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The influence of electromyographic biofeedback therapy on knee extension following anterior cruciate ligament reconstruction: a randomized controlled trial
© Christanell et al.; licensee BioMed Central Ltd. 2012
Received: 26 July 2011
Accepted: 31 October 2012
Published: 6 November 2012
Loss of knee extension and a deficit in quadriceps strength are frequently found following anterior cruciate ligament (ACL) reconstruction. The aim of this study was to investigate whether the addition of Eletromyographic Biofeedback (EMG BFB) therapy for the vastus medialis muscle to the in the early phase of the standard rehabilitation programme could improve the range of knee extension and strength after ACL reconstruction more than a standard rehabilitation programme. The correlation between EMG measurement and passive knee extension was also investigated.
Sixteen patients, all of whom underwent endoscopic ACL reconstruction using patellar tendon autograft, were randomly assigned to two groups:
• Control group (8 patients): standard rehabilitation protocol; with full weight-bearing postoperative, knee brace (0° extension, 90° flexion), electrical stimulation, aquatics and proprioceptive training.
• The EMG BFB group (8 patients): EMG BFB was added to the standard rehabilitation protocol within the first postoperative week and during each session for the next 6 weeks.
Each patent attended a total of 16 outpatient physiotherapy sessions following surgery. High-Heel-Distance (HHD) Test, range of motion (ROM) and integrated EMG (iEMG) for vastus medialis were measured preoperatively, and at the 1, 2, 4 and 6-week follow ups. Additionally, knee function, swelling and pain were evaluated using standardized scoring scales.
At 6 weeks, passive knee extension (p < 0.002) and the HHD Test were significantly (p < 0.01) better in the EMG BFB group compared to controls. Integrated EMG (vastus medialis) of the EMG BFB group also showed a significant increase after 2 (p < 0.01) and 6 (p < 0.01) weeks. At the 6-week follow up, no significant (p > 0.01) differences were found between the two groups for the assessment of knee function, swelling and pain.
The results indicate that EMG BFB therapy, in the early phase of rehabilitation after ACL reconstruction, is useful in enhancing knee extension. Improved innervation of the vastus medialis can play a key role in the development of postoperative knee extension. EMG BFB therapy is a simple, inexpensive and valuable adjunct to conventional therapeutic modalities.
Over the last two decades, the human anterior cruciate ligament (ACL) has been widely studied. Investigation has focused mainly on mechanical and biological properties . This development has been in response to the continuous increase in ACL injuries, attributing to the increase in leisure activities, such as snowboarding or roller blading [2, 3].
Extensive literature research shows that after ACL reconstruction, knee extension range deficits [4–8], decrease in quadriceps muscle strength [9–14] and anterior knee pain, [15–18] frequently create difficulties for rehabilitation. The underlying mechanism for the loss of knee extension can be summarized in preoperative , intraoperative  and postoperative  deficits. A loss of knee extension often leads to an extended rehabilitation period and additional surgeries [4, 5]. The relationship between persistent loss of knee extension and deficits of activation for vastus medialis have been described . Changes in the sensorimotor system, caused by protection from the central neuronal system, are suspected to play an important role [11, 20–24]. Positive effects of Electromyographic Biofeedback (EMG BFB) training [25, 26] on knee extension deficit have already been published. However, there were a number of shortcomings to these studies: no preoperative measurements were performed to allow the improvement between the preoperative phase and the end of the postoperative rehabilitation programme to be identified. No short term effects following the first 6-week postoperative phase were presented. Furthermore, no direct comparison between EMG measurements and extension deficit has been made. And, finally, the use of a preventive, early implementation of EMG-assisted training after ACL reconstruction has not yet been studied.
The aim of this study was to investigate whether Electromyographic Biofeedback (EMG BFB) therapy for the vastus medialis muscle has an add-on effect to the standard rehabilitation programme in improving knee extension range and strength in the early phase of rehabilitation after ACL reconstruction. The correlation between EMG measurement and passive knee extension was also investigated.
Sixteen patients were recruited from the University Clinic of Innsbruck (Austria), of which 12 were male and 4 female. They had a mean age of 30 years (range: 20–49 years). All participants underwent endoscopic ACL reconstruction by the same surgeon and the same surgical protocol using patellar tendon autograft.
Between March and October, 2005, consecutive patients of the orthopaedic surgeon conducting the operations were included in the study. The inclusion criteria included free range of motion, pain free and without inflammation, isolated ACL rupture and cartilage damage < level 2.
The patients were enrolled for therapy by telephone through the secretary of the surgeon. The recruited patients were chronologically randomized into one of two treatment groups; with EMG BFB (n = 8), or, without EMG BFB (n = 8). The treating therapist had no knowledge of the group to which a patient was allocated, and the patients were previously unknown to him. After randomisation the therapist knew who gets BFB-therapy and who not.
Division key of sessions: 16 Physiotherapy (PT), 8 Electrostimulation (ES), 8 Manual Lymphatic drainage (MLD), 8 Underwater Hydrotherapy (UWT)
3 × PT
3 × PT
3 × PT
3 × PT
2 × PT
2 × PT
3 × ES
3 × ES
2 × ES
3 × MLD
3 × MLD
2 × MLD
2 × UWT
2 × UWT
2 × UWT
2 × UWT
High-Heel-Distance (HHD) Test, range of motion (ROM) and integrated EMG (iEMG) for vastus medialis were measured 1 week before surgery and at the 1,2,4 and 6-week post-operative follow-ups. Integrated Electromyography was used to measure muscle activation across surface electrodes. It is the common method used in scientific studies for muscle activation measurement . Additionally, knee function, swelling and pain were evaluated using standardized scoring scales. A standardised questionnaire was utilized to evaluate socio-demographic parameters, sport activity and irritability, according to the guidelines of the International Knee Documentation Committee (IKDC) [24, 28]. Ethical approval was given by the hospital ethical board and all patients signed a written informed consent form before entering the study.
Measuring the knee extension and tension of vastus medialis
For a better test-retest reliability, the positions of the electrodes were calculated as the following: 20% of the distance between the medial knee joint gap and ASIS (anterior superior iliac spine) . In addition to this imagined line, the electrode position was turned 45° laterally and stuck above the vastus medialis at a distance of 20 mm. The reference electrode was positioned on the tibial tuberosity. Adhesive tape was used to attach the cable onto the patient’s skin. The integrated EMG measurement (Soft- and Hardware by INSIGHT INSTRUMENTS – Austria) was carried out as follows: the baseline test and later tests followed the same procedure: the patient was required to isometrically extend their knee to maximum extension for 4 seconds, then rest for 10 seconds, the procedures were repeated for 3 times. During the 4 seconds of maximum isometric knee extension, mean values of EMG raw signal (wide pass filter 25–1000 Hz) were measured. The best mean value (Microvolt) of the three trials was taken for statistical calculations.
Within 1 week after preoperative testing, all 16 patients obtained an ACL reconstruction operation by an experienced surgeon from the Medical University of Innsbruck (Austria). All patients wore thrombosis stockings and a knee brace (0° extension, 90° flexion) following 6 weeks after operation. Ambulant physiotherapy started after the first postoperative week with the first retest. The same tests were performed after 2, 4 and 6 weeks.
The rehabilitation protocol
The rehabilitation protocol following surgery comprised a total of 16 outpatient physiotherapy sessions of 40 minutes, 8 electrical stimulations (ES) of 20 minutes, 8 manual lymphatic drainages (MLD) of 30 minutes and 8 aquatic underwater therapies (UWT) of 30 minutes, supervised by the same therapist. Table 1 shows all sessions divided between the first 6 weeks after ACL reconstruction.
All data were summarised with standard descriptive statistics (mean value ± SD). Differences between the groups were calculated using Student’s t-test for parametric and Mann–Whitney-U-test for nonparametric variables. Correlations were calculated using Pearson’s coefficient of correlation test. For calculating interaction effects between the two groups, a two-way repeated measures ANOVA was used including Bonferroni adjustment; with reference to the Bonferroni adjustment, p-value less than 0.01 was considered significant. All p-values were two-tailed. The statistical analysis was performed with version 12.0 of SPSS for Windows; the graphics were created with Microsoft EXCEL (version XP 2002 for Windows).
All patients attended all the therapy sessions. There were no drop-outs or adverse events. The amount of therapy was identical for both groups, except that the EMG BFB group combined manual therapy and exercises with acoustic and visual Biofeedback for vastus medialis in the first week.
Comparison of social demographic parameters (mean value ± standard deviation) between the two groups; *Denotes a significant difference at p < 0.05
EMG BFB group
BMI (kg/m 2 )
Number of therapy sessions (mean value ± SD) during 6 weeks of rehabilitation; PT = physiotherapy, ES = electrical stimulation, MLD = lymphatic drainage, UWT = aquatics)
EMG BFB group
16 (± 0)
8 (± 0)
9 (± 1)
7.5 (± 0.9)
16 (± 0)
8 (± 0)
8.5 (± 0.9)
7.4 (± 0.9)
Range of motion (ROM)
The results of ROM reported on mean values with SD of passive motions in the sagittal plane. Passive flexion and extension (with goniometer) and HHD Test were measured 1 week before surgery and at the 1,2,4 and 6-week post-operative follow-ups comparing the involved and non-involved leg.
The passive knee flexion deficit of both groups was preoperatively similar (mean 8.1° ±8.7°). Flexion deficit of 68% of all patients was preoperative 0-5°. Both the EMG BFB group (<40°) and Control group (<40°) showed their largest deficits after 1 week. After 6 weeks the BFB group (20°) had a reduced, but not significantly different, flexion deficit compared to the Control group (26.3°).
Subjective functional evaluation
The results of standard functional scoring scales (IKDC guidelines; [20, 31] showed no significant differences between the groups during the first 6 weeks of rehabilitation. Both groups started preoperatively between “nearly normally” and “abnormally” knee joint function. After 1 week postoperative, both groups showed the strongest restriction (EMG BFB group 3.3 ± 0.7 / Control group 3.3 ± 0.9) and both reached a “nearly normally” knee function after 6 weeks (EMG BFB group 2.0 ± 0 / Control group 2.1 ± 0.6). The parameter “Activity of daily life” showed similar results. After 6 weeks of rehabilitation, both groups reported between “no” and “a bit” of ADL restriction. It seems that EMG BFB therapy had no influence on “joint function” (p = 0.72) and “ADL” (p = 0.96).
Mean values and standard deviation of the visual analog scale (VAS 0–10) and knee swelling (categories 1 = none, – 4 = severe) also showed no significant differences between the two groups after 6 weeks rehabilitation. All patients reported their main pain under the patella, in the knee hollow, or in the wound area where the graft was taken. After 6 weeks the pain value of both groups was nearly “1”. The main knee swelling was located above the patella and at the medial joint gap. The strongest swelling was after 1 week (EMG BFB group 3.3 ± 0.5 / Control group 3.3 ± 0.9).
Integrated EMG (vastus medialis)
The deficit in knee extension (measured with goniometer) (Figure 5) showed a significant positive impact from BFB therapy on passive knee extension after 6 weeks of rehabilitation (p < 0.002). This result was corroborated by the HHD Test (Figure 6), which also reported significant differences after 6 weeks (p < 0.01), thus confirming the positive effect of BFB therapy on knee extension. Due to the significant reduction in the loss of knee extension, the concerns associate with premature muscle-strengthening training  and increased activities of ASMA (actin isoform alpha-smooth muscle actin) in the wound area  through the use of BFB therapy, can be rejected. However, our experience gained during therapy confirmed that wound healing periods, especially the first 3 postoperative weeks (phase of proliferation), should be respected. A high correlation between the HHD Test and knee extension (measured with goniometer) after 6 weeks (0.83 = p < 0.01) indicates that these tests are important and efficient instruments for use in the evaluation of ACL rehabilitation. EMG BFB therapy could help patients be more aware of correct muscle activity. The performance assessment may be an important source of motivation for patients. The results indicate that the application of EMG BFB therapy, in the early phase of rehabilitation, is useful in enhancing knee extension after ACL reconstruction.
Questions on the functional evaluation (joint function, ADL and “Giving way”) of the involved knee were asked 1 week before surgery and at periods of 1, 2, 4 and 6 weeks after reconstruction. The division of such parameters were based on IKDC categories, which are commonly used for the evaluation of ACL rehabilitation [14, 24]. The results indicated that BFB therapy had no significant effects on joint function (p = 0.721) and ADLs (p = 0.959) compared to controls after 6 weeks rehabilitation. Our study does not show whether these findings could be different after a longer follow up period. The outcomes indicated that these parameters are not useful in evaluating knee function during the early phase of rehabilitation. Using a longer follow up of 12 months, Mihaela  showed a similar improvement in her study by using the “Knee Injury and Osteoarthritis Outcome Score” (KOOS).
BFB therapy showed positive effects on “Symptomatic giving way” (Figure 7) after 1 and 2 weeks (p = 0.028). This effect could be explained by the reduction of neuromuscular quadriceps deficit with BFB therapy, described by Krogsgaard & Solomonow  and Kanemura . The application of BFB therapy could have positive influences on reduced afferent inputs (ligament, muscle, tendon, skin) as a protective function created by the central neural system. In addition, the results raised the question about the time period for using a brace. After 4 weeks, both groups showed almost no “Symptomatic giving way”. At this moment, most patients described wearing a brace as a restriction to the development of physiologic gait. This finding leads to a consideration of whether the recommended time period for wearing a brace should be less than 6 weeks.
Symptoms (knee pain – VAS; knee swelling) of the involved knee were measured 1 week before surgery and at the 1,2,4 and 6-week post-operative follow-ups. Neither the results of knee pain, nor of knee swelling, showed a positive effect from EMG BFB therapy after 6 weeks of rehabilitation, although many authors [34–36] confirm the relationship between knee pain, quadriceps inhibition and loss of knee extension. Concerning symptoms, it remains unclear whether BFB therapy could have an influence on later rehabilitation development. Muller et al.  investigated the wound area of Ligamentum patellae after 4.3 years (where the autograft was taken) and discovered that there was some coherence between knee pain and sub-optimal rehabilitation (shortening of Ligamentum patellae, increased quadriceps inhibition). The results of flexion deficit are of interest. The BFB therapy showed a positive, but not significant, influence on flexion deficit after both 2 (p = 0.05) and 6 (p = 0.05) weeks postoperatively.
This positive effect of BFB therapy, without additional knee flexion techniques, leads to the conclusion that therapy content for improving flexion could place more emphasis on reinforced strength and/or proprioceptive sessions during BFB therapy.
The literature has shown many EMG measurements, which were used as diagnostic [36, 38–41] and/or therapy methods [25, 31, 42, 43]. Selection of the vastus medialis muscle was based on Jarvela , who reported that the inferior fibres of vastus medialis are the most important for dynamic knee stability and the reduced load of passive knee structures. Because of different inter-individual EMG values (μV), to gain a better comparison the preoperative values were calculated relatively as 100%. The subsequent measurements at 1, 2, 4 and 6 weeks after ACL reconstruction were compared with this preoperative starting level.
The results of Figure 8 showed the biggest μV reduction 1 week postoperative (BFB Group 16.7%; Control group 9.7%). These findings are similar to other results, which also describe a protection mechanism by the central neural system in the form of quadriceps inhibition after ACL reconstruction [8, 11, 20, 21] . Following the first postoperative week, the BFB Group improved their EMG values compared to the Control group, so that after 6 weeks the BFB Group (124.9% ± 52%) raised their preoperative level by +24.9%, while the Control group (70.3% ± 45.8%) deteriorated by −29.7%.
These results indicated the positive influence of BFB therapy on vastus medialis activation after both 2 (p < 0.01) and 6 weeks (p < 0.01). Recent studies have reported a long term effect of this training [5, 45]. In addition, significant correlation between EMG and HHD Test (−0.602 = p < 0.01), and EMG and passive knee extension (−0.712 = p < 0.002), confirmed the relationship between vastus medialis innervation and knee extension. It is still unknown, whether there is a relationship between increased EMG values and improved quadriceps strength. Onishi , for example, found a correlation coefficient of 0.87 – 0.94 between EMG and strength.
Although it should be clear that EMG values are dependent on compliance and daily constitution of patients, and of innervation zones under attached electrodes., The fact is, that many authors [5, 12, 40, 44, 45, 47] have described the persistent weakness of quadriceps as one of the biggest problems of ACL rehabilitation. In addition, they are convinced that reduced quadriceps strength can be traced back to sub-optimal rehabilitation, due to a persistent spinal or supraspinal reflex inhibition. It may be that improved activation of the vastus medialis through EMG BFB therapy could lead to a subsequent support for improved quadriceps strength. It remains unknown whether EMG BFB therapy in the early phase of rehabilitation could have a positive influence on subsequent increased knee-loading, on return to sporting activities. Recent studies conclude that an early functional EMG BFB therapy can have a positive effect for a better return to sport activities [5, 45].
The sample size of 16 subjects was small. The patients and the assessor were not blinded. The treating therapist also performed the measurements and may have introduced bias. Future studies should include larger samples, blinded assessors and longer follow up times.
These results indicate that EMG BFB therapy, in the early phase of rehabilitation, is useful in enhancing knee extension after ACL reconstruction. Improved innervation of the vastus medialis seems to play a key role in the development of postoperative knee extension. EMG BFB is a simple, inexpensive and valuable adjunct to conventional therapeutic modalities.
The study was supported by the South Tyrolean institute for young researchers; with a grant from the Austrian Trauma Insurance Association (AUVA) and an educational grant from the Innsbruck School of Physiotherapy (AZW).
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