Comparison of vastus medialis obliquus:vastus lateralis muscle integrated electromyographic ratios between healthy subjects and patients with patellofemoral pain.

Disturbances of patellofemoral mechanism have been identified as one of the most commonly encountered abnormalities involving the knee joint. [1] Abnormal lateral tracking of the patella is one suggested cause of patellofemoral pain (PFP). Abnormal lateral tracking of the patella may elevate patellofemoral contact pressures and precipitate pathology in patellofemoral articular cartilage. [2-4] Proposed mechanisms for abnormal lateral patellar tracking are abnormalities in the static soft-tissue restraints of the patella, such as the retinacula [5]; variability in the bony congruency between the patellar and femoral articular surfaces [6]; and increased Q angle, or increased angulation between the femoral longitudinal axis and the orientation of the patellar ligament. [5] One other suggested mechanism for abnormal lateral tracking of the patella is an imbalance in the activity of the vastus medialis obliquus muscle (VMO) relative to the vastus lateralis muscle (VL). [3] Several investigators have examined the electromyographic (EMG) activity of the vastus medialis muscle (or the VMO) and the VL or have constructed VMO:VL ratios to assess imbalances for various groups of individuals or for various groups muscle contractions. Mariani and Caruso [7] reported that, during terminal knee extension, subjects with subluxating patellae had decreased vastus medialis muscle EMG activity compared with VL EMG activity. Healthy subjects in the study by Mariani and Caruso had equal EMG levels for both muscles during terminal knee extension. Other investigators [8,9] have examined VMO and VL EMG levels in patients with PFP, but have not used control groups. Investigations that lack control groups are not able to provide documented differences in the activity of components of the quadriceps femoris muscle in individuals with PFP and in individuals who do not have patellofemoral pathology.

Treatment protocols for patients with PFP often incorporate exercises to strengthen the VMO selectively. [1,3,10] Exercises designed to effect changes in the activity of components of the quadriceps femoris muscle are based on the existence of differences in activation patterns between healthy individuals and individuals with PFP. Only the study by Mariani and Caruso [7] supports the claim that patients with PFP differ from healthy individuals concerning VMO and VL activation patterns.

The previously described exercise protocols are also based on the assumption that specific exercises that elicit relatively larger VMO:VL EMG ratios can effect a change in the activation patterns of the quadriceps femoris muscle components during the performance of functional activities. We were unable to identify studies that have documented whether VMO:VL EMG ratios elicited by isotonic muscle contractions during functional movements differ from VMO:VL ratios elicited by isometric contractions. Darling [11] has documented that VMO:VL integrated electromyographic (IEMG) ratios do not differ among isometric and non-weight-bearing isotonic muscle contractions or among non-weight-bearing muscle contractions performed on an isokinetic exercise device. Blanpied [12] has suggested differences in VMO:VL IEMG ratios among non-weight-bearing eccentric, and isometric exercises. The statistical significance of these differences, however, was not evaluated by appropriate post hoc testing.

The purpose of this study was to test the null hypothesis that VMO:VL IEMG ratios (1) would not differ between healthy subjects and patients with unilateral PFP, (2) would not differ among several isotonic and isometric quadriceps femoris muscle contraction conditions, and (3) would not be affected by an interaction between type of knee condition and type of muscle contraction.

Method

Subjects

Subjects for this study were 6 men and 10 women ranging in age from 18 to 35 years (X=28.06, SD=5.97). The subjects were assigned to one of three groups on the basis of type of knee condition. In group 1, which consisted of 7 healthy control subjects (5 men, 2 women; age range=21-36 years, X=29.14, SD=5.98) with no history of knee pathology, both knees were tested. Subjects in this group were recruited from the faculty and student population of The University of North Carolina at Chapel Hill. In group 2, which consisted of 9 patients with unilateral PFP (1 man, 8 women; age range=18-35 years; X=27.22, SD=6.18), only the affected (ie, painful) knee was tested. Subjects in this group either were being treated for PFP by medical personnel at the Student Health Service at the university or were recruited from the community. All group 2 subjects complained of pain that they localized to the anterior aspect of the knee. In group 3, which consisted of the same subjects who comprised group 2, only the unaffected (ie, nonpainful) knee was tested. Each subject signed a statement of informed consent prior to participating in the study.

The investigators excluded from group 2 potential subjects who reported (1) pain on palpation of the quadriceps tendon or the patellar ligament, (2) a snapping sensation at the knee or reported tenderness on palpation medial to the patella suggestive of plica, [13] or (3) pain onn palpation of the knee joint line or with application of the McMurray's test for meniscal involvement. [14] Subjects in group 2 reported they had not experienced any trauma or surgical procedures to the affected knee during the 2 years prior to testing. Group 2 subjects also had no joint effusion of the affected knee at the time of testing. Joint effusion was defined operationally as midpatellar knee girth of the affected knee being greater than 105% of the girth of the unaffected knee. [15]

Instrumentation

The EMG data were acquired using the Therapeutics Unlimited GCS-67 multichannel EMG system. (*) This system uses two 8-mm-diameter silver-silver chloride electrodes with an on-site solid-state amplifier embedded in a plastic enclosure. The interelectrode distance is 22 mm between each member of the electrode pair. The preamplified signals are transmitted by "hard wiring" and amplified again. Total gain settings available range between 500 and 10,000. The frequency response of the system is 40 to 4,000 Hz, the common mode rejection ratio is 87 dB at 60 HZ, and the input impedance is greater than 15 M[Omega] at 100 Hz. The raw EMG signals were recorded on a Vetter Model E FM tape recorder. (+) A Tektronix 5111A/R511A storage oscilloscope (++) was used to monitor EMG signals qualitatively and to provide each subject with visual feedback regarding torque production for submaximal isometric contractions.

The EMG data were acquired from the FM recorder at 1,000 HZ using a Tecmar Labmaster analog-to-digital converter (*1) and were processed using an IBM PC/XT computer (*2) and a customized software program written using AYST programming software. (#) The DC offset was removed from the raw EMG data acquired from each muscle. The data were then processed using full-wave rectification, followed by mathematical integration.

A Cybex[R] II isokinetic dynamometer (**) was used to measure maximal and submaximal isometric knee extension torque. Stairs consisting of three steps were used to elicit concentric and eccentric isotonic quadriceps femoris muscle contractions. Each step was 25.4 cm (10 in) high and 20.3 cm (8 in) deep. Switches powered by 1.5-V batteries were secured to the left and right sides of the middle step to provide signals that would indicate the beginning and end of foot contact for each lower extremity. Electromyographic signals and voltage signals from each switch were recorded simultaneously on FM tape. A metronome was used to assist each subject in maintaining a standard stepping rate (92 steps/min) for ascending and descending the stairs.

Procedure

The midpatellar girth of the tested knee was measured for each subject in groups 2 and 3. Girth measurements were performed with each subject in the supine position. We did not assess the reliability of the girth measurements. Individuals who did not meet the previously described criteria for joint effusion were excluded from the study.

We prepared each subject for EMG electrode placement by shaving the skin of each electrode site with a safety razor and then wiping the site with isopropyl alcohol. Double-sided adhesive tape and conducting gel were applied to each electrode. A longitudinal electrode placement, as described by Basmajian and Blumenstein, [16] was used for the VL. The VMO electrode was placed over the distal portion of the vastus medialis muscle [17] and was oriented transversely to the muscle fibers because of the short course of the muscle.

The order of testing right and left lower extremities was determined randomly for each subject. We positioned each subject for submaximal and maximal isometric quadriceps femoris muscle contractions (SICs and MICs, respectively) of the first test leg. Each subject sat in the Cybex[R] II test chair with hips flexed at approximately 80 degrees and the tested knee flexed at 10 degress. We selected 10 degrees of knee flexion for the test position based on research that indicates patellofemoral contact pressure decreases for isometric quadriceps femoris muscle contractions performed at terminal knee extension. [4,18] We used a universal goniometer to determine knee position for the first muscle contraction and then relied on the goniometric dial on the Cybex[R] II dynamometer to maintain consistent knee joint positioning for subsequent muscle contractions.

Each subject performed several SICs to become familiar with the testing apparatus and then performed three MICs. Each subject maintained each MIC for approximately 5 seconds and rested for 2 minutes between contractions. Peak torque production during these contractions was recorded on the storage oscilloscope. Electromyographic data were not acquired during these muscle contractions. These trials were used only to identify a torque level for each subject that was 25% of maximal torque production (ie, 25% relative to the largest torque from the three trials). We did not assess the reliability of the torque measurements. The investigators placed an oscilloscope channel marker at a position corresponding to 25% of the largest torque produced during the three MICs.

Each subject then performed three SICs at this 25% level using the oscilloscope feedback to maintain a consistent torque level. Subjects maintained each SIC for 5 seconds and rested for 1 munite between contractions. Electromyographic and torque data were recorded on FM tape for the three contractions. Computer software was used as previously described to determine the IEMG value for the middle 3 seconds of each SIC. The principal investigator (DRS) then calculated the average IEMG value across the three SIC trials for each muscle. The average IEMG value for each muscle was used for data analysis.

Each subject then performed three MICs, maintaining each muscle contraction for 5 seconds and resting for 2 minutes between contractions. Electromyographic and torque data were recorded on FM tape for the three MIC trials. The average IEMG value was calculated for each muscle for the MIC trials in the same manner described for the SIC trials. We repeated the MIC and SIC data-collection procedure for each subject's opposite lower extremity after data collection for the MIC and SIC trials had been completed for the first test leg.

Following isometric testing, we collected data for concentric and eccentric quadriceps femoris muscle contractions (CONs and ECCs, respectively) performed by the subjects during ascent and descent of the stairs. The order of right- and left-leg testing was the same as the order used for the isometric testing conditions. Subjects had approximately 10 minutes of rest between isometric and isotonic testing conditions. Each subject ascended the stairs and performed the CON by making contact with each successive step first with the test leg and then with the opposite leg. We elicited an ECC from each subject by having the subject descend from each step in a nonreciprocal fashion. Each subject descended the stairs and performed the ECC by making contact with each successive step first with the opposite leg and then with the test leg.

Prior to data collection, each subject practiced ascending and descending the stairs several times using the metronome to maintain a stepping frequency of 92 steps/min. We recorded EMG and footswitch data on FM tape for three CONs and three ECCs for each leg. Integrated electromyographic values were calculated for the CONs beginning with foot contact of the test leg and ending with foot contact of the opposite leg during ascent of the stairs. The IEMG values for the three CONs were averaged and used for data analysis. The average duration of the three CONs also was calculated for each muscle. Integrated electromyographic values were calculated for the ECCs beginning with foot contact of the opposite leg and extending for the average duration of the three CONs. The IEMG values for the three ECCs were averaged and used for data analysis.

[TABULAR DATA OMITTED]

Data Analysis

The IEMG value for each muscle was normalized for the CON, ECC, and SIC conditions by dividing the IEMG value for each condition by the muscle's IEMG value for the MIC condition. The purpose of data normalization was to allow comparison among the three groups. [19] The dependent variable assessed was the normalized VMO:VL ratio. The two independent variables were type of knee condition and muscle contraction condition. An ANOVA was conducted with repeated measures on one factor: muscle contraction condition. The alpha level for the ANOVA procedure was .05. A Tukey's Honestly Significant Difference (HSD) Test was used to perform post hoc contrasts.

Results

Mean VMO:VL IEMG ratios for the analysis of normalized data are presented in Table 1 and Figure 1, and the ANOVA results are shown in Table 2. The interaction between type of knee condition and muscle contraction condition was not statistically significant, and no differences were identified across the three groups. Significant differences were identified, however, across the three muscle contraction conditions. The Tukey HSD analysis indicated that the VMO:VL IEMG ratios for the CON and the ECC conditions were significantly greater than the VMO:VL IEMG ratios for the SIC condition (HSD - .147, df=58, P<.01).

Discussion

The results for the normalized VMO:VL IEMG data indicate no differences in VMO:VL IEMG ratios across the three groups and suggest that patients with PFP may not differ from healthy individuals with regard to VMO:VL activation patterns. Following the initial data analysis, however, we realized a potential threat to comparison among the three groups for the analysis of the normalized data. This potential threat to validity involves an effect of type of knee condition on the normalizing MIC. Each normalized VMO:VL IEMG ratio is expressed as a percentage of the VMO:VL IEMG value for the normalizing MIC. If subjects within a group had the same abnormal VMO:VL IEGM ratios for the normalizing contraction and the other three muscle contraction conditions, potential differences among the three groups could have gone undetected. A second analysis was conducted, therefore, using nonnormalized IEMG data from the four muscle contraction conditions (SIC, MIC, CON, and ECC). Conclusions based on this analysis of nonnormalized data are independent of the assumption of no effect of type of knee condition on a normalizing contraction.

Mean VMO:VL IEMG ratios for the analysis of nonnormalized data are presented in Table 3 and Figure 2, and the ANOVA results are shown in Table 4. The variables type of knee

Table 2. Analysis-of-Variance Results for Effects of Group and Condition on Normalized Vastus Medialis Obliguus: Vastus Lateralis Muscle Integrated Electromyographic Ratios

Source                  df  SS    MS     F
Between subjects
  Group                  2  0.55  0.27    2.71
  Error                 29  2.94  0.10
Within subjects
  Condition              2  1.13  0.56   14.96 (a)
  Condition x group       4 0.08  0.02    0.51
  Error                 58  2.19  0.04
  (a) P<.001.

condition and muscle contraction condition did not interact signifcantly to affect the VMO:VL IEMG ratios. Significant differences were present across the three groups. The Tukey HSD analysis (HSD=.157, df=29, P<.01) indicated that the VMO:VL IEMG ratios for group 1 were significantly greter than the ratios for groups 2 and 3. Significant differences also were present across the four muscle contraction conditions. The Tukey HSD analysis inidcated that the VMO:VL IEMG ratios for the ECC (HSD=.048, df=87, p<.05) and CON (HSD=.059, df=87, p<.01) conditions were significantly greater than the VMO:VL IEMG ratios for the SIC and MIC conditions.

An interesting aspect of the results for the two analyses pertains to the effect of type of knee condition on VMO:VL IEMG ratios. Significant differences were detected across the three groups for the analysis of nonnormalized data, but not for the analysis of normalized data. Differences in the VMO:VL ratios among the groups for the normalizing contraction may have resulted in the inability to detect differences among the three groups for the analysis of the normalized VMO:VL ratios for the SIC, CON, and ECC conditions.

The major threat to the validity of detecting differences across groups in the analysis of nonnormalized data is the effect of electrode placement on IEMG values. We did not conduct a reliability analysis to assess the significance of this threat to the validity of nonnormalized measurements. The nonnormalized results for between-groups comparisons, therefore, should be interpreted with caution until the reliability is established for the analysis of nonnormalized data acquired with the method described in this article.

Exercising the previously noted caution, the significant differences across groups revealted by analysis of the nonnormalized data suggest that individuals with PFP may differ from healthy individuals with regard to quadriceps femoris muscle activation

[TABULAR DATA OMITTED]

patterns. The patients with unilateral PFP had lower VMO:VL IEMG ratios for both knees than the VMO:VL IEMG ratios of the control subjects. This result supports the findings of Mariani and Caruso, [7] who reported decreased vastus medialis muscle EMG values in both knees of eight subjects with unilateral subluxating patellae as compared with healthy subjects. The presence of diminished VMO:VL IEMG ratios in both knees of the patients with unilateral PFP suggests that abnormal muscle activation patterns may interact with biomechanical factors in explaining the cause of unilateral PFP. These factors might include unilateral structural abnormalities (eg, leglength discrepancy, foot deformities, patellar asymmetry) or histories of activities that place asymmetrical demands on the lower extremities (eg, running on a surface with a consistent camber or a jumping sport that involves push-off with only one lower extremity). [10,20,21]

Conclusions based on the effect of muscle contraction condition on VMO:VL IEMG ratio are independent of the previously identified dilemmas associated with normalization and electrode placement. All subjects contributed data to all conditions. The VMO:VL IEMG ratios for the isotonic conditions were greater than the VMO:VL IEMG ratios for the isometric contractions for both analyses. Differences in VMO:VL IEMG ratios across activities may have important implications under certain circumstances. If an abnormal VMO:VL activation pattern facilitates the development of PFP, then clinicians might identify a change in VMO:VL activation pattern as a goal of rehabilitation. A series of research questions is indicated in following this line of reasoning. Does a consistent program of exercise involving activities that elicit relatively large VMO:VL IEMG ratios cause an increase in VMO:VL IEMG ratios for those activities? Does this effect transfer to other activities or exercises that elicited relatively low VMO:VL IEMG ratios prior to initiation of the exercise program? Does the exercise program also cause a reduction in pain

Table 4. Analysis-of-Variance Results for Effects of Group and Condition on Nonnormalized Vastus Medialis Obliquus:Vastus Lateralis Muscle Integrated Electromyographic Ratios

Source                  df  SS    MS     F
Between subjects
  Group                  2  0.86  0.43    8.76 (a)
  Error                 29  1.42  0.05
Within subjects
  Condition              3  0.23  0.08   14.49 (b)
  Condition x group      6  0.02  0.00    0.51
  Error                 87  0.46  0.00
  (a) p<.01.
  (b) p<.001.

levels and differences in the arthrokinematics of the patellofemoral joint?

The results indicate that the isotonic quadriceps femoris muscle contractions in this study elicited greater VMO:VL IEMG ratios than did isometric quadriceps femoris muscle contractions. Clinicians may be tempted to apply this information in treating patients who have PFP by using isotonic rather than isometric quadriceps femoris muscle contractions in treatment regimens. We caution that the previously identified research questions should be addressed prior to concluding that such treatment planning decisions are appropriate.

A final point for discussion concerns the gender distribution of subjects in this study. Most of the patients with unilateral PFP in this study were women. Most of the subjects in the control group were men. We are unaware of any data that suggest a gender difference in VMO:VL activation patterns. Future studies may indicate whether such a gender difference exists and whether this difference has an effect on PFP etiology.

Conclusions

The results of this study suggest that individuals with PFP may differ from healthy individuals with regard to VMO:VL activation patterns. This factor may interact with biomechanical factors in explaining the cause of PFP syndrome. The isotonic quadriceps femoris muscle contractions tested in this study elicited larger VMO:VL IEMG ratios than did the isometric quadriceps femoris muscle contractins. This result suggests the need for a series of studies to investigate the efficacy of different exercise programs for the treatment of PFP.

D Souza, MS, PT is in private practice at Sanger Orthopedic and Sports Physical Therapy, 2570 Jensen St, #108, Sanger, CA 93657. He was a graduate student, Division of Physical Therapy, The University of north Carolina at Chapel Hill, when this study was conducted. This study was completed in partial fulfillment of the requirements for Mr Souza's master's degree, Department of Medical Allied Health Professions, School of Medicine, The University of North Carolina at Chapel Hill.

M Gross, PhD, PT, is Assistant Professor, Division of Physical Therapy, The University of North Carolina at Chapel Hill, CB #7135 Medical School Wing E 222H, Chapel Hill, NC 27599-7135 (USA). Address all correspondence to Dr Gross.

(*) Therapeutics Unlimited Inc, 2835 Friendship St, Iowa City, IA 52240.

(+) AR Vetter Co, PO Box 143, Rebersburg, PA 16872.

(++) Tektronix Inc, Howard Vollum Industrial Park, PO Box 500, Beaverton, OR 97077.

(*1) Techmar Inc, Personal Computer Products Div, 6225 Cochran Rd, Solon, OH 44139.

(*2) International Business Machines Corp, PO Box 1328-S, Boca Raton, FL 33432.

(#) MacMillan Software, 866 3rd Ave, New York, NY 10022.

(**) Cybex, Div of Lumex Inc, 2100 Smithtown Ave, Ronkonkoma, NY 11779.

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