Research Report
Comparison of Vastus Medialis 0bliquus:Vastus ~aterdisMuscle Integrated Electromyographic Ratios Between Healthy Subjects and Patients with Patellofemoral Pain
fie purpose of this study was to compare vastus medialis obliquus:vastuslateralis muscle (VM0:VL)integrated electromyographic (IEMG) ratios of healthy subjects and patients with unilateral patellofemoral pain (PFP) under isotonic and isometric quadricepsfemoris muscle contraction conditions. Subjects ranging in age from 18 to 35 years @=28.06, SD=5.97) were assigned to one of three groups on the basis of type of knee condition. In group I, which consisted of seven healthy control subjects with no history of knee pathology, both knees were tested. In group 2, which consisted of nine patients with unilateral PFP, only the painful knee was tested. In group 3, which consisted of the same nine patients who comprised group 2, only the nonpainful knee was tested Nonnormalized and normalized VM0:VLIEMG ratios were computed for ascending stairs, descending stairs, submaximal isometric contraction, and maximal isometric contraction (nonnormulized only). A two-way analysis of variance for repeated measures indicated VM0:VLratiosfor isotonic stair-climbing activities were signiJicantly greater than VM0:VLratiosfor isometric contractions.Nonnormulized VM0:VL ratios in group I were sgniJicant&greater than nonnormalized VM0:VLratios in the other two groups. Patients with PFP may have abnormal VM0:VLactivation pattern, and isotonic quadncepsfemoris muscle exercise may elicit more favorable muscle activation patterns than isometric exercbe for patients with PFP. [Souza DR, G r m M7: Comparison of vastus medialis ob1iquus:vastuslateralis muscle integrated electromyographic ratios between healthy subjects and patients with patellofeemoral pain. Phys Ther. 1991;71:310-320.1
Daniel R Souza Michael T Gross
Key Words: E1ectrom~)ography; Exercise, general; Lower extremity, knee; Muscle perfomzance, measurement;Pain.
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. This project was approved by the Committee on the Protection of the Rights of Human Subjects at The University of North Carolina at Chapel Hill.
This article was submitted July 28, 1989, and was accepted November 20, 1990.
58/310
Disturbances of the patellofemoral mechanism have been identified as one of the most commonly encountered abnormalities involving the knee joint.' 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.24 Proposed mechanisms for abnormal lateral patellar tracking are abnormali-
Physical Therapy /Volume 71, Number 4 /April 1991
ties in the static soft-tissue restraints of the patella, such as the retinacula5; variability in the bony congruency between the patellar and femoral articular surfaces6; 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.).' Several investigators have examined the electromyographic (EMG) activity of the vastus medialis muscle (or the VMO) and the VL or have constructed VM0:VL ratios to assess imbalances for various groups of individuals or for various types of muscle contractions. Mariani and Carus07 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 (luring terminal knee extension. Other investigators8a9 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 riot have patellofemoral pathology. Treatment protocols for patients with PFP often incorporate exercises to strengthen the VMO selectively.l23>l0 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 Carus07 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 VM0: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 VM0:VL EMG ratios elicited by isotonic muscle contractions during functional movements differ from VM0:VL ratios elicited by isometric contractions. DarlingH has documented that VM0:VL integrated electromyographic (IEMG) ratios do not differ among isometric and nonweight-bearing isotonic muscle contractions or among non-weightbearing muscle contractions performed on an isokinetic exercise device. Blanpied12 has suggested differences in VM0:VL IEMG ratios among non-weight-bearing concentric, 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 VM0: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 @=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
'Therapeutics Unlimited Inc, 2835 Friendship St, Iowa City, IA 52240.
Physical Therapy /Volume 71, Number 4 /April 1991
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,l3 or (3) pain on palpation of the knee joint line or with application of the McMurray's test for meniscal involvement.'* 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 ehsion of the affected knee at the time of testing.Joint efusion 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 silversilver chloride electrodes with an onsite 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 MR at 100 Hz. The raw EMG signals were recorded on a Vetter Model E FM tape recorder.+A Tektronix 5111A/R511A storage oscilloscopes was used to monitor EMG signals qualitatively and to provide each subject with visual feedback regarding torque production for submaximal isometric contractions.
in maintaining a standard stepping rate (92 steps/min) for ascending and descending the stairs.
The EMG data were acquired from the FM recorder at 1,000 Hz using a Tecmar Labmaster analog-to-digital converters and were processed using an IBM PCBT computed1and a customized software program written using ASYST 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.
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,16was used for the VL. The VMO electrode was placed over the distal portion of the vastus medialis muscle" and was oriented transversely to the muscle fibers because of the short course of the muscle.
A Cybefl I1 isokinetic dynamometers*
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
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.
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 CybexB I1 test chair with hips flexed at approximately 80 degrees and the tested knee flexed at 10 degrees. We selected 10 degrees of knee flexion for the test position based on research that indicates patellofemoral contact pressure decreases for isometric
+AR Vetter Co, PO Box 143, Rebersburg, PA 16872. *Tektronix Inc, Howard Vollum Industrial Park, PO Box 500, Beavcrton, OR 97077. '~echrnarInc, Personal Computer Products Div, 6225 Cochran Rd; Solon, OH 44139 l11ntcrnationalBusiness Machines Corp, PO Box 13284, Boca Raton, FL 33432. # ~ a c ~ i l l aSoftware, n 866 3rd Ave, New York, NY 10022. **Cybex,Div of Lurnex Inc, 2100 Smithtown Ave, Ronkonkoma, NY 11779.
quadriceps femoris muscle contractions performed at terminal knee extension.4JWe used a universal goniometer to determine knee position for the first muscle contraction and then relied on the goniometric dial on the CybexmI1 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 minute 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
Physical Therapy /Volume 71, Number 4 /April 1991
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 berween 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. Elach 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 thn:e 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.
.
Table 1 Normalized Vastus Medialis 0bliquus:VastusLateralis Muscle (VMOVL) Integrated Electromyographic (IEMG) Ratios for Groups, Muscle Contraction Conditions, and Group x Condition Interactionf Groupb 1
2
3
Condltlonc Means
CON ECC
1.1820.22
1.26e0.26
1.4320.24
1.2720.25
1.1520.36
1.1320.30
1.2520.27
1.17c0.31
SIC
0.9620.17
0.95r0.10
1.1320.14
l.OOrt0.16
Group means
1.10e0.27
13020.25
1.10rt0.26
"Data are meanskstandard deviations for IEMG ratios formed by dividing normalized VMO IEMG value by normalized vastus medialis muscle IEMG value. hGroup l=healthy control subjects (both knees); group 2=patients with patellofemoral pain (affected knees); group 3=patients with patellofemoral pain (unaffected knees). 'CON=concentric quadriceps femoris muscle contraction elicited during ascent of steps; ECC=eccentric quadriceps femoris muscle contraction elicited during descent of steps; SIC=submaximal isometric quadriceps femoris muscle contraction.
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.'9 The dependent variable assessed was the normalized VM0: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 VM0: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
Physical Therapy /Volume 71, Number 4 /April 1991
HSD analysis indicated that the VM0:VL IEMG ratios for the CON and the ECC conditions were significantly greater than the VM0:VL IEMG ratios for the SIC condition (HSD=.147, df=58, P
Comparison of Vastus Medialis 0bliquus:Vastus ~aterdisMuscle Integrated Electromyographic Ratios Between Healthy Subjects and Patients with Patellofemoral Pain
fie purpose of this study was to compare vastus medialis obliquus:vastuslateralis muscle (VM0:VL)integrated electromyographic (IEMG) ratios of healthy subjects and patients with unilateral patellofemoral pain (PFP) under isotonic and isometric quadricepsfemoris muscle contraction conditions. Subjects ranging in age from 18 to 35 years @=28.06, SD=5.97) were assigned to one of three groups on the basis of type of knee condition. In group I, which consisted of seven healthy control subjects with no history of knee pathology, both knees were tested. In group 2, which consisted of nine patients with unilateral PFP, only the painful knee was tested. In group 3, which consisted of the same nine patients who comprised group 2, only the nonpainful knee was tested Nonnormalized and normalized VM0:VLIEMG ratios were computed for ascending stairs, descending stairs, submaximal isometric contraction, and maximal isometric contraction (nonnormulized only). A two-way analysis of variance for repeated measures indicated VM0:VLratiosfor isotonic stair-climbing activities were signiJicantly greater than VM0:VLratiosfor isometric contractions.Nonnormulized VM0:VL ratios in group I were sgniJicant&greater than nonnormalized VM0:VLratios in the other two groups. Patients with PFP may have abnormal VM0:VLactivation pattern, and isotonic quadncepsfemoris muscle exercise may elicit more favorable muscle activation patterns than isometric exercbe for patients with PFP. [Souza DR, G r m M7: Comparison of vastus medialis ob1iquus:vastuslateralis muscle integrated electromyographic ratios between healthy subjects and patients with patellofeemoral pain. Phys Ther. 1991;71:310-320.1
Daniel R Souza Michael T Gross
Key Words: E1ectrom~)ography; Exercise, general; Lower extremity, knee; Muscle perfomzance, measurement;Pain.
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. This project was approved by the Committee on the Protection of the Rights of Human Subjects at The University of North Carolina at Chapel Hill.
This article was submitted July 28, 1989, and was accepted November 20, 1990.
58/310
Disturbances of the patellofemoral mechanism have been identified as one of the most commonly encountered abnormalities involving the knee joint.' 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.24 Proposed mechanisms for abnormal lateral patellar tracking are abnormali-
Physical Therapy /Volume 71, Number 4 /April 1991
ties in the static soft-tissue restraints of the patella, such as the retinacula5; variability in the bony congruency between the patellar and femoral articular surfaces6; 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.).' Several investigators have examined the electromyographic (EMG) activity of the vastus medialis muscle (or the VMO) and the VL or have constructed VM0:VL ratios to assess imbalances for various groups of individuals or for various types of muscle contractions. Mariani and Carus07 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 (luring terminal knee extension. Other investigators8a9 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 riot have patellofemoral pathology. Treatment protocols for patients with PFP often incorporate exercises to strengthen the VMO selectively.l23>l0 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 Carus07 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 VM0: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 VM0:VL EMG ratios elicited by isotonic muscle contractions during functional movements differ from VM0:VL ratios elicited by isometric contractions. DarlingH has documented that VM0:VL integrated electromyographic (IEMG) ratios do not differ among isometric and nonweight-bearing isotonic muscle contractions or among non-weightbearing muscle contractions performed on an isokinetic exercise device. Blanpied12 has suggested differences in VM0:VL IEMG ratios among non-weight-bearing concentric, 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 VM0: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 @=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
'Therapeutics Unlimited Inc, 2835 Friendship St, Iowa City, IA 52240.
Physical Therapy /Volume 71, Number 4 /April 1991
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,l3 or (3) pain on palpation of the knee joint line or with application of the McMurray's test for meniscal involvement.'* 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 ehsion of the affected knee at the time of testing.Joint efusion 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 silversilver chloride electrodes with an onsite 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 MR at 100 Hz. The raw EMG signals were recorded on a Vetter Model E FM tape recorder.+A Tektronix 5111A/R511A storage oscilloscopes was used to monitor EMG signals qualitatively and to provide each subject with visual feedback regarding torque production for submaximal isometric contractions.
in maintaining a standard stepping rate (92 steps/min) for ascending and descending the stairs.
The EMG data were acquired from the FM recorder at 1,000 Hz using a Tecmar Labmaster analog-to-digital converters and were processed using an IBM PCBT computed1and a customized software program written using ASYST 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.
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,16was used for the VL. The VMO electrode was placed over the distal portion of the vastus medialis muscle" and was oriented transversely to the muscle fibers because of the short course of the muscle.
A Cybefl I1 isokinetic dynamometers*
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
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.
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 CybexB I1 test chair with hips flexed at approximately 80 degrees and the tested knee flexed at 10 degrees. We selected 10 degrees of knee flexion for the test position based on research that indicates patellofemoral contact pressure decreases for isometric
+AR Vetter Co, PO Box 143, Rebersburg, PA 16872. *Tektronix Inc, Howard Vollum Industrial Park, PO Box 500, Beavcrton, OR 97077. '~echrnarInc, Personal Computer Products Div, 6225 Cochran Rd; Solon, OH 44139 l11ntcrnationalBusiness Machines Corp, PO Box 13284, Boca Raton, FL 33432. # ~ a c ~ i l l aSoftware, n 866 3rd Ave, New York, NY 10022. **Cybex,Div of Lurnex Inc, 2100 Smithtown Ave, Ronkonkoma, NY 11779.
quadriceps femoris muscle contractions performed at terminal knee extension.4JWe used a universal goniometer to determine knee position for the first muscle contraction and then relied on the goniometric dial on the CybexmI1 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 minute 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
Physical Therapy /Volume 71, Number 4 /April 1991
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 berween 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. Elach 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 thn:e 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.
.
Table 1 Normalized Vastus Medialis 0bliquus:VastusLateralis Muscle (VMOVL) Integrated Electromyographic (IEMG) Ratios for Groups, Muscle Contraction Conditions, and Group x Condition Interactionf Groupb 1
2
3
Condltlonc Means
CON ECC
1.1820.22
1.26e0.26
1.4320.24
1.2720.25
1.1520.36
1.1320.30
1.2520.27
1.17c0.31
SIC
0.9620.17
0.95r0.10
1.1320.14
l.OOrt0.16
Group means
1.10e0.27
13020.25
1.10rt0.26
"Data are meanskstandard deviations for IEMG ratios formed by dividing normalized VMO IEMG value by normalized vastus medialis muscle IEMG value. hGroup l=healthy control subjects (both knees); group 2=patients with patellofemoral pain (affected knees); group 3=patients with patellofemoral pain (unaffected knees). 'CON=concentric quadriceps femoris muscle contraction elicited during ascent of steps; ECC=eccentric quadriceps femoris muscle contraction elicited during descent of steps; SIC=submaximal isometric quadriceps femoris muscle contraction.
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.'9 The dependent variable assessed was the normalized VM0: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 VM0: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
Physical Therapy /Volume 71, Number 4 /April 1991
HSD analysis indicated that the VM0:VL IEMG ratios for the CON and the ECC conditions were significantly greater than the VM0:VL IEMG ratios for the SIC condition (HSD=.147, df=58, P
