Training Program and Additional Electric Muscle Stimulation for Patellofemoral Pain Syndrome: A Pilot Study

Participants 

Between June 2003 and August 2005, 64 participants were referred by outside orthopedic surgeons, and 40 fulfilled the eligibility criteria. Inclusion criteria were bilateral anterior knee pain for 6 to 120 months and at least 3 of the 4 following clinical criteria: pain associated with prolonged sitting with bended knees, descending stairs, kneeling and squatting, or sports activities.

Exclusion criteria were clinical evidence of patellar dislocation or subluxation, periarticular bursitis or tendonitis, ligamentous instability, or intra-articular pathology. Before beginning therapy, all patients were thoroughly clinically examined. Those who did not reveal any obvious reason for a systemic disorder like patellar or lower-extremity alignment problems or benign joint hypermobility syndrome were not excluded. To rule out osteoarthritic changes or hypoplastic femoral trochlea, radiographs were performed.

Additional exclusion criteria were: pregnancy, a history of knee surgery, or oral or intra-articular administration of drugs within the last 3 months. Two of the recruited participants withdrew for personal reasons. Thirty-eight subjects (14 men, 24 women) diagnosed with bilateral PFPS by 2 independent physiatrists were randomly assigned to group PT training or group PT training and EMS (table 1). Random allocation of the patients to the 2 treatment groups was performed by using shuffled sealed envelopes.

Table 1. Demographic Data of Patients

		Group
	Demographics	PT	PT and EMS
	Age (y)	23.7±5.5	27.0±7.7
	Height (cm)	168.7±4.9	174.5±9.3
	Weight (kg)	59.4±5.7	68.8±13.7
	Sex (men/women)	5/14	9/10
	Duration of pain complaints (mo)⁎	16(6–24)	12(6–24)

NOTE: Values are mean ± SD.

⁎ Values are median (quartiles).

The study was approved by the local ethics committee. All subjects provided written informed consent.

Outcome Measures 

The assessments were performed before treatment with follow-ups after 3 months of therapy and after 1 year.

Training Protocol 

Training was instructed and performed as described by Thomeé.11 The 12-week training program consisted of isometric, concentric, and excentric leg raises and pulls as well as stepping and squatting exercises. Balance exercises started from week 4 onward and consisted of standing on 1 leg for 2 minutes each. To increase the exercise demand, patients were instructed to draw circles in the air with the free contralateral leg from week 6 onward. From week 8 onward, patients had to do the 1-legged balance exercises in a toe-raised position with drawing circles with the contralateral leg in weeks 11 and 12.

Static stretching exercises of the calf and thigh muscles consisted of 3 sets of 10-second passive sustained stretching for each muscle group that were performed by the patients themselves at the end of each training session from weeks 4 to 12.

Appendix 1 provides a detailed description of the training program. Patients were instructed for daily training during the first 2 weeks and had a group session once a week under the supervision of the same physical therapist. From the third week on, they were instructed to train with higher loads 3 times a week for a total of 12 weeks. All subjects received an exercise instruction booklet with detailed instructions for every training session (type of exercise, number of repetitions and sets) and were instructed to report about their pain during every training session. Training protocols were controlled once a week for compliance. The information was used for further progression of exercises. A pain level of up to 2 on the VAS during the exercises was accepted for further increasing the loads. In case of reported pain levels of 3 or 4, the loads were kept constant; in case of a reported pain level above 5, during the exercise the loads were reduced accordingly.

EMS Protocol 

For EMS of the quadriceps, a commercially available portable battery-operated stimulation device (N607 E.M.S.)a was used. The EMS device is a 2-channel stimulator, which produces asymmetric biphasic pulses for a duration of .26ms. Maximal amplitude is 80mA, with maximal output of 50V at 500Ω. The stimulation frequency was set at 40Hz, with a duty cycle of 5 seconds on and 10 seconds off. The patients randomized to the EMS group were carefully instructed in a home-based self-stimulation program by a physician. Four self-adhesive electrodes were placed respectively on both ends of the quadriceps muscles (50×130mm; total area, 130cm2 on each thigh). The daily stimulation protocol consisted of two 20-minute sessions with a minimum of 60-minute rests between each session. The protocol was followed for 12 weeks. The intensity of the stimulation was kept as high as possible; however, pain tolerance and patient discomfort were modifying factors. Patients were given instruction protocols of electrode placement, and they were asked to record each stimulation session to ensure compliance.

Statistical Analysis 

Pain was described by the largest value of the 2 VAS evaluations, VAS 1 and VAS 2. The KPS was used to assess knee pain and function. Differences between values before treatment and after 3 months of therapy were considered as main outcome parameters measuring the treatment effect.

Mean values ± SDs are given in case of normally distributed variables. Median (quartile) values are mentioned, describing variables with skewed distributions.

The Spearman correlation coefficient was used to describe the correlation between pain, function, and strength before and after training. The paired t test was used to test for differences between pre- and post-treatment values. To calculate the different effects of the 2 therapies (with vs without EMS) and differences between men and women, the unpaired t test was used. To evaluate the influence of the potential prognostic factors, the duration of pain complaints (log-transformed), and sports activities (measured on an ordinal scale with codes 0=no sports, 1=1h sports a week, 2=2–3h sports a week, 3=>3h sports a week) on the treatment effect, univariate and multiple linear regression models were estimated. Two-sided P values smaller than .05 were considered statistically significant.

Pain and Function 

In the supervised PT group, we started with 19 patients. One patient did not return for the 3-month control for personal reasons. Eighteen patients were analyzed after 3 months of training. In the PT group, 13 patients completed the 1-year follow-up. Of the 5 patients missing, 1 was diagnosed with rheumatoid arthritis, 1 had to undergo knee surgery because of his PFPS, and 3 patients did not return for the 1-year follow-up investigations.

Nineteen patients started in the PT and EMS group, receiving PT and additional EMS. One patient withdrew. The remaining 18 patients were analyzed after 3 months of training and EMS. Sixteen subjects of this group finished the study and participated and completed the 1-year follow-up. One did not want to return for the 1-year follow-up, and 1 could not be contacted (fig 1).

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Fig 1. Progress of patients through the study.

The duration of pain complaints before participation in the study was comparable in the 2 treatment groups: median (quartile) values (in months): 16 (6−24) in PT group and 12 (6−24) in PT and EMS group.

There was a statistically significant reduction of pain in both groups from pretraining values to the 3-month measurements. The mean decrease ± SD in the VAS evaluation (maximum value of VAS 1 and VAS 2) was −2.84±3.50 (P=.003) in the PT group and −3.39±3.43 (P<.001) in the PT and EMS group. At the 1-year follow-up, the reduction of pain remained constant. Values are given in table 2.

Table 2. Maximum Values of VAS 1 and VAS 2 in Both Groups

	Maximum VAS	PT	PT and EMS
	Before treatment	5.3(2.9–7.6)	5.6(3.5–8.3)
	After 3mo of treatment	1.3(0.4–3.3)	1.5(0.3–2.8)
	One-year follow-up	0.4(0.2–3.4)	1.8(0.1–3.6)

NOTE. Values are median (quartiles).

There was also a statistically significant improvement of the KPS. Values improved in both therapy groups from pretraining to the 3-month control. The improvement was 8.4±7.9 (P<.001) in the PT group and 12.1±11.9 (P<.001) in the PT and EMS group. Improvement in KPS values persisted at the 1-year follow-up. Values are given in table 3.

Table 3. KPS Values in Both Groups

	KPS	PT	PT and EMS
	Before treatment	83(72–88)	73(66–85)
	After 3mo of treatment	90(85–95)	89(82–96)
	One-year follow-up	95(85–96)	94(88–96)

NOTE. Values are median (quartiles).

There was no statistically significant difference of improvement between the 2 treatment groups (VAS evaluation, P=.64; KPS, P=.29). Moreover, we did not find a statistically significant difference between men and women with respect to the VAS score and KPS (P=.32, P=.79, respectively). Univariate and multiple linear models revealed no statistically significant influence of the duration of pain complaints (both univariate and multiple model, P=.16) and sports activities (P=.80, P=.88, respectively) on the decrease in the VAS evaluation.

Evaluating the correlation between pain, function, and strength, the only significant correlation was between pain and the KPS (before treatment, ρ=−.54, P<.001; after 3mo, ρ=−.77, P<.001; after 12mo, ρ=−.64, P<.001), showing that lower pain values correlate with a higher function score. No correlation was detected between pain and strength or between the KPS and strength (ρ range, −0.3 to 0.3).

Isometric Muscle Strength 

In the PT group, the mean isometric muscle strength in 30° knee flexion was 94.7±23N before training and 89.7±20.8N after the 3-month training program. With 60° knee flexion, isometric strength was 152±45N before and 149±33N after training.

In the PT and EMS group, isometric strength with 30° was 108.7±29N before and 128±49N after 3 months of training and EMS. At 60° knee flexion values were 188±77N before and 199±77N after training and EMS.

There were no significant differences of pretraining and posttraining values of isometric strength for both tested knee flexion angles in both groups.

Study Limitations 

In addition, we should be concerned about the potential ceiling effect of one monotherapy when polytherapy is applied. This could be the case in our study such that exercise therapy reached a ceiling effect, and, therefore, it was not possible to document an additional effect of EMS. Another explanation could be that the statistical power was too low to reveal distinct differences.

Power analysis revealed that the power of the study was only 24% and thus too small to detect a difference of 1.5cm in the VAS improvement (ie, the difference in the VAS evaluation between values before and after therapy), considering the sample size of 18 patients in each group and the observed SD of 3.5.

It is widely considered that pain relief through EMS in patients with PFPS is caused by an increase of extensor strength and compensation of imbalances of the vasti muscles. Morrissey24 argued that pain inhibition occurred through a better muscular balance, whereas Melzack and Wall40 considered that it was caused by modification of pain-related input by posterior spinal cord column according to their gate control theory.

Irrespective of the good effects on pain relief and improvement of function, we did not find any changes of isometric strength values of the knee extensor muscles in our study. The improvement of quadriceps strength is believed to be a reason for pain relief in PFPS patients, but our patients showed a clear reduction of pain without changes in strength parameters. It is postulated that the reason for this could be that the improvement in the clinical condition refers more to neurophysiologic changes than to strength gains. In contrast, Werner et al32 found small (<6.2% from baseline values) but significant increases in concentric peak torques of the quadriceps muscle measured on an isokinetic dynamometer after a 10-week EMS procedure. One reason for this could be that they chose a study population with a hypotrophic VMO. The lower the baseline value, the greater the potential improvement. Callaghan and Oldham33 showed converse effects on isometric strength and isokinetic strength in their EMS groups. These conflicting results have been explained by their testing and stimulation protocol, but the results also show that an improvement of function and a decrease of pain can be achieved without relevant changes of strength parameters.

In this study, we did not investigate neurophysiologic mechanisms of pain. Therefore, we cannot rule out altered sensorimotor behavior neuromuscular activity because we did not investigate the time relation between activity onset of vasti muscles, as it has been mentioned by Cowan et al.39 Delayed onset of surface electromyographic activity of VMO compared with vastus lateralis during stair stepping was detected in patients with PFPS, and disturbed timing of onset of vasti muscles was suspected to be an assignable cause for dysfunction of knee extensors.39 Thus, it is not possible to rule out an additional effect of EMS and training on the timing and activation pattern of the knee extensor muscles.

The results measured after 3 months in both groups remained constant at the 1-year follow-up. There was no difference in the treatment effects with respect to sex, duration of pain complaints, or sports activities.