Eledromyographic Analysis of Knee Rehabilitation Exercises Stephen M. Cryzlo, MD' Robert M. Patek, MD' Marilyn Pink, MS, PTZ lacquelin Perry, MD3

T he knee is a frequently injured area that often requires assessment by a physician and a phys- ical therapist. Some in-

juries need only rest o r reassurance, while others require some form of therapy to return the patient to his o r her preinjury state. Others may need surgical intervention and post- operative rehabilitation (2 1).

Knee rehabilitative exercises continue to be defined and rede- fined by clinical experience, observa- tions, and basic science research. T h e general goals of knee rehabilita- tion are to increase strength, endur- ance, and range of motion. T o reach these goals, the exercises must be performed through a safe range of motion and be efficient and effective (24).

Electromyography (EMG) is an appropriate tool to measure the rela- tive intensity of muscle activity oc- curring during exercises o r func- tional activities (1 6). Soderberg and Cook and Soderberg e t al, in two separate studies during the 1980s. analyzed the surface EMG activity of four lower extremity muscles during quadricep femoris muscle setting (quad sets) and straight leg raises (SLR) (23,24). In both reports, more muscle activity was found in the vas- tus medialis, biceps femoris, and glu- teus medius during quad sets than during SLR. T h e rectus femoris was

Many exercises are used to strengthen the knee muscles, yet limited studies that evaluate the exercises exist. The purpose of this study was to describe and compare the muscle firing patterns in five knee muscles during five rehabilitative exercises, which were presumed either to strengthen a specific muscle group or to elicit a cocontraction. During short-arc knee extension, the medial and lateral vasti were significantly more active during the last 15" of extension than during other arcs (p < .05). The short-arc knee extension with hamstring cocontraction demonstrated significantly more activity in the rectus fernoris, vastus rnedialis oblique, and vastus lateralis than the biceps femoris and semimembranosus in the final 15" of extension (p < .05). With isometric knee cocon- tractions, no significant difference in muscle activity occurred in any of the arcs tested. During squatting, the rectus femoris, vastus medialis oblique, and vastus lateralis were significantly more active than the biceps femoris and semimembranosus during the descending, holding, and arising phases (p < .05). This information offers suggestions in selecting optimal knee rehabilitation exer- cises.

Key Words: electromyograph y, rehabilitative exercises, cocon tractions ' Clinical Instructor, Orthopaedic Surgery, Northwestern University Medical School, Chicago, I1

Director and Assistant Administrator, Biomechanics Laboratory, Centinela Hospital Medical Center, 555 E. Hardy St., Inglewood, CA 90301 'Consultant, Biomechanics laboratory, Centinela Hospital Medical Center, Inglewood, CA

more active during SLR than during quad sets. This corroborated a find- ing by Skurja et al, who reported more EMG activity from the vasti muscles (vastus medialis obliques and vastus lateralis) during isolated iso- metric knee extension than during SLR and, conversely, more increased EMG values for the rectus femoris during SLR than during isometric knee extension (22). It is obvious from these studies that different lower extremity muscles a re active a t varying degrees of intensity with dif- ferent exercises.

A current area of controversy in knee rehabilitation is the program

for the patient with an anterior cru- ciate ligament (ACL) reconstruction (10). All authorities agree that reha- bilitation is important, but the spe- cific approach remains an enigma (2). In the last I 0 years, many changes in the postoperative ACL reconstruction therapy program have occurred. Emphasis has shifted from rigid immobilization and non- weight bearing to immediate contin- uous passive motion and weight bearing as tolerated to even full knee extension on the first postoperative day and participation in sports by 4- 6 months (1 5,19,20). Sachs et al found the most common complica-

Volume 20 Number I *July 1994 *JOSPT

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tions following ACL surgery to be quadriceps weakening, flexion con- tracture, and patellofemoral pain (1 8). They stated that all three were interrelated.

Rehabilitation aimed at quadri- ceps strengthening and improved ex- tension placed the reconstructed ACL a t risk by increasing strain on the ligament and increasing anterior tibial translation (2,5,7-10,17,27). A potential answer to this problem was simultaneous hamstring and quadri- ceps contraction-so-called cocon- traction (26). Draganich e t al re- ported hamstrings/quadriceps cocon- traction in healthy subjects during knee extension (6). In their study, the greatest hamstring activity oc- curred a t less than 10" of flexion. T h e authors concluded that the hamstrings act in concert with the ACL to prevent anterior tibial trans- lation. Additionally, in a cadaveric study, Renstrom et al reported a po- tential strain reduction role of the hamstrings when coactivated with the quadriceps in regards to the ACL (16). Finally, an in vivo study by Kain e t al determined that if hamstring muscles fired before quadriceps muscles and if simultane- ous contractions of both were sus- tained, then a decreased strain on the ACL occurred (1 0). These stud- ies suggest that cocontractions of the hamstring/quadriceps group can re- duce stress on the ACL. Clinically, this concept of cocontraction has been incorporated into the rehabili- tation program for ACL reconstruc- tions ( l , l9 ) .

T o date, EMG studies (I 2,14, 23-25) on lower extremity muscle activity have concentrated on only a few rehabilitation exercises, namely the SLR and quad sets exercises. Many exercises remain untested. At this time, objective information on coactivation of hamstring and quadr- icep muscles is available during sim- ple knee motions (3.6) but not dur- ing rehabilitative exercises. Addi- tionally, previous research (I 2,14, 23-25) has used surface electrodes

to record lower extremity EMG ac- tivity during the different knee mo- tions. A problem with surface EMG is potential crosstalk o r contamina- tion of the recording signals by neighboring muscle activity (3). In- dwelling wire electrodes can avoid this concern.

T h e purpose of this paper is two- fold. T h e first purpose is to describe and compare the EMG activity of the rectus femoris, vastus lateralis, vastus medialis oblique, biceps femoris, and semimembranosus with the use of fine-wire electrodes during five re- habilitative exercises. T h e second purpose is to compare the EMG ac- tivity within a muscle throughout the arc of motion for each exercise.

Significant acfivify, however, did occur in

the knee extensors during all phases of

the squat.

MATERIALS AND METHODS

This study was conducted in the Biomechanics Laboratory a t Centi- nela Hospital Medical Center in In- glewood, CA. Twelve healthy sub- jects volunteered for participation in the study (nine males and three fe- males), with ages ranging from 25 to 32 years (mean age = 29). None of the participants had a history of knee injury. They all had a full range of motion, and no atrophy of the lower extremity muscles was present.

Muscle activity of the right vas- tus medialis oblique, vastus lateralis, rectus femoris, semimembranosus, and the long head of the biceps fem- oris was recorded using indwelling wire electrodes. Recording of the signal utilized the Basmajian (4) sin- gle needle technique. Following proper skin preparation and isolation

of the specific muscle, dual 50-p in- sulated wires with 2-3-mm bared tips were inserted into the muscle using a 25-gauge hypodermic needle as a cannula. T h e wires from each muscle were attached to ground plates and taped to the patient's body. T h e signals from the leads were transmitted using an FM-FM telemetry system (Model 4200-A, Bio-Sentry Telemetry, Torrance, CA), which was capable of transmit- ting up to four muscles simultane- ously. Correct wire electrode place- ment was confirmed by a manual test specific to the inserted muscle with the telemetry signal monitored on an oscilloscope (Model 5 1 1 1 A, Tek- tronix, Beaverton, OR). Each subject wore a battery-operated FM trans- mitter belt pack oriented to prevent any restrictions in bodily move- ments. T h e EMG information was bandpass filtered at 100- 1,000 Hz and recorded on a multichannel in- strumentation recorder (Model 3968A. Hewlett-Packard, Palo Alto. CA) for later retrieval and review.

One 16-mm high-speed motion picture camera (Model DBM-55, Te- ledyne Camera System, Arcadia, CA) operating a t 50 frames per second was positioned a t a right angle to the subject and recorded his o r her per- formance. Marks were electronically placed on the film and EMG data to allow for synchroni7ation. Later, the films were reviewed and the exercises were divided into arcs of motion.

T o begin the testing, resting EMGs were first recorded. T h e EMGs were then taken during a maximal manual muscle test (MMT) for each muscle. T h e muscle test po- sitions were in accordance with standard physical therapy guidelines (1 1). T h e subjects were instructed through the series of exercises being evaluated to ensure proper perform- ance. Two trials of each exercise were taken, and a rest averaging 3 minutes was given between each trial, thus eliminating the potential for fatigue.

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T h e exercises were performed in the following sequence:

Straight Leg Raise With the subject supine, a straight leg raise (SLR) was performed until a hip flexion angle of 75" was reached. No weight was attached to the leg in order to mimic an early stage of a rehabilitation program.

Short-Arc Knee Extension T h e subjects were seated and a 12-inch roll was placed behind their right thigh. Their right knee was flexed to 45" off the end of the table. An iso- tonic short-arc quadricep exercise (SAEX) was performed from 45" of knee flexion to full extension at a constant rate with a 12.5-lb weight attached to the ankle. A 12.5-Ib weight was utilized as it mimicked an early stage of a rehabilitation pro- gram.

Short-Arc Knee Extension with Hamstring Cocontraction T h e sub- jects were asked to perform the same exercise as above and this time add hamstring cocontractions (SAEXHS). This was accomplished by isometrically contracting their hip into the thigh roll as they extended their knees from 45" of flexion to full extension. Again, a 12.5-lb weight was attached to the ankle for the exercise in order to mimic an early stage of a rehabilitation program.

Squat T h e subjects stood and were asked to descend to 90" of knee flexion. They held this position for 3 seconds and then ascended.

Isometric Knee Cocontraction T h e subjects returned to the supine position. A 12-inch roll was placed behind their right thigh, and their right knee was placed off the end of the table. An isometric quadricep with hamstring cocontraction exer- cise (ICO) was performed a t 15, 30, and 45" of knee flexion and held for a count of five.

After the films were processed, they were reviewed and the isotonic exercises were divided into arcs of motion which were synchronized with the EMG. T h e SLR was divided

into 15" arcs of hip flexion as meas- ured by a goniometer. T h e SAEX and SAEXHS were divided into 15" arcs of knee extension as measured by a goniometer. T h e squat was di- vided into the descend phase, the hold phase, and the ascend phase.

Throughout all of these exer- cises, the EMG activity of the five muscles was stored for later retrieval and review. T h e EMG data were converted from analog to digital sig- nals by computer (PDP 1 1/23. Digi- tal Equipment Corporation, Bed- ford, MA) at a sampling rate of 2,500 Hz, rectified, and quantitated by computer integration. After ex- cluding the noise identified by the resting recording, the peak 1 -second EMG signal during a maximal iso- metric MMT was selected as a nor- malizing value (1 00% MMT). Activ- ity patterns during the 15" arcs were assessed every 20 msec and ex- pressed as a percentage of the nor- malization base.

Data Analysis

T h e percent MMT for each muscle during each arc was averaged for the two trials in each subject. A mean and standard deviation were calculated. For each exercise, the arc with highest EMG activity for each muscle was identified and called the

peak arc. One-way repeated meas- ures analyses of variance (ANOVAs) with repeated measures were exe- cuted ( p < .05). Twenty-one sepa- rate ANOVAs were done to com- pare the EMG activity throughout the arc of motion within a muscle for each exercise. For example, the SLR has five arcs of motion (0- 1 5 " , 1 5- 30°, 30-45". 45-60". and 60-75"). and the vastus medialis oblique EMG activity was compared throughout these arcs (ANOVA 1). T h e EMG activity throughout these arcs was compared separately for the vastus lateralis, as it was for the rectus fem- oris (ANOVAs 2 and 3; note the rows in Table 1). Analyses of vari- ance were done for the other exer- cises as well (note the rows in Tables 2-5). Seventeen ANOVAs were exe- cuted to compare the EMG activity between the muscles during each arc of motion for each exercise. For ex- ample, during the SLR, three mus- cles were tested. Thus, the vastus medialis oblique, vastus lateralis, and rectus femoris were compared at the 0-15" arc, and then at the 15-30" arc, the 30-45" arc, the 45-60' arc, and the 60-75" arc (note the col- umns in Table 1). Thus, for this ex- ercise, five separate ANOVAs were done to compare the EMG activity between the muscles during each arc of motion. Likewise, ANOVAs were done for the other exercises (note

AK of Motion (Degrees of hip flexion) Comparison

0-15' 15-30' 30-45' 45-60' 60-75' of Arcs - - - - - ( p c . 0 5 ) - - 1 SO X SD X SO 1 SO SO

VMO 2 6 f 23 2 6 f 1 8 2 5 f 1 8 24+18 2 4 f 21 NSD VL 2 9 f 3 4 2 8 f 2 6 2 9 f 2 5 28+23 27+23 NSD R F 4 2 f 1 8 3 8 f 1 5 3 6 f 1 5 3 3 f 1 3 3 4 f 2 0 NSD

Comparison NSD NSD NSD NSD NSD of muscles

NSD = No signiiicant diiierence. VMO = Vastus medialis oblique. VL = Vastus lateralis. RF = Rectus femoris.

TABLE 1. Means and standard deviations for the straight leg raise exercise (SLR) (muscle activity as a percent of manual muscle test).

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Arc of Motion (Degrees of knee extension)

Comparison of Arcs 45-30' 30-15' 15-0' ( P < .W

Ti SD -

Ti SD X SD

VMO VL RF

16 + 9 28+ 16 56 + 23 15-0' > 30-IS', 45-30" 14f 8 27 + 19 58 f 32 15-0' > 30-IS", 45-30' 24 f 18 28 f 27 41 f 29 NSD

Comparison of muscles NSD NSD NSD ( p < .05)

NSD = No significant difference. VL = Vastus lateralis. VMO = Vastus medialis oblique. RF = Rectus femoris.

TABLE 2. Means and standard deviations for the short-arc knee extension exercise (SAEX) (muscle activity as a percent of manual muscle test).

An: of Motion (Degrees of knee extension)

Comparison of Arcs 45-30' 30-15' 15-0' ( p < .05)

- Ti SD X SD Ti SD

VMO VL RF BF SM

29 + 24 45 f 24 83 f 27 15-0' > 30-IS', 45-30" 19f 8 39f 18 81 f 21 15-0' > 30-IS', 45-30' 15 f 14 25 f 24 50 f 34 15-0' > 30-1 So, 45-30' 25f 19 21 f 16 12f 8 45-30' > 15-0' 17 + 17 16 f 14 13 f 1 1 NSD

Comparison of muscles NSD NSD VM0,VL > RF > BF,SM ( p < .05)

NSD = No significant difierence. RF = Rectus femoris. VMO = Vastus medialis oblique. BF = Biceps femoris. VL = Vastus lateralis. SM = Semimembranosus.

TABLE 3. Means and standard deviations for the short-arc knee extension with hamstring cocontraction exercise (SAEXHS) (muscle activity as a percent of manual muscle test).

Arc of Motion

Descend Hold A d Comparison of A m 0-90' 90' 90-0' ( P < .05)

VMO VL RF BF SM

20+ 10 31 + 13 38 f 16 hold, ascend > descend 16f 1 1 28 f 18 31 f 18 hold, axend > descend 22 + 8 38 + 21 16 + 9 hold > descend 1+2 2 f 2 4 f 5 NSD 1 + 2 1 f 2 3 f 4 NSD

Comparison VMO,VL,RF of muscles VMO,VL,RF VMO,VL,RF > BF,SM

( P < .05) > BF,SM > BFISM VM0,VL

NSD = No significant difference. RF = Rectus femoris. VMO = Vastus medialis oblique. BF = Biceps femoris. VL = Vastus lateralis. SM = Semimembranosus.

TABLE 4. Means and standard deviations for the squat exercise (squat) fmuscle activity as a percent of manual muscle test).

the columns in Tables 2-5). Five ANOVAs were done to compare EMG activity of the peak arc of the five exercises for each muscle (Table 6). When a significant difference was seen, a post hoc Tukey multiple comparison was done. The level of significance was accepted at p < .05.

Straight Leg Raise

During the SLR, the rectus fem- oris consistently had more muscle ac- tivity than the vastus medialis oblique and vastus lateralis in each 15 ' arc tested (Table I , Figure 1). This was best seen during the first 15" of the SLR when the rectus femoris measured 42% MMT, while the vastus medialis oblique and vas- tus lateralis measured 26 and 29% MMT, respectively. However, this difference and the others seen dur- ing the remaining arcs of motion were not statistically significant ( p > .05). When further analyzing the EMG activity of each individual mus- cle from arc to arc, again no signifi- cant difference was seen, although all three muscles demonstrated a trend of less EMG activity as the leg was progressively raised (ie., rectus femoris 42% MMT at 0-1 5' and 34% MMT at 60-75').

Short-Arc Knee Extension

During this exercise, the muscle activity of the rectus femoris, vastus medialis oblique, and vastus lateralis increased as the knee approached full extension (Table 2. Figure 2). However, when comparing the activ- ity of the three muscles during each arc, no one muscle was significantly more active than the next. When further analyzing the individual mus- cle activity from arc to arc, signifi- cance was found. During the 15-0" arc, the vastus medialis oblique had significantly more activity (56% MMT) than during the previous two arcs (28 and 16% MMT). The same

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Arc of Motion

45' 30' 15' Comparison of Arts

( p < .05) Ti so K so Ti so

VMO 23 + 14 26 + 17 40 + 19 NSD VL 19 + 12 21 f 17 36 + 28 NSD RF 2 4 f 19 25+ 16 26 f 18 NSD BF 21 f 21 2 4 f 16 28 f 16 NSD SM 33 + 23 3 4 f 19 34 f 22 NSD

Comparison NSD NSD NSD of muscles

NSD = No significant difference. RF = Rectus lemoris. VMO = Vastus medialis oblique. BF = Biceps fernon's. VL = Vastus lateralis. SM = Semimembranosus.

TABLE 5. Means and standard deviations for the isometric knee cocontraction exercise (KO) (muscle activity as a percent of manual muscle test).

Peak Arc % MMT Comparison of Exercises

SLR SAEX SAEXHS Squat ICO

VMO 26 56 83 38 40 SAEX,SAEXHS > SLR,Squat,lCO

3 1 36 SAEX,SAEXHS > SLR,Squat,lCO

38 26 No significant differences between exercises

4 28 IC0,SAEXHS > SAEX,SLR,Squat

3 34 ICO > SAEX,SAEXHS,SLR,Sauat

VMO = Vastus medialis oblique. SLR = Straight leg raise exercise. VL = Vastus lateralis. SAEX = Short-arc knee extension exercise. RF = Rectus femoris. SAEXHS = Short-arc knee extension with hamstring cocontraction exercise. BF = Biceps femoris. 1CO = Isometric knee cocontraction exercise. SM = Semimembranosus.

TABLE 6. Percent manual muscle test and significant differences for the peak arc of each muscle during each exercise.

was true for the vastus lateralis, dem- onstrating significantly more activity during the 15-0" arc (58% MMT) than earlier in the exercise (27 and 14% MMT). Although the activity of the rectus femoris increased as the knee extended (4 1 % MMT at 15-0" compared with 28 and 24% MMT at 30- 15" and 45-30" arcs, respec- tively), this trend was not significant.

Short-Arc Knee Extension With Hamstring Cocontraction

No one muscle had significantly more EMG activity than the others

during the 45-30" o r 30-15" arc (Table 3, Figure 3). However, dur- ing the 15-0" arc, the activity of both the vastus rnedialis oblique and vastus lateralis (83 and 8 1 % MMT, respectively) was greater than the rectus femoris (50% MMT), biceps femoris ( 1 2% MMT), and semimem- branosus (1 3% MMT, p < .05). Ad- ditionally, the EMG activity of the rectus femoris (50% MMT) during this arc when compared with the bi- ceps femoris (1 2% MMT) and semi- membranosus ( 1 3% MMT) was sig- nificantly more. When comparing in- dividual muscle activity from arc to

arc, the vastus rnedialis oblique had significantly more activity (83% MMT) in the 15-0" arc than during the 45-30" (29% MMT) o r 30-1 5" arcs (45% MMT). This was also true of the vastus lateralis. Its EMG activ- ity was greater during the 15-0" arc (8 1 % MMT) than during the 45- 30" (19% MMT) o r 30-15" arcs (39% MMT). T h e rectus femoris also demonstrated more activity dur- ing the 15-0" arc (50% MMT) than during the 45-30" (15% MMT) o r 30-1 5" arcs (25% MMT). T h e bi- ceps femoris had greater activity ( p < .05) during the 45-30" arc (25% MMT) than during the 15-0" arc (12% MMT).

Squat

In all phases of this exercise (de- scend, hold, and ascend), the vastus medialis oblique, vastus lateralis, and rectus femoris had more activity ( p < .05) than the biceps femoris and semimembranosus (Table 4, Figure 4). T h e vastus medialis oblique and vastus lateralis (38 and 31 % MMT. respectively) demonstrated more ac- tivity than the rectus femoris (1 6% MMT) during the ascend phase (90- 0") of the squat ( p < .05). During the descending and holding phase of the squat, there was no statistical dif- ference between the vastus medialis oblique, vastus lateralis, and rectus femoris. When comparing individual muscle activity from phase to phase, the vastus medialis oblique and vas- tus lateralis had significantly more activity during the hold phase (3 1 and 28% MMT, respectively) and as- cend phase (38 and 3 1 % MMT, re- spectively) than during the descend phase (20 and 16% M MT, respec- tively). For the rectus femoris, the most activity occurred during the hold phase (38% MMT) when com- pared with the descend (22% MMT) and ascend (16% MMT, p < .05) phases. There was no significant dif- ference in the activity of the biceps femoris o r semimembranosus from phase to phase. T h e most activity for

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STRAIGHT LEG RAISE EXERCISE (SLR)

0-15' 15-30' 30-45' 45-60' 60-75'

ARCS OF MOTION

OVMO ~ V L ~ R F

FIGURE 1. Muscle activity during the straight leg raise (means f standard deviations). VMO = Vastus medialis oblique, VL = Vastus lateralis, RF = Rectus lemoris.

SHORT ARC KNEE EXTENSION EXERCISE (SAEX)

- 45-30' 30-15' 15-0'

ARCS OF MOTION

FIGURE 2. Muscle activity during the short-arc knee extension (means + standard deviations). VMO = Vastus medialis oblique, VL = Vastus lateralis, RF = Rectus lemoris.

these two muscles, the biceps femoris and semimembranosus, was during the ascend phase of the squat (4 and 3% MMT, respectively).

Isometric Knee Cocontradion

At each flexion angle tested (1 5, 30, and 45"). there was no signifi- cant difference in the EMG activity of any of the five muscles (Table 5, Figure 5). When comparing individ- ual muscle activity from angle to an- gle, once again, none of the five

muscles showed significant differ- ences ( p > .05).

Comparison of Exercises

Upon comparing all five of the exercises, the vastus medialis oblique and vastus lateralis demonstrated sig- nificantly more activity during the 15-0" arc of the SAEX (56 and 58% MMT) and SAEXHS (83 and 8 1 % MMT) exercises than during any portion of the SLR, squat, o r ICO exercises ( p < .05). With regard to

the rectus femoris, no exercise re- vealed significantly more activity than the next. T h e biceps femoris demonstrated the most activity dur- ing the ICO exercise at 15" (28% MMT) and also during the 45-30" arc (25% MMT) in the SAEXHS ex- ercise ( p < .05). T h e semimembra- nosus revealed the greatest activity during the ICO at 15" (34% MMT, p < .05) (Table 6).

DISCUSSION

Unlike work by Soderberg and Cook (23) and Soderberg et al (24), this study did not find the SLR exer- cise to provide significantly more EMG activity for the rectus femoris than the vastus medialis oblique and vastus lateralis. In fact, none of the five exercises examined during this study provided significantly more ac- tivity for the rectus femoris than the rest. Rather, the rectus femoris was equally active when compared with the other muscles throughout all five exercises. This difference may repre- sent the use of fine wire electrodes used in this study compared with the use of the surface electrodes in other studies, o r it may be due to low sta- tistical power with a sample size of only 12 subjects.

O u r results during the SAEX ex- ercise reflect the findings of Lieb and Perry (I 3). Their study con- cluded that a 60% increase in force was needed to complete the final 15" of extension. It was during those same final 15" in our study that we recorded a doubling of EMG activity for the vastus medialis oblique (28 to 56% MMT) and VL (27 to 58% MMT) when compared with the motion arc of 30- 15". Ad- ditionally, this exercise portrayed the vastus medialis oblique and vastus lateralis with identical muscle action throughout the motion arcs.

Our study suggests that the SAEX and SAEXHS exercises are the most effective exercises in gener- ating vastus medialis oblique and vas- tus lateralis EMG activity. Clinically,

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R E S E A R C H S T U D Y

SHORT ARC KNEE EXTENSION WITH HAMSTRING CO-CONTRACTION (SAEXHS)

- 45-30° 30-15' 15-0"

ARCS OF MOTION

OW0 H V L QRF N B F H S M

FIGURE 3. Muscle activity during the short-arc knee extension with hamstring cocontraction (means + standard deviations). VMO = Vastus medialis oblique, VL = Vastus lateralis, RF = Rectus femoris, BF = Biceps femon's, SM = Semimembranosus.

SQUAT

OW0 H V L URF H B F H S M

FIGURE 4. Muscle activity during the squat (means + standard deviations). VMO = Vastus medialis oblique, VL = Vastus lateralis, RF = Rectus femoris, BF = Biceps femoris, SM = Semimembranosus.

ISOMETRIC KNEE CO-CONTRATION (ICO)

KNEE FLEXION ANGLE

FIGURE 5. Muscle activity during the isometric knee cocontraction (means + standard deviations). VMO = Vastus medialis oblique, VL = Vastus lateralis, RF = Rectus femoris, BF = Biceps femoris, SM = Semimembranosus.

if knee motion is not contraindi- cated, patients would benefit most with these two exercises to restore vastus medialis oblique and vastus lateralis strength as well as rectus femoris strength.

Of major interest when this study began were the cocontraction exercises: SAEXHS and ICO. With regard to the ICO exercise, our study demonstrated balanced o r matched EMG activity for both the knee flexors and extensors. No sin- gle muscle o r muscle group studied revealed significantly more activity than the other at each angle tested, including 30 and 15" of flexion.

During the SAEXHS exercise, balanced EMG activity occurred be- tween the hamstrings and quadriceps groups during the 45-30" and 30- 15" motion arcs. Balance, however, did not carry over into the final 15- 0" motion arc. Rather, significantly more extensor activity was demon- strated than hamstring activity.

Clinically, knee cocontraction exercises have been advocated in the rehabilitation program for patients who have undergone ACL recon- structive surgery (1 9). T h e theory is that hamstring contraction during knee extension will prevent anterior tibia1 translation (27) as well as pre- vent increased strain on the ACL graft (2,8,10,17) and possibly avoid knee flexion contractions. We agree with this assumption and further suggest that effective hamstring ac- tivity must occur for the exercise to be considered a cocontraction. Other studies have revealed EMG activity in the hamstring group (less than 10% MMT) with knee exten- sion (3.6). However, we believe this level of EMG activity is too small to be considered a balanced cocontrac- tion. O u r study revealed no signifi- cant differences between quads and hamstring activity a t every arc ex- cept during the 15-0" arc of SAEXHS. This would suggest that the ICO exercise and SAEXHS exer- cise minus the final 15" of extension may be safe to perform in the ACL- reconstructed patient; also, as afore-

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mentioned, there was a low statistical power.

T h e squat, o r 90" knee bend, i s another exercise used for the patient w i th an ACL reconstruction (20). T h i s study clearly indicates the v i r - tually nonexistent EMG activity o f the biceps femoris and semimembra- nosus muscles dur ing all phases o f this exercise. Significant activity, however, d id occur in the knee ex- tensors (rectus femoris, vastus medi- alis oblique, and vastus lateralis) du r - ing all phases o f the squat. T h i s evi- dence o f a lack o f cocontraction suggests that the therapist should uti l ize the squat w i th caution in the patient w i th a reconstructed ACL.

T h i s study provides descriptive evidence for muscle activity in healthy subjects dur ing early-stage rehabilitation programs. These ref- erence data offer a baseline upon which t o compare fu ture studies done o n patients w i th specific knee disorders.

CONCLUSIONS

T h r e e key points can be re- ported f r om this study:

1 ) T h e final 15" o f extension dur ing the short-arc knee extension exercise o r short-arc knee extension w i th hamstring cocontraction exer- cise demonstrated the greatest EMG activity for the vastus medialis oblique and vastus lateralis.

2) Balanced cocontractions oc- curred w i th short-arc knee extension w i th hamstring cocontraction and isometric knee cocontraction exer- cises at all angles and arcs except dur ing the final 15" o f extension in the short-arc knee extension w i t h hamstring cocontraction exercise.

3 ) T h e squat exercise provided all quadriceps activity but n o ham- string coactivity. JOSPT

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