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Effects of Electromyographic Biofeedback on Quadriceps Strength: A Systematic Review

Lepley, Adam S; Gribble, Phillip A; Pietrosimone, Brian G

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Journal of Strength and Conditioning Research: March 2012 - Volume 26 - Issue 3 - p 873-882
doi: 10.1519/JSC.0b013e318225ff75
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Quadriceps strength is a vital component of lower extremity function. Quadriceps dysfunction has been associated with altered gait (34) and landing kinematics (30). Optimizing quadriceps force generation capabilities is often the focus of both therapeutic exercise interventions after knee joint injury and programs aimed at increasing athletic performance in healthy individuals (5,24). A variety of different modalities have been used in conjunction with exercise in an effort to maximize the outcomes of quadriceps strengthening interventions. Electromyographic biofeedback (EMGBF) is a modality that intends to specifically target decreasing cortical mechanisms associated with generating strength (3,9,33). The EMGBF has been hypothesized to increase muscular strength and neuromuscular control of the quadriceps muscle group (9,11,14,26); therefore, it may be valuable for clinicians to understand the true effect EMGBF has on quadriceps muscle strength to effectively target quadriceps weakness.

The EMGBF uses surface electrodes to measure underlying muscular activity. The EMG signals can be converted to either an auditory (9) or visual cue (36), revealing to the individual a representation of his or her muscular contraction, affording individuals the opportunity to quantify a physiological event (36). These cues allow for alterations in neuromuscular control, permitting a patient to reach a desired goal of muscular contraction. It has been hypothesized that EMGBF can potentially affect strength by improving motor unit recruitment (9,25,26) as well as optimizing firing rates through cortically generated mechanisms (7,25).

The EMGBF has been reported to positively affect quadriceps strength (7,9,21,26,29) and activation and timing of different muscles (9,10). However, there is confounding evidence on whether EMGBF training can increase quadriceps strength better than exercise alone can. Because appropriate neuromuscular control is an essential component to protecting the knee from potentially hazardous positions and traditional strength training and rehabilitation have not reported sustained improvements in strength (15), it is important to identify interventions that can help restore and magnify quadriceps strength gains. A novel rehabilitation approach that specifically targets the neuromuscular system may produce superior gains in quadriceps muscle function and strength. Currently, there are only a few modalities that are capable of successfully targeting the neural system, yet EMGBF has been touted to be able to affect cortical levels of the neuromuscular system (3,9,33). Unfortunately, the effectiveness of EMGBF has not been systematically evaluated throughout the literature, and at this point, the clinical usefulness of EMGBF is unknown.

Therefore, the purpose of this investigation was to systematically review the current literature and determine the magnitude of the treatment effect for EMGBF on quadriceps strength compared with that of placebo and traditional exercise-only interventions in both healthy and pathological populations. This review could potentially highlight the benefits, or lack thereof, of using EMGBF in a clinical setting. It is important to evaluate the concentration of data on EMGBF before moving forward in further research and continued application of this clinical modality.


Experimental Approach to the Problem

An online search using Web of Science and ProQuest was performed to obtain peer reviewed articles published between 1975 and August 25, 2010. The search strategy consisted of the terms biofeedback OR electromyographic biofeedback OR EMG biofeedback AND quadriceps OR quadriceps strength OR quadriceps training OR quadriceps electromyography OR quadriceps EMG OR quadriceps maximum voluntary isometric contraction OR quadriceps rehabilitation. References from pertinent articles were crossreferenced to locate any further relevant articles not found with the initial search.


There were no subjects used for this article. For a complete description of articles included in this review, please refer to Table 1.

Table 1
Table 1:
Characteristics and methodological assessment of included articles.*

Inclusion Criteria

To be included, a study needed to meet the following criteria: (a) Design: Only randomized controlled trials were included. (b) Interventions: The EMGBF and a comparative exercise-only and/or placebo intervention used to increase quadriceps strength were included. (c) Study population: Participants could be healthy or suffer from an acute or chronic knee joint pathology. (d) Outcome measure: The study needed to investigate isometric quadriceps strength in response to the previously stated interventions. (e) Language: The search was limited to original research that was written in English.

Methodological Quality Assessment

All included articles were evaluated using the Physiotherapy Evidence Database (PEDro) scale and the Oxford Centre for Evidence Based Medicine (CEBM) levels of evidence. Two raters (A.S.L. and B.G.P.) independently assessed the quality of the studies and met to resolve discrepancies. The PEDro scale is a widely used 10-point assessment tool, used to critically assess methodological issues within articles, whereas the CEBM categorizes these studies into different levels of evidence.

Statistical Analyses

Preintervention and postintervention means and SDs were collected from all the interventions used in each study (EMGBF, placebo, traditional exercise–only groups). Standardized effect sizes (d = [postintervention − preintervention]/pooled SDs) and 95% confidence intervals (95% CI) were calculated. One included article, Durmus et al. (13) reported the measurements for right and left legs of the participants; therefore, means and SDs for each limb were pooled to calculate 1 pooled effect size. These standardized effect sizes allowed us to compare the results across the 6 different studies. Based on the articles included, data were separated by type of intervention (EMGBF, exercise-only, placebo), length of intervention (<4 and > 4 weeks), and population (healthy, pathological). Effect sizes were classified as weak (d ≤ 0.2), small (d = 0.2–0.5), moderate (d = 0.5–0.8), and strong (d ≥ 0.8) (8).


Literature Search

The original electronic database search and crossreferencing yielded 3,305 initial studies. There were 3,285 articles excluded based on title or abstract. The remaining 20 articles were retrieved and examined for inclusion and exclusion criteria. Fourteen additional articles were excluded on the basis of reported data (11,12,22,25), measurements of instantaneous EMGBF (2,7,29), or outcome measures not of interest in this particular study (10,14,20,21,23,28,31) (Figure 1). A total of 6 articles (6,9,13,26,36,37) were included in this review (Table 1).

Figure 1
Figure 1:
Flow chart of evidence search.

Five (9,13,26,36,37) of the 6 included studies reported measures from interventions lasting between 19 days and 4 weeks. Two (6,37) of the 6 studies investigated interventions lasting >4 weeks (6–14 weeks). Also, 4 (6,13,36,37) of the 6 studies included individuals with a knee joint pathology, which included patellofemoral pain syndrome (PFPS) (37), osteoarthritis (OA) (13,36), and anterior knee pain (AKP) (6). The remaining 2 (9,26) articles looked at interventions on healthy subjects. These 2 articles also compared placebo interventions. A total of 17 data sets were extrapolated from the 6 included articles. Effect sizes and 95% CI of these 17 data sets are illustrated in Figures 2–4.

Figure 2
Figure 2:
Forest plot representing the effects of electromyographic biofeedback (EMGBF), exercise-only, and placebo interventions lasting <4 weeks.

Methodological Quality

Each article received an Oxford CEBM score of 1b. All of the studies included were classified as level 1, indicating their use of a randomized controlled trial design. The average PEDro score of the articles included was 6.0 ± 1.03, representing a good quality of research (Table 1).

Interventions Lasting from 19 Days to 4 Weeks

Five studies (9,13,26,36,37) were combined for 11 data sets that examined EMGBF, exercise-only, and placebo interventions lasting between 19 days and 4 weeks (Figure 2). The EMGBF intervention group showed a homogeneous, positive effect. There were 2 definitively strong effects (d = 1.61, 5.56) with 95% CI that did not cross zero. The remaining 3 data sets ranged from weak to strong (d = 0.01–0.86), all with 95% CI that crossed zero, meaning it was no more effective than the reference interventions were (Table 2).

Table 2
Table 2:
Type of interventions assessed in the included articles.

The exercise-only group showed a heterogeneous effect, with effect sizes ranging from a weak negative effect to a strong positive effect (d = −0.12 to 1.18) with all 95% CI crossing zero. The placebo group also demonstrated a heterogeneous effect (d = −0.02, 1.38). One of these effects (d = 1.38) revealed a conclusive 95% CI that did not cross zero.

Interventions Lasting >4 Weeks (6–14 Weeks)

Two (6,37) studies provided a total of 6 data sets that looked at EMGBF and exercise-only interventions lasting >4 weeks (Figure 3). The EMGBF intervention group showed a homogeneous, positive effect that ranged from small to strong (d = 0.28, 0.36, 1.26). Both 95% CI of the small effect sizes crossed zero. The exercise-only groups also showed a homogeneous effect (d = 0.43, 1.00, 1.07) with only one 95% CI that crossed zero. There were no placebo interventions lasting >4 weeks.

Figure 3
Figure 3:
Forest plot representing the effects of electromyographic biofeedback (EMGBF) and exercise-only interventions lasting >4 weeks.

Pathological Vs. Healthy Populations

There were a total of 11 data sets from 4 articles (6,13,36,37) that investigated the effects of EMGBF vs. exercise-only on pathological populations (Figure 4). The EMGBF demonstrated a homogeneous, positive effect in pathological populations, with all 6 effect sizes being greater than zero. However, the effect sizes ranged from weak to strong (d = 0.01–5.56) with two 95% CI that did not cross zero (6,13).

Figure 4
Figure 4:
Forest plot representing the effects of electromyographic biofeedback (EMGBF), exercise-only, and placebo interventions on healthy vs. pathological populations.

There were 5 data sets that reported the outcomes of exercise interventions in pathological populations. Exercise-only also showed a homogeneous, positive effect in pathological patients, with small to strong effect sizes (d = 0.17–1.07). Similar to EMGBF, only two 95% CI did not cross zero.

There were 2 articles (9,26) that compared the effect of EMGBF, exercise-only, and placebo interventions on healthy individuals. The EMGBF was the only intervention that demonstrated a homogeneous, positive effect on healthy individuals. The EMGBF intervention showed strong effects (d = 0.86, 1.61); however, one of these effect sizes had a corresponding 95% CI that crossed zero. Exercise-only in healthy individuals had a heterogeneous effect (d = −0.12, 1.18) with both 95% CI crossing zero. The placebo group in healthy populations was similar to exercise-only, which resulted in a heterogeneous effect (d = −0.02, 1.38) but with only one 95% CI that crossed zero.


As a group, the effects were the strongest for the EMGBF intervention compared with the effect sizes for the exercise-only and placebo interventions. However, definitive evidence that EMGBF is beneficial for increasing quadriceps strength cannot be concluded based on wide 95% CI and the wide range (weak to strong) of effect sizes. Effect sizes and 95% CI of all interventions overlap, revealing that definitive effects for increased quadriceps strength between EMGBF and exercise only or placebo interventions may not exist.

The data sets obtainable in each article allowed the outcomes to be broken up into interventions lasting from 19 days to 4 weeks and interventions lasting >4 weeks (ranging from 6 to 14 weeks). Neural adaptations contribute to initial strength gains in muscle tissue, with this early phase of strength acquisition lasting from 2 to 6 weeks (16,27). Beyond this time frame, strength gains become increasingly dependent on muscle hypertrophy (27). The EMGBF is hypothesized to affect initial strength gains by improving motor unit recruitment (9,25,26) and/or optimizing firing rates through cortically generated mechanisms (7,25). Therefore, if a difference were to be seen, it would be reasonable to expect EMGBF to have greater effects in the early stages of strength training contributing to these neural adaptations.

The EMGBF group demonstrated a greater number of strong effect sizes (4 out of 5 data sets) in the 19-day to 4-week intervention as compared with that of the exercise-only group (1 out of 4). During this same time frame, the EMGBF group showed a homogeneous sample of positive effect sizes, with two 95% CI not crossing zero. In contrast, the exercise-only group not only demonstrated a heterogeneous effect but it also showed that every 95% CI crossed zero. No distinct differences were found during interventions lasting >4 weeks. The homogeneous collection of weak to strong effect sizes demonstrated by the EMGBF group illustrates a possible benefit for quadriceps strength. However, the effectiveness of EMGBF on strength acquisition is inconclusive, regardless of the length of the intervention.

Another objective of this study was to determine the effect of EMGBF on strength acquisition in those with knee joint pathology. This systematic review compared a variety of pathologies including PFPS, OA, and AKP. There have been numerous articles documenting strength deficits that are present after knee joint injury (17-19,32), prompting clinicians to investigate new tools that may help to increase strength. In our current review, there were no definitive differences in the effect that EMGBF and exercise-only interventions had on strength gains among pathological populations. Both the EMGBF and exercise-only groups exhibited homogeneous, positive effects with similar effect sizes and 95% CI width.

Additionally, there were no apparent differences between EMGBF interventions on quadriceps strength in healthy vs. pathological populations. These findings are interesting, considering that EMGBF is often used clinically with pathological populations. However, there is an abundance of research supporting the existence of arthrogenic muscle inhibition after knee joint injury (17-19,30,34). Arthrogenic muscle inhibition is defined as an ongoing, reflexive response after an injury is sustained, which results in an inability to completely contract a muscle despite there being no structural damage to that muscle or supplying nerve (17). This evidence suggests that there is a dysfunction among the reflexive capabilities of the muscle; therefore, any interventions intended to exploit cortically aimed signals to increase strength, such as EMGBF, may be blocked by this inhibition. Subsequently, the muscle may need to be disinhibited before the use of EMGBF, or other cortically aimed interventions would be beneficial on quadriceps strength. Also of note, and possibly related, is that in the 6 studies examined, the 2 (9,26) that investigated healthy populations concluded that EMGBF interventions increased strength significantly from the exercise-only group. The 4 (6,13,36,37) studies that examined pathological populations concluded that the EMGBF group increased strength significantly; however, this difference was not statistically significant when compared with that of the exercise-only groups.

It was interesting to observe that one of the placebo groups demonstrated a strong effect size with a 95% CI that did not cross zero (d = 1.38, CI = 0.13–2.43). Although the Croce et al. (9) placebo group demonstrated increases in peak torque, the EMGBF group produced a significantly greater torque than did both placebo and exercise-only interventions. This specific placebo group underwent a detuned, sham ultrasound session before exercise, as opposed to that in Lucca and Recchiuti (26) (d = −0.02), who used inactive EMGBF electrodes. Previous research exists that promotes the effects of massage, or even light muscle stroking, on neuromuscular excitability and muscle function (1,4,35). Placebo ultrasound treatment has even been shown to have a beneficial effect on delayed onset muscle soreness (4) and increases in hand grip strength with treatments over the forearm muscles (1). The additive effect of muscle stroking with the ultrasound head may have played a role in the strong effect size associated with this particular placebo group (1,4,35).

Although no irrefutable differences were found between EMGBF and exercise-only, there are other signs of promise for EMGBF interventions. Durmus et al. (13) presented EMGBF effect sizes that were significantly higher compared with that of the other interventions evaluated (d = 5.56, CI = 4.26–6.68), meaning that lower limits of the EMGBF 95% CI did not intersect the upper limits of 95% CI for the placebo or exercise-only interventions. There was a methodological difference noted between this specific study and the other 5 included in this review. The subjects were all female patients between the ages of 42 and 74 years, diagnosed with OA. Also, as stated in the Methods section of this current review, data were reported for both the right and left limbs individually. These reported values were pooled together for a total combined effect. Although never directly stated, this particular study infers that the subjects received EMGBF bilaterally during intervention. Also, this study never indicates which limb is reported as dominant nor injured. If EMGBF was applied bilaterally, it is interesting that it produced a significantly greater effect size and 95% CI than in the rest of the literature. Nonetheless, these results cannot be generalized to larger populations and is more relevant to 42- to 74-year-old women diagnosed with osteoarthritis. Future research studies should investigate the effect of bilateral EMGBF training and the possible crossover effect it may have at the cortical level.

Inconclusiveness in the data represent a potential need for future research on EMGBF and its effects on strength and other measures of neuromuscular function. To understand the clinical application of EMGBF, we must first determine how it impacts specific neural pathways. Although hypothesized to increase motor unit recruitment and optimize firing rate through cortically aimed mechanisms (7,9,25,26), researchers do not fully understand the role cortical level pathways have on neuromuscular dysfunction and strength deficits. Future research needs to establish the effect cortical level pathways has on strength and neuromuscular function before it can assess interventions to specifically target these mechanisms. This knowledge will not only lead to more effective applications for EMGBF but also lead to new, innovative techniques in targeting cortical level pathways to restore proper neuromuscular function.

In addition to understanding cortical level pathways on neuromuscular function, future research should look to establishing the effect EMGBF has on different populations. The limited data presented in this review suggest that EMGBF may have a greater effect on patients with OA (13); however, it is important for further randomized control trials to determine exactly what type of populations would have a greater benefit from the application of this treatment.

One must also take into consideration that the purpose of this review was to determine the effect of EMGBF on measures of quadriceps strength. Strength is by no means a complete indicator of neuromuscular control and physical function. Future investigations should explore the effect of EMGBF on other measures of physical function, such as joint kinematics, muscle activation, dynamic stability, and self-reported outcomes of function.

The findings of this research have the potential to impact both clinicians and researchers. Investigators examining neuromuscular control, human performance, injury rehabilitation, etc., can use these conclusions to advance the current evidence on muscle function. An avenue is also opened for the literature to establish other interventions that are more efficient at increasing quadriceps strength, in addition to furthering research on EMGBF. Currently, neuromuscular deficits that are present and persistent after knee joint injury have not been able to be restored with traditional exercise, possibly leading to early onset and progression of knee joint osteoarthritis (17-19). This investigation adds to the existing gap in the literature, potentially leading to innovative techniques aimed at restoring proper neuromuscular function.

Practical Applications

Using a systematic review technique, it is unclear as to whether EMGBF has a greater effect on increasing quadriceps muscle strength in either healthy or pathological individuals. Although this review of the current literature cannot definitively support its use, harmful effect sizes for EMGBF were not reported. This suggests that further examination of EMGBF should be conducted to determine its true effectiveness. The current review does not produce robust support for the use of EMGBF on increasing strength and also does not provide evidence to deter its use. Even though EMGBF did not have significant gains from traditional strength training alone, it did show a homogeneous positive effect on strength from baseline measurements. The limited evidence examined in this review suggests that EMGBF may have a stronger effect on healthy populations (9,26) and those with OA (13); however, confirmation of this finding is warranted through future research. Pending further investigation, EMGBF could be used as a strengthening intervention to supplement and add variety to traditional strength training or rehabilitation programs. However, if EMGBF is to be used for increasing quadriceps strength, it should be used with clinical judgment and open consideration.


There was no external financial support for this project. No competing interests are noted.


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knee joint musculature rehabilitation; knee injury; knee joint osteoarthritis; muscle strength; physiological feedback

© 2012 National Strength and Conditioning Association