris, vastus lateralis, vastus medialis, and hamstrings, including the biceps femoris, semitendinosus/semimembranosus, and gluteus medius. Each muscle group will perform 3 maximum isometric voluntary contraction trials, with each trial requiring 3 s of peak force maintenance. After signal denoising, the root mean square values are extracted, and the average value of root mean square across the 3 trials is computed. The flexion angle and extension angle of the knee joint are measured, and the range of motion (ROM) of the knee joint is calculated by determining the difference between these two angles. Functional near-infrared spectroscopy (fNIRS) can be used to detect cortical excitability by monitoring changes in the concentration of oxygenated hemoglobin, with a focus on the frontal, parietal, and occipital lobes. For the assessment of brain network connectivity, a continuous movement paradigm will be adopted: participants will actively flex and extend the affected knee joint for 3 min. For the assessment of brain activation, a block design will be used: each block will include a 15-s task of active flexion and extension of the affected knee joint followed by a 20-s rest period, and this task‒rest sequence will be repeated 5 times. Isokinetic technology features high accuracy and excellent repeatability. Muscle strength will be measured via a multi-joint isokinetic testing system (IsoMed 2000, manufactured in Germany), with peak torque at a test angular velocity of 60°/s defined as the maximum muscle strength of the knee flexors and extensors. Test indicators will include concentric and eccentric contraction strength of the quadriceps femoris and hamstrings in both lower limbs. Knee proprioception will be assessed via a multi-joint isokinetic testing system (IsoMed 2000, Germany), with only position sense measured. The tests will target preset knee flexion angles of 30°, 60°, and 90°, with 3 trials per angle; the average difference between the actual angle achieved by the subjects and the preset angle will be recorded. A smaller angle difference indicates stronger knee proprioceptive ability. Gait will be assessed via the Tecnobody Gait Testing System (WalkView, Italy). Subjects will first determine a test speed close to their daily walking pace on the treadmill, and then undergo the test. At least 2 valid datasets will be collected per subject, with the average value used for analysis. Gait parameters, including step length, stride length, step frequency, and gait cycle, will be recorded. Balance will be quantitatively assessed using a Tecnobody (PK252, Italy) balance instrument. The key test indicators include the displacement length and displacement area of the center of pressure. The smaller their values are, the better the patients’ static postural control, and the knee joint achieves better control accordingly. The single-leg hop test is conducted by taking off and landing on the same foot. The uninjured leg is always tested first, followed by the affected leg. Each leg is permitted three attempts, with the longest jump distance recorded for each separately. The Limb Symmetry Index is then calculated as the percent quotient between the longest jump of the affected leg and that of the uninjured leg. Considering the interindividual variability in post-operative recovery trajectories, not all participants were able to complete every functional assessment at the post-intervention evaluation. The feasibility of each test was determined based on the following predefined criteria. For weight-bearing and functional assessments (e.g., gait, balance), these were administered only to patients who had achieved full weight-bearing status and could safely maintain unassisted single-leg stance. Participants who remained non-weight-bearing or had only recently progressed to partial weight-bearing (e.g., due to concomitant meniscal root repair, post-operative thrombosis, significant swelling, or poorly controlled pain) were excluded. For balance testing, a valid attempt was defined as the successful completion of at least one of three standardized trials. Regarding isokinetic muscle strength and joint angle reproduction assessments, both measurements were performed using an isokinetic dynamometer. Testing would not be performed if the patient reported severe pain (≥ 4/10 on the VAS) during the procedure, to avoid symptom exacerbation or other injury. Other outcome measures The laxity of the reconstructed ACL in patients can be evaluated using the Lachman test and anterior drawer test, both of which have high sensitivity and specificity [ 39 ]. Participant timeline Participants will follow a structured timeline covering interventions and assessments, with specific study procedures detailed in Table 2 and the study flowchart presented in Fig. 2 . Fig. 2. Open in a new tab Flow diagram of the study protocol Sample size The sample size calculation for this study was performed via G*Power software. On the basis of previous research data [ 40 ], the mean and standard deviation of the primary outcome measure, the Knee Injury and Osteoarthritis Outcome Score (KOOS), were extracted. A two-independent-sample t test (two-tailed test) will be adopted, with α set to 0.05 and β set to 0.2. This calculation yields a baseline sample size of at least n = 32 per group. Considering a 20% follow-up attrition rate, 40 participants were planned for final inclusion in each group, with a total of 80 participants to be enrolled in this trial. Recruitment Patients who meet the inclusion criteria will be identified through daily screening of medical records. They will receive verbal and written information about the study, and those willing to participate will subsequently be required to sign the informed consent form. Methods: assignment of intervention Allocation and sequence generation In this study, participants will be consecutively randomized into two groups (the active stimulation group and the sham stimulation group) at a 1:1 allocation ratio. The allocation sequence will be computer-generated, with the entire process managed by a data manager independent of the research team. No stratification will be implemented during the randomization. Allocation concealment and implementation The 1:1 random allocation sequence (active/sham stimulation groups) will be computer-generated and managed by an independent data manager (not involved in recruitment/assessment). For each eligible participant, the principal investigator will send their serial number to the manager online; the manager will then separately inform the stimulation operator of the group assignment. Blinding The electrical stimulation operator will be unblinded and will only implement active or sham stimulation as assigned but will not participate in outcome assessment or communicate with participants about group assignments. The principal investigator, participants, and outcome assessors will remain fully blinded to group allocations throughout the study. To maintain participant blinding, active and sham stimulation will follow identical operational procedures, including the same device appearance, electrode placement, and stimulation duration, except for the core stimulation parameters that distinguish active from sham intervention. No unblinding will occur unless a serious adverse event occurs, with such cases documented in detail. Methods: data collection, management, and analysis Data collection methods and management Prior to commencing data collection, all outcome assessors will complete two comprehensive training sessions to standardize protocols for all objective outcome measures. For all participants, data collection will be conducted at baseline, post-intervention, and during the 6-month post-operative follow-up (including both self-reported outcomes and objective assessments). Supplementary follow-ups focusing on self-reported outcomes will be administered online at 12 and 18 months. Upon completion of the assessments, the outcome assessors will document the clinical trial data and objective examination findings in the CRF. During supervised exercise and tsDCS interventions, researchers will record intervention-specific data in printed CRFs. Statistical methods The full and detailed statistical analysis plan for this trial is fully integrated and described in the “Methods” section of this article, which provides complete specifications for all planned analyses. We will perform both short-term and long-term outcome analyses in accordance with the study objectives. Baseline characteristics comparison Baseline data will be compared between the experimental and control groups to verify randomization balance. For continuous variables (age, BMI, pre-operative VAS score, pre-operative ROM, etc.), normally distributed data are expressed as the means ± SDs and are compared via independent samples t -test; non-normal data via Mann–Whitney U test. For categorical variables (expressed as frequencies and percentages), comparisons use Pearson’s chi-square test or Fisher’s exact test. Primary outcome analysis The primary outcome is the 18-month post-intervention KOOS4 score. First, the normality and variance homogeneity of the KOOS 4 scores will be verified. For between-group comparisons, normal and homogeneous data use independent samples t tests (95% CI of the mean difference), whereas for non-normal or heterogeneous data, the Mann-Whitney U test (median difference with 95% CI) will be used. The primary analysis uses intention-to-treat (including all randomized patients regardless of protocol adherence); per-protocol (only patients completing 18-month follow-up and adhering to intervention) analysis serves as a sensitivity analysis to exclude the impact of protocol violation. Secondary outcome analysis The following secondary outcomes are analyzed by time point: Continuous variables: time-point indicators (4-week isokinetic muscle strength, 6-month single-leg hop test, etc.): normal data via t tests and non-normal data via the Mann-Whitney U test. Repeated measurements (VAS, KOOS, Tampa scale scores, ROM, etc.): A mixed-effects linear model is used to analyze group, time, and group-time interaction effects (adjusted for baseline values and confounders); Bonferroni-corrected post hoc pairwise comparisons are conducted if the interaction is significant. Categorical variables (anterior drawer test, adverse events, etc.): Between-group incidence is compared via the chi-square test or Fisher’s exact test; binary logistic regression is used if confounders need adjustment. Subgroup analyses Exploratory subgroup analyses explore effect modifiers (surgical approach: autologous vs. allogeneic tendon; concurrent meniscus injury: with vs. without). Stratified mixed-effects linear model or stratified logistic regression is used, with interaction terms added to the model to test intervention effect homogeneity across subgroups. Safety analysis Safety is evaluated by adverse events (AEs), such as the AE type, occurrence time, severity (mild/moderate/severe), and handling outcome; the incidence of adverse events per group (compared via Fisher’s exact test); and the AE‒intervention relationship (causal/incidental/unrelated) as judged by investigators. Significance level and data management The two-sided test significance level is set at 0.05. For follow-up missing values (e.g., 18-month loss to follow-up), primary handling uses multiple imputation (5 datasets, assuming missing at random); complete case analysis serves as a sensitivity analysis to verify the robustness of the results. Methods: oversight and monitoring The monitoring of this study is designed to ensure protocol compliance, data accuracy, and participant safety, with simplified procedures outlined as follows: Daily operation monitoring, on-site inspection and data quality control were performed by the research team (Han Chen, Lu Wang, Lucan Jiang). Site coordination was undertaken by the research team (Peng Tang, Jingjing Yang). Overall steering and academic supervision were provided by the research team (Jingming Hou, Peng Tang, Jiang Ma). No interim analyses are planned for this trial. A dedicated study coordinator will be responsible for daily monitoring, including verifying the completeness of CRFs and ensuring timely documentation of outcome indicators. A Clinical Research Associate, appointed by the study team, will conduct onsite monitoring to assess study compliance. Data quality monitoring: Randomly review 20% of CRFs against original medical records (e.g., range of motion measurements, visual analog scale scores) to confirm consistency and accuracy; missing or ambiguous data must be resolved within 3 working days. Participant safety monitoring: Review AE records to ensure that all AEs (e.g., knee pain exacerbation, local skin irritation) are documented with details of occurrence time, severity, and management measures; serious adverse events must be reported to the Institutional Review Board within 24 h in accordance with regulatory requirements. Ethics and dissemination Research ethics approval The study has been reviewed and approved by the Ethics Committee of the First Affiliated Hospital, Army Medical University, People’s Liberation Army of China, with approval number A-KY2023188. To ensure transparency and compliance with international clinical research standards, the study has been registered in the Chinese Clinical Trial Registry (ChiCTR), with the registration number ChiCTR2400079530. The trial protocol and any subsequent amendments (if applicable) will be updated in the registry in a timely manner to maintain the accuracy and completeness of the study information. Patients will receive information about the study and its details via both verbal and written informed consent, which will be secured by the study coordinator. Dissemination policy The study findings will be published in peer-reviewed international journals and presented at national and international conferences. The results will be published in accordance with the CONSORT guidelines [ 41 ]. Co-authorship eligibility will follow the Vancouver rules. Discussion Current ACLR rehabilitation protocols, despite their standardization, often fail to fully address functional impairments associated with the initial injury and subsequent surgery. These persistent deficits are closely linked to underlying neural dysfunctions, including reduced corticospinal excitability, heightened spinal reflex excitability, and diminished motor cortex activation [ 42 , 43 ]. These neurophysiological abnormalities disrupt the normal flow of sensorimotor signals, impairing motor control and limiting functional recovery even after intensive rehabilitation. Mounting evidence suggests that future advancements in post-ACLR care must prioritize the normalization of neural pathways to restore effective motor drive, an approach that targets not only peripheral musculoskeletal recovery but also central nervous system regulation [ 42 , 43 ]. The tsDCS, an emerging non-invasive neuromodulatory technique, has shown promise in this context. It can effectively regulate the excitability of the spinal tract and reflexes, modulate neural function, alleviate pain, and enhance motor performance [ 44 – 46 ]. This makes tsDCS theoretically viable for improving functional recovery after ACLR, offering a valuable alternative to conventional rehabilitation protocols by addressing the neural underpinnings of post-surgical deficits. Feasibility is supported by prior evidence: tsDCS, as a non-invasive neuromodulatory technique, has been shown to be safe in neurorehabilitation studies, with adverse events limited to mild, transient local skin irritation [ 47 ]. More importantly, we previously conducted a pilot study on cathodal tsDCS in post-ACLR patients. Its preliminary results (manuscript under review) showed good safety and suggested potential improvements in corticospinal excitability. The planned sample size of 80 patients is practical for recruitment via Southwest Hospital, ensuring successful completion of this RCT. The study is clinically interesting, as it focuses on a critical unmet need in post-ACLR care: resolving persistent functional deficits by targeting neural dysfunctions such as reduced corticospinal excitability that conventional rehabilitation often overlooks. The novelty lies in challenging the general understanding that post-ACLR rehabilitation should focus solely on peripheral musculoskeletal recovery; instead, we propose integrating tsDCS to modulate spinal–cortical pathways, offering a potentially transformative adjunct to current care. Ethically, the study maintains high standards: both groups receive evidence-based conventional rehabilitation, with the control group receiving sham stimulation (non-therapeutic but safe, avoiding redundant or inferior treatment). This design ensures that all participants benefit from standard care, while the approved ethics clearance further validates its ethical compliance. Clinically, the findings will inform optimized neuromodulation-based rehabilitation protocols for ACLR, directly supporting the application of ERAS in sports medicine and enhancing its relevance to clinical practice. Trial status The final version (V6.0, 18 December 2023) of the study protocol was approved by the Ethics Committee of the First Affiliated Hospital, Army Medical University, People’s Liberation Army of China. The trial was registered in the Chinese Clinical Trial Registry in January 2024. The recruitment of participants commenced in February 2024. As of early December 2025, 77 patients have been randomized into the study. Recruitment for the study is expected to be completed by December 2025. Supplementary Information Additional file 1. (29.3KB, doc) Additional file 2. (17.3KB, xlsx) Additional file 3. (466.4KB, pdf) Additional file 4. (134.9KB, pdf) Additional file 5. (3.4MB, pdf) Acknowledgements The authors would like to express gratitude to Professors DEJIE FU and LIN GUO for their contributions to anterior cruciate ligament reconstruction (ACLR) surgeries in this study, ensuring the standardized implementation of surgical procedures. We would also like to thank Professor XIANGJI JIANG, as well as QIUWEN GONG, DONGYUN ZHANG, XIAOYU WU, and JINGJING YANG for their efforts in coordinating the screening and enrollment of participants, which laid a solid foundation for the smooth progress of the study. We would like to thank HAN CHEN, LU WANG and LUCAN JIANG for their contributions to the daily operation monitoring, on-site inspection and data quality control of this trial. We are also grateful to Professor MINMIN WANG from Zhejiang University for providing the transcutaneous spinal direct current stimulation (tsDCS) equipment required for the study interventions. Finally, we sincerely thank all the participants who agreed to be involved in this project—their participation is essential to the completion of this research. Abbreviations ACL Anterior cruciate ligament ACLR Anterior cruciate ligament reconstruction MT Resting motor threshold MEP Motor evoked potential tsDCS Transcutaneous spinal direct current stimulation KOOS Knee Injury and Osteoarthritis Outcome Score RCT Randomized controlled trial PCL Posterior cruciate ligament BMI Body mass index ROM Range of motion CRF Case report form VAS Visual analog scale VISA-P Victorian Institute of Sport Assessment-Patella TMS Transcranial magnetic stimulation fNIRS Functional near-infrared spectroscopy AE Adverse event ERAS Enhanced recovery after surgery Authors’ contributions JH conceived the study concept and overall design, served as the principal investigator of the study, and took charge of the overall advancement of the research. As the grant holder, JH also provided statistical expertise in formulating the primary and secondary statistical analysis protocols, ensuring the rigor of the study’s analytical design. SW, XY, and HL were responsible for the assessment of all outcome measures throughout the study, including baseline, post-intervention, and follow-up evaluations, and ensured the accuracy and standardization of outcome data collection. LC and ST undertook the implementation of transcutaneous spinal direct current stimulation interventions for participants, strictly following the study’s predefined operational procedures to guarantee the consistency of intervention delivery. QL, CH, CD, and GL were in charge of administering the standardized conventional rehabilitation training for both the experimental and control groups, providing guidance to participants during training and documenting training adherence. DF performed all anterior cruciate ligament reconstruction (ACLR) surgeries involved in the study, ensuring the standardization and quality of surgical procedures in line with the study protocol. JX was responsible for generating the 1:1 random allocation sequence via a computer and implementing allocation concealment, safeguarding the integrity of the randomization process. JM,PT and JH jointly conducted training sessions on testing procedures for all study personnel, unifying operational standards to minimize inter-operator bias. HC was responsible for daily operation monitoring, on-site inspection and data quality control. PT undertook site coordination. JH, PT and JM provided overall steering and academic supervision. JM also led the drafting of the study protocol and subsequent manuscripts for publication. All authors participated in the critical revision of the study protocol and manuscripts for important intellectual content and have read and approved the final version of the protocol. Funding This work was supported by the Special Project for the Improvement of Scientific and Technological Innovation Capabilities of Army Medical University (project number: 2023XJS22). The sponsor and funder is Army Medical University. The funder/sponsor played no role in study design; collection, management, analysis, and interpretation of data; writing of the manuscript; and the decision to submit the manuscript for publication. Data availability The datasets used and/or analyzed in the current study will be available from the corresponding author upon reasonable request following the publication of the main study findings. Declarations Ethics approval and consent to participate This study was ethically approved by the Ethics Committee of the First Affiliated Hospital, Army Medical University, People’s Liberation Army of China, complying with the Declaration of Helsinki. Written informed consent was obtained from all participants prior to enrollment. Consent for publication Written consent for publication was obtained from all participants. Competing interests The authors declare no competing interests. Footnotes Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Peng Tang and Jingming Hou are co-corresponding authors. Peng Tang is the first corresponding author. Contributor Information Peng Tang, Email: 498157055@tmmu.edu.cn. Jingming Hou, Email: jingminghou@tmmu.edu.cn. References 1. Reider B, Arcand MA, Diehl LH, Mroczek K, Abulencia A, Stroud CC, et al. 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[ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] Associated Data This section collects any data citations, data availability statements, or supplementary materials included in this article. Supplementary Materials Additional file 1. (29.3KB, doc) Additional file 2. (17.3KB, xlsx) Additional file 3. (466.4KB, pdf) Additional file 4. (134.9KB, pdf) Additional file 5. (3.4MB, pdf) Data Availability Statement The datasets used and/or analyzed in the current study will be available from the corresponding author upon reasonable request following the publication of the main study findings. 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