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induced by co-contraction of muscles in3 areas of the body to the muscle usewith exercise on commercial weight lift-ing equipment. The areas of the bodyundergoing isometric exercise were thearm, trunk, and leg muscles. Subjectsexercised these areas for 25 secondscompared to 3 loads on a commercialTuffStuff Apollo 5900 gym system(TuffStuff Inc, Pomona, CA). The resultsof the experiments showed that isomet-ric co-contraction of muscle while thesubjects were standing still resulted in 5times greater work then exercise on acommercial exercise gym for the musclegroups. Thus isometric exercise againstagonist and antagonist pairs provides agood exercise regime.

Vol. 6, No. 4, 2006 • The Journal of Applied Research

Muscle Use During Isometric Co-contraction of Agonist-AntagonistMuscle Pairs in the Upper andLower Body Compared toAbdominal Crunches and aCommercial Multi Gym ExerciserJerrold Petrofsky, PhD, PTA, JD*†

Jennifer Batt, BS†

Hye Jin Suh, DPT*

Ryan Jones, BS†

Natalia Ushak, BS†

James P. Tucker, BS†

Luke Gentry, BHK†

Vincent Kambe, BS†

Tamara Billings, BS, AS†

*Department of Physical Therapy, Loma Linda University, Loma Linda, California†Department of Physical Therapy, Azusa Pacific University, Azusa, California

KEY WORDS: exercise, exertion,muscle, performance, strength

ABSTRACTIsometric exercise can be used forstrength training. Generally, strengthtraining requires heavy weights to belifted on large pieces of gym equipment.However, the use of co-contraction ofagonist and antagonist muscle pairswhile the subject is standing has beensuggested as a means of isometricstrength training.

In the present investigation, themuscle activity, as assessed by the elec-tromyogram, was examined in 6 maleand 11 female subjects aged 22 to 28years old to compare isometric exercise

The Journal of Applied Research • Vol. 6, No. 4, 2006 301

INTRODUCTIONIt is well established that isometric andisokinetic strength training increasesmuscle strength more than does dynam-ic exercise training.1 By using isometricstrength training with the joints at vary-ing angles, even though the person ismotionless during each exercise, specificmuscles can be trained through therange of motion.1 This type of isometricstrength training can be useful in a vari-ety of clinical situations such as aiding inthe repair of shoulder damage caused byinjuries.2

Strength training is specific to themuscle being exercised3 as is the angleof the joint during training.4 If isometricexercise is accomplished at a fixed jointangle, strength increases little at otherjoint angles. By exercising muscles iso-metrically at various angles, this phe-nomenon can be overcome and strengthcan be increased at all joint angles.4While some of the increase in muscleperformance and strength previously hasbeen linked to motor skill training asso-ciated with isometric exercise,5 morerecent studies show that muscle strengthtraining does increase actin and myosinsynthesis,6 and increases muscle enzymessuch as lactic acid dehydrogenase andcreatinine phosphokinase.1

While isometric strength decreaseswith age7 and increases with body fatdue to the additional load of liftinggreater body weight,8 strength trainingretards much of the aging loss.3 Evenwhen endurance varies during the men-strual cycle, isometric strength is con-stant.9

Isometric strength training is some-what specific. Only sustained isometriccontractions cause an increase in isomet-ric endurance and isometric strengthtogether.10 Dynamic exercise does nothave this effect.1 Some people feel thatthe rapid increase in strength due to iso-metric training is just a change in theelastic properties of muscle. But numer-

ous studies show that there is no changein the elastic property of muscles withisometric training11 and the increase instrength and motor performance isincontrovertibly due to a synthesis ofactin and myosin in muscle.12

Given the above, when musclestrength is weak, as is commonly seenwith bed rest,13 spinal cord injury, stroke,or even athletic injuries, isometric exer-cise has been recommended as astrength training modality.14,15 It hasbeen used with the geriatric populationto increase strength.14,15 It is also usedduring spaceflight where access to largepieces of exercise equipment is not prac-tical.16,17 Isometric training is oftenaccomplished in people who have had aspinal cord injury or stroke by usingelectrical stimulation to elicit musclecontractions.18 Electrical stimulation isalso used for isometric training in non-paralyzed subjects.19

Specific strength training protocolshave great benefits for sports injuries aswell.20 Strength training is beneficial forpreventing back injury and removinglower back pain.21-23 Even in children,increased muscle strength is correlatedto a decreased incidence of back pain.24

There seems to be no difference in theability to train strength in younger andolder women25 and men, therefore iso-metric exercise has a beneficial effect onall populations.

Thus, there seems to be a generalagreement in the literature that isomet-ric strength training develops strengthfast and is beneficial for both preventingand treating many types of therapeuticinjuries. However, most strength trainingregimes involving isometric exercise useeither free weights or large pieces ofequipment such as a roman chair.26

Many other studies use large commer-cial gyms such as Cybex exercise train-ers (Cybex International, Inc; Medway,MA)14,27 or other complex pieces ofexercise equipment.17 Most people are

cient to achieve statistical significanceby power analysis. Medical screeningwas conducted prior to participation toassure subjects were free of cardiovascu-lar, neurological, or orthopedic injuries.The general characteristics of the sub-jects are listed in Table 1. All protocolsand procedures were approved by theInstitutional Review Board of AzusaPacific University and all subjects signeda statement of informed consent.

MethodsIsometric Exercise – Isometric exercisewas accomplished in 3 areas of the body.Subjects were told to simultaneouslycontract agonist and antagonist pairs ofmuscles for 25 seconds and then rest for2 minutes. The exercise included;

Set 1- Contracting the biceps and tri-ceps groups together with the elbowsand 90° and arms forward as shown inFigure 1.

Set 2- Contracting the biceps and tri-ceps groups together with the elbows at90° at the elbow and the shoulderextended by 90° as shown in Figure 2.

Set 3- Contracting the quadricepsand hamstring groups and gluteus max-imus and medius muscles with the kneebent at 5°, the person erect but leaningforward about 15° as shown in Figure 3.

In addition, 10 subjects completed 2subsets of exercise in the set 1 and 2experiments. For these exercises, thesubjects repeated the exercise asdescribed above but were told to con-centrate on using abdominal (rectusabdominus) and back extensor muscles.

Determination of muscle activity – Todetermine muscle activity, the elec-tromyogram (EMG) was used. EMG

Vol. 6, No. 4, 2006 • The Journal of Applied Research302

either reluctant to join a gym or do nothave access to such equipment.

However, in other studies unrelatedto strength training, it was found that co-activation of agonist-antagonist musclepairs, such as the ankle musculature,occurs naturally during maximal isoki-netic dorsiflexion.28 Agonist-antagonistco-contraction is also seen and correlat-ed to strength in stroke patients.29 Thissame phenomenon of co-activation isalso seen for the vastus lateralis musclein pubertal children and adults.30 Thusactivation of agonist and antagonistpairs is normally found in children andadults. For those with a disability, activa-tion of the two muscle groups simultane-ously helps maintain strength.31 In somestudies positive results have been seenwhen electrical stimulation of antagonistmuscle has been used with voluntaryagonist contraction to strength train.32,33

Therefore, rather than using com-plex exercise equipment, activation ofagonist and antagonist pairs simultane-ously seems to be a way of generatingan isometric contraction. At the sametime, this activation of muscle pairswould be quite useful for strength train-ing. The purpose of the present investi-gation was to compare muscle useduring isometric co-contraction of threedifferent muscle groups in the body andcompare these to the muscle activityduring weight lifting with commercialexercise equipment and abdominalcrunches.

MATERIALS AND METHODSSubjectsThe subjects in this study were 6 malesand 11 females in the age range of 22 to28 years old. The sample size was suffi-

Table 1. General Characteristics of the Subjects

Age (years) Height (cm) Weight (kg)Mean ± standard deviation 24.8 ± 1.9 182.8 ± 65.2 73.2 ± 16.4


was recorded by two electrodes and aground electrode placed above theactive muscle.34-39 The relation betweentension in muscle and surface EMGamplitude is linear.35,40 Thus, the ampli-tude of the surface electromyogram canbe used effectively as a measure ofactivity of the underlying muscle by sim-ply normalizing the EMG in terms of amaximal effort. Muscle activity wasassessed by first measuring the maxi-mum EMG of the muscle during a maxi-mal effort and then, during any exercise,assessing the percent of maximum EMGto calculate the percent of muscle activi-ty.35-37 The electrical output from themuscle was amplified with a biopotentialamplifier (EMG 100C amplifier, BiopacInc., Goletta, CA) with a gain of 2000and frequency response, which was flatfrom DC to 1000 Hz. The amplifiedEMG was digitized with a 16-bit analogto digital converter (MP150, BiopacInc.) and sampled at a frequency of 500

Figure 1. A subject using co-contraction ofthe biceps-triceps muscle groups.

Figure 2. A subject using co-contraction ofthe biceps-triceps but with the elbows at 90degrees.

Figure 3. A subject using co-contraction ofhamstrings-quadriceps groups.


samples/sec. The software to analyzedthe EMG was the Acknowledge 3.8.1package (Biopac Inc., Goletta, CA).

Commercial Weight Lifting Equipment –A TuffStuff Apollo 5900 gym system(TuffStuff Inc, Pomona, CA) was usedfor these studies. The exercises usedwere the chest press, biceps curl, tricepscurl, lat pull down, abdominal extension,abdominal flexion, leg extension, legcurl, and leg press (Figure 4).

Abdominal floor crunches – This exer-cise was done supine, with the feet onthe floor, heels 12-18 inches apart, andthe knees flexed. The abdominals wereflexed to lift the shoulders and head offthe floor to an angle of 30 degrees. Thearms were crossed on the chest.

ProceduresThree series of exercises were per-

formed. Prior to any measurements, amaximal isometric contraction wasexerted to measure the maximum EMG

during a 100% effort. This was used tonormalize EMG muscle activity duringexercise to assess the use of the muscles.

Series 1 – During a 25-second iso-metric co-contractions of the appropri-ate muscle groups, EMG activity wasmeasured. Three separate sets of isomet-ric contractions were conducted asdescribed under methods for 25 secondsand repeated 4 times. Also as describedunder methods, two subsets of isometriccontractions was performed in the upperbody exercises to increase abdominalmuscle use. EMG for upper body exer-cise was measured in the biceps, triceps,deltoid, pectoralis major, rectus abdomi-nus, lumbar muscles, and lattisimus dorsimuscles. For lower body exercise, themuscles examined were the quadriceps,hamstring, gastrocnemius, gluteus max-imus, rectus abdominus, and lumbarmuscles.

Series 2 – This series involvedabdominal floor crunches. EMG wasmeasured as described above.

Series 3 – Lastly, subjects used com-mercial weightlifting equipment andcontracted each individual muscle groupagainst 3 different loads as shown inTable 2. EMG was measured duringthese contractions. Subjects were askedto exercise at a normal rate for weightlifting for a period of at least 30 secondsand, from these data, a 25-second aver-age of muscle use was computed.

Data AnalysisData analysis included the calculation ofmeans, standard deviations and t-tests aswell as analysis of variance (ANOVA).The level of significance was P<0.05.

RESULTSSeries 1, Isometric ExerciseWhen the arms were in the forwardposition (Figure 1), isometric exercisewas used to contract agonist-antagonistpairs in the upper body. The EMG activ-ity was high throughout the entire 25-

Figure 4. A subject exercising on the commer-cial weight lifting equipment


second period. The muscle use, asassessed by the EMG is shown for a typ-ical subject in Figure 5. During the 25-second period that the contraction wasmaintained, EMG activity was continu-ous. As shown in Figure 6, even at thebeginning of the 25-second period, mus-cle activity was close to 80% for thebiceps and triceps muscle as well as thepectoralis major muscles. While muscleactivity was low for the rectus abdomi-nus, the deltoids had considerable mus-cle activity as did the back extensors andlatissimus dorsi. Further, during the 25-second period, there was a small butprogressive and significant (P<0.01,ANOVA) increase in muscle activity inthe biceps, triceps, and pectoralis majormuscles, as well as the deltoid musclesshowing muscle fatigue. For example, forthe biceps muscle, the average for thesubjects was 78.6 ± 14.2% muscle activi-ty for the biceps at the onset of the 25-second period. After 15 seconds, EMGactivity increased to 95.8 ± 14.2% andafter 25 seconds, muscle activity hadincreased to 97.6 ± 14.3% of muscleactivity for the biceps.

While the muscle activity on mus-cles like the rectus abdominus were lowaveraging only about 20% of muscleactivity, there was still significant muscleactivity sustained over the 25-second

period. Muscle activity for the backextensors, which average 52.2 ± 19.4%,was significantly higher than the rectusabdominus muscles (P<0.01) for thesubjects examined here. This amountedto half the muscle activity of the backextensors showing significant muscleactivity.

As shown in the bottom of Figure 6,by multiplying the percent muscle activi-ty by the duration of the contraction, thework index can be determined. Thework was found to be was 2266 for thebiceps, 2382 for the triceps, 2120 for thepectoralis major muscles, and for thedeltoid 2122 units. Muscle activity wasless for the rectus abdominus averaginga total work of 472.1. For the backextensors and latissimus dorsi the aver-age was approximately 1200 work units.Thus, for the average of all muscles asshown in Figure 6 by the bar on theright hand side of the Figure, the aver-age work was 1792 units.

In the second isometric exercise (set 2), with the arms facing posterior(Figure 2, triceps workout), muscle usewas also high as shown in Figure 7. Withthe arms placed toward the posterior ofthe body, muscle use was highest for thedeltoid muscles. As was shown in Figure6, muscle activity increased continuouslyover the 25-second period showing mus-

Table 2. Exercise and Workloads

Exercise Trial 1 Trial 2 Trial 3Weight Weight Weight

Chest Press 20 lbs, 9.072 kg 40 lbs, 18.143 kg 60 lbs, 27.215 kgBiceps Curl 20 lbs, 9.072 kg 30 lbs, 13.608 kg 40 lbs, 18.143 kgTriceps Curl 20 lbs, 9.072 kg 30 lbs, 13.608 kg 40 lbs, 18.143 kgLat Pull Down 20 lbs, 9.072 kg 30 lbs, 13.608 kg 40 lbs, 18.143 kgAbdominal Extension 20 lbs, 9.072 kg 40 lbs, 18.143 kg 60 lbs, 27.215 kgAbdominal Flexion 20 lbs, 9.072 kg 40 lbs, 18.143 kg 60 lbs, 27.215 kgLeg Extension 20 lbs, 9.072 kg 40 lbs, 18.143 kg 60 lbs, 27.215 kgLeg Curl 20 lbs, 9.072 kg 40 lbs, 18.143 kg 60 lbs, 27.215 kgLeg Press 60 lbs, 27.215 kg 120 lbs, 54.431 kg 180 lbs, 81.647 kg


cle fatigue for all muscles examined.However, the deltoid muscle use herewas significantly higher than in the set 1experiment (P<0.01). In addition, tricepsactivity was near maximal throughoutthe entire exercise.

One difference in this exercise wasthat the biceps activity was significantlylower than the previous series 1 experi-ment (P<0.05). As shown as Figure 7,rectus abdominus activity was still lowbut was not significantly different thanin the previous set 1 experiments(P>0.05). Back extensor activity was sig-nificantly higher in this series averaging75% of muscle activity over the 25-sec-ond period in the data shown in Figure 6(P<0.01). Latissimus dorsi activity wasalso higher. In this series of experiments,the latissmus dorsi activity was nearlymaximal throughout the entire 25-sec-ond period whereas in the first series ofexperiments it was on the average using50% of muscle activity. This differencewas significant (P<0.01). Looking at thebottom of Figure 7, muscle work aver-aged 2401 work units and the greatestwork done was for the latissimus dorsi,back extensors, and triceps muscles inthis series.

When the protocol was modified in10 subjects to increase the abdominalmuscle activity in the 2 upper body exer-cise protocols (series 1a and 2a), theresults are shown in Figures 8 and 9.

Figure 8 illustrates the first upper bodyisometric series and Figure 9 the secondseries. During the first isometric series,when subjects concentrated on addition-al abdominal use, rectus abdominalactivity increased to an average of 55%maximum muscle activity. Back exten-sors increased to 76% muscle activity.This increased muscle work to 1370 and1900 units, respectively. The increase inabdominal flexor and extensor activitywas significant (P<0.01). In the secondmodified set, a similar increase was seenboth in muscle use and total work. Herework was significantly higher for theback extensor and rectus abdominusmuscles as shown in Figure 9. Othermuscle groups did not statisticallychange when comparing series 1 to 2.

Isometric exercise was accomplishedin the lower body by contracting themuscles while leaning forward as wasshown in Figure 3. A typical EMG isshown in Figure 10. Muscle use is shownin Figure 11 in the upper panel andwork in the lower panel. When lowerbody exercise was accomplished, thegreatest muscle activity was in thequadriceps muscle. Muscle activity aver-aged 93.2 ± 16.7% at the beginning ofthe 15-second period and increased to98.5 ± 26.3% at the end of the 15-secondperiod in the quadriceps. Muscle activityfor the rectus abdominus was lowestaveraging at 17.3% of total muscle activ-ity. Rectus abdominus activity in allthree series (1, 2, and 3) of experimentswas not significantly different from eachother (ANOVA, P>0.05) except asdescribed for series 1 and 2a. Whenseries 1 and 2 were repeated with con-scious core muscle use, there was a sig-nificant increase in rectus abdominusmuscle activity. Gastrocnemius activitywas also high averaging 85% of muscleactivity over the 25-second period. Backextensor muscle activity was low averag-ing only 30% of muscle activity. Whenexamining the work in the lower panel

Figure 5. The EMG during upper body isomet-ric contraction-co-contraction of muscle


Figure 6. This graph shows the muscle use of the biceps, triceps, pectoralis major, rectus abdomi-nus, deltoid, back extensors, and latissimus dorsi at the start, halfway through, and at the end of25 seconds of a sustained isometric co-contraction of the upper body for the series 1 experi-ment described under methods. While the actual muscle use is shown in the bar graph in theupper panel, the muscle work (present muscle use times the duration in seconds) as an averageover the 25-second period is shown in the lower panel. Each point is shown with the standarddeviation.


Figure 7. This graph shows the muscle use of the biceps, triceps, pectoralis major, rectus abdomi-nus, deltoid, back extensors, and latissimus dorsi at the start, halfway through, and at the end of25 seconds of a sustained isometric co-contraction of the upper body for the series 2 experi-ment described under methods. While the actual muscle use is shown in the bar graph in theupper panel, the muscle work (present muscle use times the duration in seconds) as an averageover the 25-second period is shown in the lower panel. Each point is shown with the standarddeviation.


Figure 8. This graph shows the muscle use of the biceps, triceps, pectoralis major, rectus abdomi-nus, deltoid, back extensors, and latissimus dorsi at the end of 25 seconds of a sustained isomet-ric co-contraction of the upper body for the series 1 experiment described under methods whenabdominal use was increased by a different protocol. While the actual muscle use is shown inthe bar graph in the upper panel, the muscle work (present muscle use times the duration inseconds) as an average over the 25-second period is shown in the lower panel. Each point isshown with the standard deviation.


Figure 9. This graph shows the muscle use of the biceps, triceps, pectoralis major, rectus abdomi-nus, deltoid, back extensors, and latissimus dorsi at the end of 25 seconds of a sustained isomet-ric co-contraction of the upper body for the series 2 experiment described under methods witha protocol to increase abdominal muscle use. While the actual muscle use is shown in the bargraph in the upper panel, the muscle work (present muscle use times the duration in seconds)as an average over the 25-second period is shown in the lower panel. Each point is shown withthe standard deviation.


of Figure 11, total work here was signifi-cantly less then the other two series ofexperiments, averaging 1266 work units(P<0.01). The greatest work was accom-plished for the quadriceps at 1996 workunits followed by the gastrocnemius at872 work units. For the hamstring, workwas also high at 1660 work units. Themuscles with the lowest activity were therectus abdominus muscles followed bythe back extensors.

Series 2 - Abdominal crunchesFigure 12 shows the abdominal crunchresults. Here the majority of the muscleuse was seen in the abdominal muscles.Total work was 233.8 units for this exer-cise and muscle use was 28.7% of themuscle activity. The duration of eachcrunch from the start of one to the startof the other was 2.1± 0.4 seconds.

Series 3- Commercial Weight LiftingEquipment Exercising on the commercial weightlifting equipment resulted in muscle usethat was more specific than which wasseen for the isometric exercise. Musclework in the chest press is shown inFigure 13. The average muscle work dur-ing the chest press for the lowest workload averaged 117.6 units of work. Thegreatest activity was for the triceps, pec-toralis major and the latissmus dorsimuscles. Even with these muscle groups,total work only averaged about 176.8

units. This was due to the fact that mostmuscle activity was during 1-second win-dows during the concentric phase of theexercise with little work at the onset andeccentric phase of each lift. Subjectshere lifted continuously at a natural rateselected by the subject. Total work hereand below was calculated over a 25-sec-ond work period.

Activity of the biceps averaged16.1% whereas for the triceps it aver-aged 38.1%. For pectoralis major, it was44.2%; rectus abdominus, 10.6%; backextensors, 30.3%; and for the latissimusdorsi, 37.1%. Thus, the muscle was notextensively active during exerciseagainst a 20 pound work load for any ofthe muscle groups examined. Therefore,is not surprising that in Figure 13, forthe chest press, the average total workwas very low. In contrast, as the workload was increased to 40 and 60 pounds(Table 2), muscle use increased. Thus, forthe highest work load of 60 pounds, theaverage work increased to 329.4 workunits as shown in the bottom panelabove this Figure. The work of the tri-ceps increased to 451.7 and pectoralismajor to 441.1 units. Here the greatestmuscle activity was only during 25% ofa given contraction cycle when concen-tric work was at its greatest.

During initial flexion, sustained lift-ing and eccentric contraction, muscleactivity was low. Thus by normalizing thework over a 25-second period as wasdone in Figure 13, the total work overthat period of time was substantiallylower than that of any of the isometricseries of contractions (P<0.01). There-fore total work was about 25% of thatof an isometric contraction even against60 pounds of weight on the chest pressexercise machine. For example, for theheaviest work load, the greatest muscleactivity was seen for the triceps whichwas 97% active. However, since it wasonly active less then 25% of time, totalwork was low over this period.

Figure 10. The EMG during co-contraction ofthe lower body muscles.


Figure 11. This graph shows, in the upper panel, the percent muscle use for the gluteus maximus,hamstring, quadriceps, rectus abdominus, gastronemius, and back extensor muscles at the start,middle, and at the end of the sustained isometric co-contraction of the leg muscles over a 25-second period. While the upper panel shows muscle use, the lower panel displays the total workaccomplished by each muscle group during the 25-second period. Each point is shown with thestandard deviation.


The same pattern results were seenfor the biceps curl in Figure 14, the tri-ceps curl in Figure 15, and the lat pulldown in Figure 16. For the biceps curl,for example, at the heaviest work load,the biceps muscle was 95.3% active. Thetriceps was 45.4% active whereas therectus abdominus was 17.9% active. Thepectoralis major was 44.6% active andthe back extensor muscles were exten-sively used here and were 71.4% active.This was also true for the lats whichwere 53.8% active. In spite of this,because of the low duty cycle on work,the total work done for the heaviestwork load, only average 280.3 workunits whereas at the lowest load thework was 181.7 work units.

For the triceps, as might be expect-ed, the greatest muscle activity was forthe triceps exercise (Figure 15) with theheaviest resistance. The pectoralis majorshowed the greatest work load, averag-ing 79.8% of muscle activity, whereasback extensors average 51.6% and lats67.0% of muscle activity. This corre-

sponded to average work for the tricepsaveraging 294.5 units at the lowest workload and 524.3 units for the heaviestwork load. The average of all the musclegroup however, even if the heaviestwork load was 331.3 work units. Thesame basic results were seen for the latpull down. For the lat pull down (Figure16), even at the heaviest work load, thelats were 96.8% active but there wasalso activity in the biceps and tricepsand back extensor muscles. Back exten-sors were 63.3% active whereas the tri-ceps were 59.3% active. This translatedto a work for the lats of 326.4 units forthe lightest work load, 304.4 units forthe moderate work load and 335.2 unitsfor the heaviest work load. Oddlyenough, the triceps average was 524.3units for the heaviest work load duringthe lat pull down exercise. However, theaverage work over the 25-second periodwas only 331.3 units even for the heavi-est work load.

Similar results were seen for thelower body exercises. These are shown

Figure 12. This graph shows the muscle use compared to total work during the crunches over acomfortable 25-second period as that shown in the figures above. Each point is shown with thestandard deviation.


Figure 13. This graph shows the average work for the biceps, triceps, pectoralis major, rectusabdominus, back extensors, latissimus dorsi and the average of all muscle groups during 25 sec-onds of exercise on the chest press for the low work load (upper panel) moderate work load(middle panel), and heaviest work load (lower panel). Each point is shown with the standarddeviation.


Figure 14. This graph shows the average work for the biceps, triceps, pectoralis major, rectusabdominus, back extensors, latissimus dorsi and the average of all muscle groups during 25 sec-onds of exercise on the biceps curl for the low work load (upper panel) moderate work load(middle panel), and heaviest work load (lower panel). Each point is shown with the standarddeviation.


Figure 15. This graph shows the average work for the biceps, triceps, pectoralis major, rectusabdominus, back extensors, latissimus dorsi and the average of all muscle groups during 25 sec-onds of exercise on the triceps curl for the low work load (upper panel) moderate work load(middle panel), and heaviest work load (lower panel). Each point is shown with the standarddeviation.


Figure 16. This graph shows the average work for the biceps, triceps, pectoralis major, rectusabdominus, back extensors, latissimus dorsi and the average of all muscle groups during 25 sec-onds of exercise on the lat pull down for the low work load (upper panel) moderate work load(middle panel), and heaviest work load (lower panel). Each point is shown with the standarddeviation.


Figure 17. Illustrated here is the muscle work in the gluteus maximus, hamstring, quadriceps, gas-tronemius, rectus abdominus, and back extensor muscles as well as the average for the group(summary) showing muscle activity during a leg press of the low (upper panel) moderate (mid-dle panel) and heaviest (lower panel) workloads. Each point is shown with the standard devia-tion.


Figure 18. Illustrated here is the muscle work in the gluteus maximus, hamstring, quadriceps, gas-tronemius, rectus abdominus, and back extensor muscles as well as the average for the group(summary) showing muscle activity during abdominal flexion exercises of the low (upper panel),moderate (middle panel), and heaviest (lower panel) workloads. Each point is shown with thestandard deviation.


Figure 19. Illustrated here is the muscle work in the gluteus maximus, hamstring, quadriceps, gas-trocnemius, rectus abdominus, and back extensor muscles as well as the average for the group(summary) showing muscle activity during back extension exercises of the low (upper panel),moderate (middle panel), and heaviest (lower panel) workloads. Each point is shown with thestandard deviation.


Figure 20. Illustrated here is the muscle work in the gluteus maximus, hamstring, quadriceps, gas-trocnemius, rectus abdominus, and back extensor muscles as well as the average for the group(summary) showing muscle activity during knee extension exercises of the low (upper panel),moderate (middle panel), and heaviest (lower panel) workloads. Each point is shown with thestandard deviation.


Figure 21. Illustrated here is the muscle work in the gluteus maximus, hamstring, quadriceps, gas-trocnemius, rectus abdominus, and back extensor muscles as well as the average for the group(summary) showing muscle activity during hamstring flexion exercises of the low (upper panel),moderate (middle panel), and heaviest (lower panel) workloads. Each point is shown with thestandard deviation.


in Figure 17 (leg press), Figure 18(abdominal flexion), Figure 19 (backextension), Figure 20 (knee extension),and Figure 21 (hamstring flexion exer-cises). As shown in Figure 17, for the legpress, even at the heaviest work load,the average muscle activity was greatestfor the back extensor muscles. This aver-aged 76.5% muscle activity followed bythe quadriceps muscle which averaged73.3% muscle activity whereas the othermuscles (gluteus maximus, hamstring,and rectus abdominus) averaged lessthen 45% muscle activity even withheaviest work load. The average workfor the leg press accomplished over the25-second period for the lowest workload was 178 units. The work increasedto 216.7 units at the moderate work loadand to 275.1 units at the heaviest workload. Total work over the 25-secondperiod was significantly less than duringany of the isometric exercises (P<0.01).

As shown in Figure 18 for theabdominal flexor exercises, total workincreased from 83.6 units to a maximumof 200.6 units for the heaviest work loadeven though the rectus abdominus mus-cles average 489.9 work units due totheir high activity. Rectus abdominusmuscles averaged 95.6% muscle activityat the heaviest work load. However, therest of the muscles examined in thebody worked little during this exercise.

As shown in Figure 19, similarresults were seen for the back extensorexercises. The back extensors work start-

ed at 219 units and increased to 354units of work and were almost fullyactive during the heaviest work loadaveraging 70.8% of muscle activity. Theyand the hamstring muscles contributedmost to the total work which averaged192.5 units during exercise of the heavi-est work load.

As shown in Figure 20, for the kneeextension exercises, the quadriceps wasthe most active muscle. The work startedat 329.4 work units for the lightest workload and, for the heaviest work load,averaged 443.8 work units. Most othermuscles except the back extensors werequiescent. Back extensor activity wasapproximately half that of the quadri-ceps muscle increasing the total work to148.3 units for the 25 seconds of exer-cise.

As shown in Figure 21, similarresults were seen for the hamstring flex-ion exercises. As would be expected, forthe hamstring muscle exercises, the ham-string was most active muscle groupstarting at 332.5 work units for the light-est work load. However, to supportactivity of the hamstring muscles, theback extensors were also used with anaverage of 210.2 work units as was thegastrocnemius muscle an average of175.1 work units. For the heaviest workload, (see Table 2) hamstring work hadincreased to 505.9 work units, gastrocne-mius work was 439.0 work units, andback extensors was 413.1 work units,yielding an average work over the 25-

Table 3. Equivalent Crunches to Isometric Exercise

Exercise Set 1a 2 3 CrunchWork

25 seconds 1375 556 374 233Work

2 minutes 6600 2592 1795 1118Equivalent crunch 212 87.8 58 366-minute total 444 108


second period of 296.7 work units.

DISCUSSIONBy sustaining a contraction continuously(isometric exercise), numerous phenom-ena occur in muscle. First, blood flow ispartly or fully occluded. Unlike dynamicexercise where muscles contract andrelax and pump blood actively back tothe heart,41 by sustaining continuous iso-metric tensions, the intramuscular pres-sure increases in muscle in proportion totension.41 Tension in muscle (intramus-cular pressure) as high as 1400 mmHghas been reported in muscle during iso-metric exercise.42,43 This in itself limitsblood flow to muscle. When intramuscu-lar pressure exceeds arterial pressureblood cannot perfuse muscle throughthe arteries. In addition, in some mus-cles, the mechanical force of the tendonsand fascia above the muscle actually nipthe arteries causing occlusion of the cir-culation. For example, in the handgripmuscles, for contractions up to 70% of amuscle’s maximum strength, there is stillsome blood flow during the exercise.44 In

contrast, for muscles such as the gastroc-nemius, which are in a pennate arrange-ment, very high forces are developed inthe muscles. In these muscles, any con-traction above 40% of the muscle’s max-imum strength results in completeocclusion of the circulation.45 Thus, mus-cles exercising isometrically exerciselargely anaerobically and thus the fuelused within the exercising muscle cannot be fats or proteins and must be glu-cose and glycogen during the exer-cise.46,47 Some of the lactate during theexercise requires aerobic metabolism offats in the liver. After the exercise, thebalance of the lactate produced in mus-cle and the energy depletion from theexercise is replenished by burning fat.46-48

With the mitochondria deprived ofoxygen, free oxygen radicals generatedby products such as hydrogen peroxideenhance the production of nitric oxidein the mitochondria. Reduced ATP lev-els cause the production of AMP(adenosine mono phosphate) and acti-vated protein kinase.49,50

Sustained isometric contractions for

Table 4. Equivalent Workloads on Commercial Equipment Compared to Isometric Exercise

Maximum Work Isometric Work Muscle Low Medium High Series Series Group Load Load Load Low Medium High

Chest Press Series 2 2417 triceps 152 255 451 15.9 9.5 5.4

Biceps Curl Series 2 2380 biceps 418 431 476 5.7 5.5 5.0

Triceps Curl Series 2 2417 triceps 294 327 524 8.2 7.4 4.6

Lat Pulldown Series 2 1357 lats 326 328 335 4.2 4.1 4.1

Leg Press Series 2a 2040 back 300 352 449 6.8 5.8 4.5extensor

Abdominal Flex Series 1a 1375 rectus 200 277 489 6.9 5.0 2.8abdominus

Back Extension Series 1a 1910 back 219 327 354 8.7 5.8 5.4extensor

Quadriceps Flexion Series 3 1996 quadriceps 418 431 443 4.8 4.6 4.5

Hamstring Series 3 1662 hamstring 332 365 505 5.0 4.6 3.3

Leg Press Series 3 1388 gluts 115 188 210 12.1 7.4 6.6


as long as 20 to 30 seconds have beenassociated with increases in the synthesisof actin and myosin in the muscle,4,12

probably mediated by these same path-ways. This short reduction in cellularATP concentration seems to be pivotalin strength training. Rhythmic exercise,while increasing metabolism, does littleto deplete ATP and increase musclestrength.51

However, for isometric training tobe effective, it must be muscle specific.In other words, a single muscle mustbecome active and sustain a contractionto train.4 The normal scheme in the cen-tral nervous system is when one musclefatigues, other muscles are used.51,52 Bysubstituting other muscles, fatigue isminimized. This is a contraindication towhat would be best for muscle training.For muscle training, we would like tototally fatigue muscles, whereas for nor-mal muscle use, the central nervous sys-tem tries to prevent fatigue in muscles.By forcing continuous muscle activity,multiple muscle groups fatigue.

In the present investigation, sus-tained isometric contractions wereaccomplished with agonist and antago-nist pairs. This offers several advantages.First, weights or heavy exercise equip-ment is not needed for exercise. For air-line travelers or even pilots, lightexercise programs have been recom-mended which can be accomplishedwhile seated.53,54 The limbs are simplymoved in circles and, with no load onmuscle, little training can be accom-plished. With isometrics, exercise couldbe accomplished by using agonist andantagonist pairs. This type of exercisecould condition both muscle14 and thecardiovascular system.55 Thus, whethersimply using the leg muscles for isomet-ric training for travelers and airlinepilots, all the muscles in the body can beexercised. This is also true for spaceflight. For space flight, exercise hasalways been an issue since there is

severe atrophy of muscle associated withliving in microgravity environments.16

However, agonist and antagonist pairco-contraction would be quite useful forthe space program. Third, sedentaryindividuals who have limited mobilityand difficulty exercising on the floorwould find this type of exercise easy toaccomplish with good therapeutic bene-fits.

In the present investigation, for the3 different agonist and antagonist exer-cises examined, the muscle use was verybroad and continuous compared to com-mercial weight lifting equipment. Forabdominal crunches, as shown in Table3, the work for the 25-second period forthe rectus abdominus muscles duringcrunches and for the set 1a, 2 and 3 iso-metric contractions for the same musclegroup is shown. For a given 25-secondperiod, the work accomplished by thesubjects during abdominal crunches wasas much as 5 times less when comparedto the work for the 3 isometric regimes.The isometric protocols, while 25 sec-onds long, were repeated 4 times over a2-minute period. Extending that work to2 minutes and then examining the totalwork over the 6 minute workout with all3 exercises, the total work was 10,987units during the 6-minute period withthe 3 isometric exercise regimes but only3,354 work units for the crunches. Overa 6-minute period, if crunches were donecontinuously at the rate that the subjectsexercised here, they would have done108 crunches. In the same 6-minute peri-od, with the isometric protocol, theequivalent work on the rectus abdomi-nal muscles was equal to 444 sit ups.

A similar situation was seen for thecommercial weight lifting equipment. Tobegin with, isometric exercise utilizesmore muscle groups than the limitednumber of muscles that are exercisedeither during sit ups or during exerciseon the commercial weight lifting equip-ment. However, looking at a single mus-


cle groups, Table 4 shows only the onemuscle group with the greatest activityof all muscles examined and for eachexercise. Activity is listed as work donein the 25-second period. Work is shownfor the low, medium, and high workloads. The muscle group that exercisedthe most for a given exercise is also list-ed. These data are compared to the datafrom the isometric exercise. The exerciseset that exercised the same musclegroup the most is listed and the workfrom that data base is also shown.Finally, the isometric exercise for thatgroup is compared to the commercialweight lifting equipment exercise as aratio of isometric work to commercialweight lifting work on the same musclegroup. As shown in Table 4, the series1a, 2, 2a, and 3 exercises caused between4.1 and 15.9 times the exercise for differ-ent muscle groups than did commercialweight lifting equipment. While this isinteresting in itself, when added to thefact that more muscle groups exercisewith isometric than the commercialequipment, the isometric protocol is amuch better form of exercise. In fact,using isometric exercise for 6 minuteswould be the equivalent muscle work of30 to 35 minutes of gym work on com-mercial weight lifting equipment.

Thus, the isometric co-contraction ofmuscle used in these experiments was anexcellent method of exercising muscle. Itoffers the advantage of not relying oncommercial exercise equipment.Further, for sedentary people whowould have a difficult time lying on thefloor, the exercise can be done standing.This type of exercise can be done bysomeone traveling or almost anywhere.It is short in duration so that it does nottake much time to accomplish.

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