Effect of electrical stimulation on functional recovery of lower limbs in patients after anterior cruciate ligament surgery: a systematic review and meta-analysis

Abstract

Objectives

This study aimed to summarise the existing literature about enhancing muscle strength, lower limb function and self-reported function by electrical stimulation (ES) relative to conventional physical therapy following anterior cruciate ligament reconstruction (ACLR), and to assess the comprehensive treatment effects of ES via meta-analysis.

Design

Systematic review, meta-analysis.

Methods

This study systematically searched five electronic databases (PubMed, Web of Science, Scopus, Embase and Chinese National Knowledge Infrastructure), covering records from their inception until February 2024, adhering to a predefined search strategy. Two independent reviewers extracted and synthesised the relevant data using RevMan software (V.5.3). Due to identified heterogeneity, a random-effects model was applied for the meta-analysis. The meta-analysis calculated the effect sizes concerning lower limb function outcomes as standardised mean differences (SMD) with 95% CIs. The methodological quality of the included studies was assessed by the Physiotherapy Evidence Database scale.

Results

A total of 15 studies involving 1583 patients (between the ages of 15 and 50 years) were included. Meta-analysis results indicated that the ES group could improve the lower limb comprehensive function compared with the control group (CG) based on four clinical tests: the muscle strength (SMD=0.55, 95% CI 0.14 to 0.95, p=0.008, I²=74%), the range of motion (SMD=1.10, 95% CI 0.40 to 1.79, p=0.002, I²=89%), the Lysholm scale (SMD=1.05, 95% CI 0.36 to 1.73, p=0.003, I²=91%) and the visual analogue scale (SMD=0.87, 95% CI 0.38 to 1.37, p=0.006, I²=75%). However, there were no significant differences between the CG and the ES group in terms of leg circumference (SMD=0.61, 95% CI −0.78 to 2.00, p=0.39, I²=87%).

Conclusions

Adjunctive ES has the potential to enhance early-phase ACLR rehabilitation outcomes, particularly by improving muscle strength, lower limb function and self-reported function, despite the use of different ES modalities.

PROSPERO registration number

CRD42024549752.

Strengths and limitations of this study.

• Adjunctive electrical stimulation (ES) enhances muscle function more effectively than standard therapy alone, even with varied ES modalities.

• This study did not assess the long-term sustainability of ES benefits.

• This study did not perform subgroup analyses because of the limited literature included for each type of outcome and the ambiguous definition of ES.

• Overuse of the generic term ‘ES’ without subtype specificity limits clinical interpretation.

Introduction

The anterior cruciate ligament (ACL) is a major stabilising structure within the knee joint, playing an important role in preventing the forward translation of the tibia relative to the femur and in internal rotation.¹ ACL injuries result from a combination of biomechanical factors (eg, dynamic knee valgus), anatomical predispositions (eg, sex-specific variations), neuromuscular control deficits, high-risk movements (sudden stops/pivots) and environmental influences (eg, playing surfaces or footwear), which are prevalent in sports medicine and account for up to 2 million reported cases globally each year.² Sports injuries, traffic accidents and falls are the primary causes of ACL injuries, with sports injuries alone contributing to over 70% of these occurrences.² ACL injuries are often associated with knee instability, and prolonged instability may result in additional harm to the meniscus and cartilage. If not promptly or appropriately addressed, ACL injuries can worsen knee instability, potentially leading to complications such as patellofemoral pain and knee osteoarthritis.^{3 4}

While non-surgical management may be appropriate for select patients (eg, partial tears, low functional demands), studies indicate that delayed or inadequate rehabilitation can lead to persistent instability and secondary meniscal/cartilage damage.⁵ In cases of severe ACL damage (eg, complete rupture with concomitant meniscal injury or rotational instability), surgical reconstruction may be indicated to restore knee stability and reduce long-term degenerative risks. However, treatment decisions should be individualised, as some patients with isolated ACL tears may achieve functional recovery through structured rehabilitation programmes. ACL reconstruction (ACLR) is commonly recommended to restore functional and dynamic stability to the knee. A recent network meta-analysis indicated that ACLR using different types of tendon grafts led to improvements in anterior-posterior and rotational knee stability to varying extents.⁶ Although the mechanical stability of the knee joint can be almost completely restored after ACLR, patients may still encounter postoperative dysfunction.⁷ Typically, in clinical practice, the muscle function of individuals undergoing ACLR is assessed using an isokinetic dynamometer, with isometric peak torque serving as a frequently used parameter to gauge muscle strength.⁸ In addition, the limb symmetry index (comparing muscle strength between the affected and healthy sides) and hamstring-to-quadriceps strength (H:Q) ratio are often calculated based on the patient’s muscle strength and used as important indicators for the patient’s return to sports, restoration of dynamic stability of the knee joint⁹ and reducing the risk of secondary ACL injuries.¹⁰ However, a review has indicated that while the majority of patients do recover near-normal strength in the affected limb (reaching over 85% of the strength of the healthy limb), the percentage of patients who return to normal activity remains low.¹¹ Furthermore, there is still a significant risk of secondary ACL injury among those who resume activity, highlighting the inadequacy of current criteria for return to activity. Knee mobility, objective signs (ie, swelling, bruising), subjective symptoms (eg, pain, stiffness), muscle strength, unilateral and bilateral functional testing, muscle mass, time since surgery and laxity testing were reported as key criteria for determining readiness to progress through all phases of rehabilitation, which consists of ACLR recommendations.^{12 13} The positive impact of restoring functional activity in ACLR patients must be carefully considered.¹⁴ Additionally, studies have revealed that the incidence of chronic pain 6 months after ACLR stands at 15.9%, impacting the patient’s daily life and recovery process.¹⁵ Chronic pain not only limits the patient’s ability to engage in physical activity but also results in psychological stress, reduced quality of life and prolonged recovery time.¹⁶

Electrical stimulation (ES) encompasses all types of ES used for therapeutic purposes, involving the application of electrical currents to stimulate nerves, muscles or other tissues.¹⁷ ES serves various therapeutic objectives, such as pain management, muscle re-education, tissue healing and more.¹⁷ Functional ES specifically targets paralysed or weakened muscles to generate functional movement. Transcutaneous electrical nerve stimulation (TENS) focuses on pain relief by targeting sensory nerves, while neuromuscular electrical stimulation (NMES) directly stimulates muscle tissue to induce muscle contractions.¹⁸ TENS and NMES applied to the quadriceps muscle are used for clinical rehabilitation after ACLR. In certain studies, ES is initiated on the 3rd day postoperative and continues through the 4th and even the 12th week after surgery.^{19 20} Previous studies have reported that ES helps minimise quadriceps femoris atrophy and enhances muscle strength.^{21 22} In particular, NMES resulted in physiological improvements such as an increased cross-sectional area²³ and suppressed muscle mass loss.²⁴ However, no notable alterations in range of motion (ROM), laxity, subjective function or the duration of return to sports were observed.²⁵

Regarding the positive effects of ES in patients with ACLR, most studies have focused only on the enhancement of extensor strength with ES,^{26 27} and there has been no systematic review comparing the combined effects on muscle strength, functional performance or self-reported function after ES treatment. Further investigation is required to elucidate the comprehensive clinical advantages of ES interventions. Thus, the purpose of this study was to summarise the existing literature about the enhancement of muscle strength, lower limb function and self-reported function using ES in comparison to standard physical therapy after ACLR, and to determine the comprehensive treatment effects of ES through meta-analysis.

Methods

Search strategy

This systematic review and meta-analysis was registered (PROSPERO CRD42024549752) and completed in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analyses guidelines.²⁸ A comprehensive search was conducted across five electronic databases, namely PubMed, Web of Science, Scopus, Embase and Chinese National Knowledge Infrastructure, from their inception to February 2024. All Medical Subject Headings terms and their synonyms were employed either singularly or in conjunction: “anterior cruciate ligament”, “ACL”, “anterior cruciate ligament reconstruction”, “anterior cruciate ligament/surgery”, “ACL reconstruction”, “plastic surgery procedures”, “electric stimulation”, “transcutaneous electric nerve stimulation”, “electric stimulation therapy”, “neuromuscular electrical stimulation”, “NMES”, “ES”, “EMS” and “TENS”. The specific search strategy employed is shown in online supplemental table 1.

Eligibility criteria

The PICOS framework guided inclusion criteria specification: (1) Participants: adolescents (aged ≥12 years) and adults who have undergone ACLR. (2) Intervention: standard physical therapy augmented by ES, with explicit intervention protocols covering stimulation type, frequency, waveform characteristics, intensity levels, electrode positioning and treatment duration. (3) Control: underwent conventional physical therapy without supplementary ES intervention. (4) Outcomes: study reporting of ≥1 outcomes (muscle strength, ROM, Lysholm scores, visual analogue scale (VAS), limb circumference) with inter-group comparison. (5) Study design: exclusive to randomised controlled trials (RCTs) with accessible full-text publications.

Study selection and data extraction

The screening protocol commenced with importing all records into EndNote X9 for automated duplicate removal. Two independent reviewers (WS and TZ) conducted initial eligibility screening against the inclusion criteria. Potentially relevant full-text articles were retrieved for evaluation. Disagreements were adjudicated through consensus discussions, with unresolved issues escalated to a third investigator (RP). Using a predesigned extraction template, both reviewers independently collected: (1) participant demographics, (2) ES intervention parameters (type, frequency, waveform, intensity, electrode placement, duration) and (3) outcome measures. Missing data prompted author contact attempts. Extraction discrepancies underwent iterative review until consensus, with arbitration by RP when needed.

Quality assessment

Two independent researchers (TZ and JZ) assessed the methodological quality of the included studies using the Physiotherapy Evidence Database (PEDro) scale. This scale allocates a score from 0 (indicating a high risk of bias) to 10 (indicating a low risk of bias), based on 11 criteria (eligibility criteria, random allocation, allocation concealment, baseline similarity and blinding of subjects, therapists and assessors), which were formulated from the Delphi list to assess the quality of RCTs.²⁹ The aggregate PEDro score was determined by aggregating the evaluations for items 2–11, yielding a total score ranging from 0 to 10. Scores under 4 were categorised as ‘poor’, 4–5 as ‘fair’, 6–8 as ‘good’ and 9–10 as ‘excellent’. In the event of a disagreement, a conclusive judgement was reached in collaboration with a third author (RP).

Publication bias assessment

Visual identification of publication bias was conducted through funnel plots, which permit both qualitative analysis of small-study effects and quantitative asymmetry testing. Symmetrical plot configuration—characterised by broadly dispersed low-precision studies at the bottom and tightly clustered high-precision studies toward the top—indicates the absence of publication bias.

Meta-analysis

Statistical synthesis was conducted in RevMan V.5.3, calculating effect sizes as standardised mean differences (SMD) with 95% CIs. Effect magnitudes were defined: SMD 0.2–0.5 (small), 0.5–0.8 (medium) and ≥0.8 (large). An inverse-variance weighted random-effects model was applied throughout, accounting for between-study heterogeneity. Given anticipated high heterogeneity (I²>50%) from diverse ES modalities, this model was prioritised. All continuous outcomes were analysed with statistical significance set at p<0.05.

Certainty assessment

Two independent reviewers (WS, TZ) employed GRADE Profiler 3.6 (Cochrane IMS) for evidence quality assessment. Applying the Grade of Recommendations Assessment，Development and Evaluation GRADE framework to systematic review findings, evidence certainty was categorised into four levels: (1) high: further research very unlikely to alter efficacy conclusions, (2) moderate: further research likely to impact conclusion credibility with possible modifications, (3) low: further research highly probable to affect credibility and necessitate conclusion changes and (4) very low: efficacy assessments remain fundamentally uncertain. Disagreements were resolved through third-reviewer arbitration (JZ).

Patient and public involvement

No patient or public involvement.

Results

Study selection

In this review, a total of 1219 potential studies were searched from five electronic databases. Of these studies, a total of 425 duplicate studies were removed, and 507 studies were excluded after screening the titles and abstracts. Then, we obtained the full text of the remaining 287 studies. Furthermore, 272 studies were excluded because they did not meet the eligibility criteria. Finally, we included 15 studies that met the inclusion and exclusion criteria in this systematic review meta-analysis (figure 1).

Figure 1. Flow diagram of the study selection process according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses. RCT, randomised controlled trial.

Study characteristics

The characteristics of the included studies have been provided in online supplemental table 2. 15 studies included in this systematic review were RCTs³⁰,⁴⁴ that compared the effects of standard physical therapy and standard physical therapy plus ES on lower limb function in ACLR patients. The study population consisted primarily of ordinary people and athletes who had undergone ACLR, ranging in age from approximately 15 to 50 years, and most received the intervention within 1–5 days of ACLR, except for Labanca et al.³⁷ The intervention period exhibited differences across all included studies regarding the intervention duration (10–60 min), frequency (3–7 sessions per week) and total duration (2–12 weeks). The ROM,³²³⁴,36 38 39 41 43 VAS,^{3032 34},36 38 41 43 Lysholm scale,^{33 35 38 41 43} muscle strength of flexor and extensor^{31 33 37 39 40 42 44} and circumference^{32 36 37} were used to measure the lower limbs’ function. There were no adverse events reported among the included studies.

The details of the parameters used in the ES interventions were presented in online supplemental table 3. The definition of ES is broad and usually includes NMES and TENS, and some studies in the included literature explicitly indicate the use of NMES3335 37 38 40 42,⁴⁴ or TENS,^{34 36 40} and some studies do not indicate which ES modality was used.³⁰,^{3239 41} Electrodes for ES are usually placed on extensor muscles, flexor muscles or painful areas. The waveforms of ES included sine wave, sawtooth wave, biphasic pulse wave, single-phase pulse wave, asymmetrical bidirectional pulse wave and so on. Stimulation intensities range from 20 mA to 100 mA, and the stimulation frequency range from 20 Hz to 150 Hz, except for Song et al.⁴¹

Quality assessment

The scores of each study for the quality assessment have been shown in online supplemental table 4. Out of a maximum of 10 points, two studies scored 5 points,^{31 41} seven studies scored 6 points,^{3035 38},^{40 43 44} five studies scored 7 points^{32 34 36 37 42} and one study scored 8 points.³³ The scores of the included studies ranged from 5 to 8 and were accepted. All studies reported random allocation, baseline similarity, group comparison and point measures. Because of the environment of the intervention, no study blinded the participants and therapists.

Outcomes related to functional recovery of the lower limbs

Muscle strength

Seven studies evaluated the effects of ES and control group (CG) on muscle strength (figure 2). A total of data was extracted for 446 participants (ES group, n=220; CG, n=226). Compared with the CG, there was a significant increase in muscle strength in the ES group (SMD=0.55, 95% CI 0.14 to 0.95, p=0.008, I²=74%). However, in subgroup analysis, there was no significant increase in extensor muscle strength (SMD=0.39, 95% CI −0.03 to 0.81, p=0.07, I²=71%) or flexor muscle strength (SMD=1.06, 95% CI −0.14 to 2.25, p=0.08, I²=83%) in the ES group (figure 2).

Figure 2. Meta-analysis of ES on muscle strength after ACLR. ACLR, anterior cruciate ligament reconstruction; CG, control group; ES, electric stimulation.

Circumference

Three studies evaluated the effects of ES and CG on circumference (figure 3). A total of data was extracted for 72 participants (ES group, n=36; CG, n=36). Compared with the CG, there was no significant increase in circumference in the ES group (SMD=0.61, 95% CI −0.78 to 2.00, p=0.39, I²=87%).

Figure 3. Meta-analysis of ES on circumference after ACLR. ACLR, anterior cruciate ligament reconstruction; CG, control group; ES, electric stimulation.

Range of motion

Seven studies evaluated the effects of ES and CG on ROM (figure 4). A total of data was extracted for 378 participants (ES group, n=189; CG, n=189). Compared with the CG, there was a significant increase in ROM in the ES group (SMD=1.10, 95% CI 0.40 to 1.79, p=0.002, I²=89%).

Figure 4. Meta-analysis of ES on ROM after ACLR. ACLR, anterior cruciate ligament reconstruction; CG, control group; ES, electric stimulation; ROM, range of motion.

Lysholm scale

Six studies evaluated the effects of ES and CG on the Lysholm scale (figure 5). A total of data was extracted for 443 participants (ES group, n=221; CG, n=222). Compared with the CG, there was a significant increase in the Lysholm scale in the ES group (SMD=1.05, 95% CI 0.36 to 1.73, p=0.003, I²=91%).

Figure 5. Meta-analysis of ES on Lysholm scale after ACLR. ACLR, anterior cruciate ligament reconstruction; CG, control group; ES, electric stimulation.

Visual analogue scale

Seven studies evaluated the effects of ES and CG on VAS (figure 6). A total of data was extracted for 328 participants (ES group, n=166; CG, n=162). Compared with the CG, there was a significant decrease in VAS in the ES group (SMD=0.87, 95% CI 0.38 to 1.37, p=0.0006, I²=75%).

Figure 6. Meta-analysis of ES on VAS after ACLR. ACLR, anterior cruciate ligament reconstruction; CG, control group; ES, electric stimulation; VAS, visual analogue scale.

Certainty of evidence

The results of the GRADE system were expressed in the format of ‘Summary of findings table’, and the outcome indicators included muscle strength, circumference, ROM, Lysholm scale and VAS. The quality of evidence was moderate for all outcomes in online supplemental table 5.

Discussion

Lower limb dysfunction is mostly present after ACLR, and ES has been widely used for clinical rehabilitation after ACLR.¹⁹,²⁴ This meta-analysis aimed to compare the effects of standard physical therapy and standard physical therapy plus ES on the enhancement of muscle strength, lower limb function and self-reported function. The results of this study suggest that adjunctive ES combined with standard physical therapy may enhance improvements in muscle strength, lower limb function, self-reported outcomes and muscle circumference compared with standard therapy alone. However, the pooled analysis did not account for potential variations in ES parameters (eg, frequency, intensity, waveform), which limits direct clinical applicability.

Early ACLR is widely acknowledged as the preferred treatment for ACL injuries, although conservative treatment options are available.⁴⁵ Both surgical and non-surgical approaches advocate for a staged, structured and progressive rehabilitation protocol. The primary objectives of rehabilitation after ACLR are to address neuromuscular impairments, deficits in strength and ROM, motor control, balance and functional stability of the lower limbs, ultimately enabling a safe return to sports.^{12 46 47} To prevent quadriceps atrophy, which commonly occurs after ACLR, it is important to prioritise early strength building. This not only helps in preventing quadriceps inhibition and atrophy but also contributes to a more effective rehabilitation process, enabling patients to achieve their rehabilitation goals and successfully return to sport.

Typical interventions include vibration training, open-chain and closed-chain exercises, ES, postoperative bracing and aquatic therapy. Long-term use of ES appears to offer greater benefits compared with short-term usage.⁴⁸ There’s moderate-certainty evidence supporting the enhancement of quadriceps strength through ES.⁴⁹ ES has been suggested to influence action potentials in both intramuscular nerve branches and cutaneous receptors, thus promoting force production directly by activating motor axons and through the direct reflex recruitment of spinal motor neurons.^{50 51} When combined with early rehabilitation training, ES significantly enhances lower limb muscle strength and ROM, and alleviates knee discomfort following ACLR. The stimulation frequency varies considerably depending on treatment objectives.⁵² To prevent fatigue and discomfort, clinicians commonly employ low-frequency stimulation, maintaining muscle contraction at a low-intensity level. Typically, the recommended ES frequency ranges from 10 Hz to 50 Hz, with upper extremities commonly stimulated at 12 Hz–16 Hz and lower limbs at 18 Hz–25 Hz.⁵³ This range ensures muscle activation, prevents atrophy and helps preserve original muscle strength and most of the ES frequencies used in the studies included in this review were in the range of 20 Hz–50 Hz. A recent meta-analysis similarly revealed long-term impairment of the H:Q ratio in patients after ACLR, with quadriceps isometric force rate symmetry being more impaired than hamstring strength.⁵⁴ Decreased quadriceps force steadiness after ACLR is also related to the change in gait kinematics.⁵⁵ The maintenance of knee joint dynamic stability heavily relies on quadriceps and hamstring function. Integrating ES into standard rehabilitation protocols moderately enhances quadriceps strength but does not affect hamstring strength. However, unfortunately, our findings suggest that ES does not enhance the benefits of standard physical therapy in enhancing muscle circumference.

The recommended pathways for ACLR rehabilitation involve stages of return to participation, return to sport and return to performance.⁵⁶ In addition to focusing on the function of the muscle itself, attention needs to be paid to the overall function of the lower limb. Before granting clearance for resuming sports activities, it is advisable to conduct comprehensive tests including clinical assessment, functional assessment and quality of movement assessment. Adhering to these tests as criteria for return to sport has been demonstrated to reduce the risk of subsequent ACL graft rupture. Our findings suggest that the use of ES as an adjunctive approach is more effective than standard physical therapy alone in improving both lower limb joint mobility and clinical function. This may suggest that ES intervention can be effective in reducing the risk of ACL secondary rupture.

In the early stages, there is significant swelling of the knee joint, usually accompanied by significant pain and inflammation. ES also has a significant pain-reducing effect when low-frequency stimulation is used.^{17 57} The mechanism of pain relief by low-frequency ES is primarily attributed to the activation of opioid receptors in the central nervous system.¹⁷ This stimulation leads to the release of endogenous opioids, such as enkephalins and endorphins, which can inhibit pain signal transmission and modulate pain perception.^{15 17} Additionally, low-frequency ES may also activate descending pain modulation pathways, resulting in pain relief through the modulation of nociceptive signals.⁵⁷ Our results similarly found that ES can provide significant analgesia compared with standard physical therapy alone, which is consistent with previous studies.^{17 57} The use of wearable devices is now a trend in rehabilitation and there are now a variety of commercially available wearable ES devices. Home-based rehabilitation with wearable ES devices after ACLR has been proven to prevent muscle weakness, offering flexibility in the rehabilitation process for ACLR patients.^{58 59}

However, several limitations should be acknowledged. First, all included studies had a small sample size of <50 people, which may have affected the quality and accuracy of the results. Second, the random effects model was employed, revealing significant heterogeneity, likely stemming from variations in ES types, frequency and duration. Third, our study did not focus on the sustainable effects of ES. Finally, due to the heterogeneity in ES modalities and inconsistent reporting of stimulation parameters across studies, we focus on a pooled analysis to evaluate the overall adjunctive effect of ES. This approach assumes that diverse ES modalities share a common mechanism in enhancing rehabilitation outcomes. Subgroup analyses based on ES type were precluded due to insufficient studies per category and lack of standardised parameter reporting in primary trials.

Conclusion

Adjunctive ES has the potential to enhance early-phase ACLR rehabilitation outcomes, particularly by improving muscle strength, lower limb function and self-reported function. However, clinicians should consider the targeted application of ES for high-risk populations, such as athletes needing rapid strength recovery, while prioritising individualised rehabilitation programmes that comprehensively address functional deficits.

Data availability statement

Data extraction form and protocol available by request to 164156247@qq.com.