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Changes of quadriceps muscle injured by eccentric exercise

Functional changes of humanquadriceps muscle injured byeccentric exercise

1Departamento de Fisioterapia, Unidade de Plasticidade Muscular,Universidade Federal de São Carlos, São Carlos, SP, Brasil2Grupo de Ressonância Magnética, Instituto de Física de São Carlos,Universidade de São Paulo, São Carlos, SP, Brasil

F.V. Serrão1, B. Foerster2,S. Spada1, M.M.B. Morales1,

V. Monteiro-Pedro1,A. Tannús2 and

T.F. Salvini1

Abstract

The present study evaluated functional changes of quadriceps muscleafter injury induced by eccentric exercise. Maximal isometric torqueof quadriceps and the surface electromyography (root mean square,RMS, and median frequency, MDF) of the vastus medialis oblique(VMO) and vastus lateralis (VL) muscles were examined before,immediately after and during the first 7 days after injury. Serumcreatine kinase (CK) levels and magnetic resonance imaging (MRI)were used to identify muscle injury. The subject was used as her owncontrol and percent refers to pre-injury data. Experiments were carriedout with a sedentary 23-year-old female. Injury was induced by 4 boutsof 15 maximal isokinetic eccentric contractions (angular velocity of5º/s; range of motion from 40º to 110º of knee flexion). The isometrictorque of the quadriceps (knee at 90º flexion) decreased 52% immedi-ately after eccentric exercise and recovered on the 5th day. The highestreduction of RMS occurred on the 2nd day after injury in both VL(63%) and VMO (66%) and only VL recovered to the pre-injury levelon the 7th day. Immediately after injury, the MDF decreased by 5 and3% (VMO and VL, respectively) and recovered one day later. SerumCK levels increased by 109% on the 2nd day and were still increasedby 32% on the 7th day. MRI showed large areas of injury especially inthe deep region of quadriceps. In conclusion, eccentric exercisedecreased the isometric torque and electromyographic signals ofquadriceps muscle, which were recovered in one week, despite themuscle regeneration signals.

CorrespondenceT.F. Salvini

Departamento de Fisioterapia, UFSCar

13565-905 São Carlos, SP

Brasil

Fax: +55-16-261-2081

E-mail: tania@power.ufscar.br

Presented at the XVII Annual Meeting

of the Federação de Sociedades de

Biologia Experimental, Salvador, BA,

Brazil, August 28-31, 2002.

Research supported by FAPESP

(No. 01/12352-1) and CNPq (No.

351197/92-3). S. Spada was the

recipient of a fellowship from

PIBIC-CNPq.

Received May 8, 2002

Accepted February 11, 2003

Key words• Isometric torque• Surface electromyography• Quadriceps muscle• Muscle injury• Eccentric exercise• Magnetic resonance imaging

Eccentric exercise has been used as aphysiological model to induce muscle injuryin humans (1). It is well known that maximaleccentric muscle contractions can producedisruptions in the Z bands of sarcomeres,especially in muscles of people with seden-tary habits (2) or animals (3).

Skeletal muscle injury induced by eccen-

tric exercise has been associated with musclesoreness (4), force reduction (5), changes inthe electromyographic (EMG) signal (6), andincreased serum levels of muscle proteinssuch as creatine kinase (CK) (7). In addition,magnetic resonance imaging (MRI) of skel-etal muscle permits the determination of thelocation and extent of the injury (8).

Brazilian Journal of Medical and Biological Research (2003) 36: 781-786ISSN 0100-879X Short Communication

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Most of the experiments used to studymuscle injury and regeneration have em-ployed invasive methods in animals. Al-though many mammals have similar pro-cesses of skeletal muscle regeneration, it isnecessary to improve the evaluation of non-invasive procedures used for the analysis ofmuscle injury and regeneration in humans.Noninvasive techniques such as surface EMG(9), force (5) and MRI (10) have been used.

In the present study we evaluated themaximal isometric torque of the quadricepsfemoral muscle and the electrical activity ofthe vastus medialis oblique (VMO) and vastuslateralis (VL) daily during the first week ofthe regeneration process after the muscleinjury induced by eccentric exercise. Additi-onally, in order to detect muscle injury, wedetermined serum CK levels at three timesduring the study: before exercise and 2 and 7days after injury. The MRI of the quadricepsfemoral muscle was evaluated before exer-cise and daily during the 7-day recoveryperiod.

A 23-year-old healthy woman with a sed-entary lifestyle gave informed consent toparticipate in the study, which was approvedby the Ethics Committee for Human Studiesof the Federal University of São Carlos, SãoCarlos, SP, Brazil. The participant did nothave any orthopedic diseases and had notbeen previously involved in weight training.The subject was used as her own control andresult (%) refers to pre-injury data.

Eccentric exercise and torque measure-ments were executed using a Multi-JointSystem 2 Isokinetic Dynamometer (BiodexMedical Systems, New York, NY, USA).Muscle injury was induced by eccentric ex-ercise of the right quadriceps muscle. Thevolunteer was submitted to a total of 4 bouts,each with 15 maximal isokinetic eccentriccontractions and a 5-min rest interval be-tween bouts. The contractions were carriedout at an angular velocity of 5º/s and the kneeflexion ranged from 40º to 110º.

In order to determine the maximal iso-

metric torque of the quadriceps muscle, sixindependent maximal isometric contractionswere performed. Each contraction was main-tained for 4 s with a 2-min rest intervalbetween contractions. The volunteer waspositioned in such a way that the hip andknee formed angles of 100º and 90º, respec-tively. The mean value and standard devia-tions of the six measurements were consid-ered for the torque measurements during thestudy.

The same isometric muscle contractionswere used to evaluate both the torque and thesurface EMG. In the current experimentalsetup the instruments for torque and EMGmeasurements could not be synchronizedexactly due to manual initiation of the ex-periments. However, the small delay of somefractions of a second was negligible com-pared to the 4-s duration of each contraction.Unlike torque measurement, EMG permitsmeasurements on individual muscle compo-nents and was used in the present study tomeasure VMO and VL.

The electrical activity of the VMO andVL muscles was determined using simpleactive differential surface electrodes (LynxElectronics Technologies, São Paulo, SP,Brazil) and a 16-channel signal-condition-ing module (1000-V2, Lynx ElectronicsTechnologies). The simple active differen-tial surface electrodes consist of two rectan-gular parallel bars of Ag/AgCl (1 cm inlength, 0.2 cm in width and separated by 1cm). These bars are coupled to a rectangularacrylic resin capsule 2.2 cm in length, 1.9 cmin width and 0.6 cm high. In addition, theelectrodes have a common mode rejectionratio with a minimum of 80 dB, an internalgain of 20 times and input impedance higherthan 10 GΩ.

The signal-conditioning module (1000-V2) was interfaced with a PC computer andhad a digital analogue A/D converter (CAD12/32-60K, Lynx Electronics Technologies)with a resolution of 12 bits, an acquisitionfrequency of 1000 Hz per channel and an

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Changes of quadriceps muscle injured by eccentric exercise

Aqdados data acquisition program version4.6 (Lynx Electronics Technologies). Thisequipment also has a Butterworth type filterwith a bandpass of 10.6 to 509 Hz and a gainof 50.

The EMG data were filtered with abandpass of 10 to 450 Hz using post-pro-cessing procedures based on functions de-veloped to calculate the root mean square(RMS, in µV) and the median frequency(MDF, in Hz) and introduced in the Matlab5.0 software. A Hanning window with a90% overlap for the processing of Fast Fou-rier transformation was used to calculate theRMS and the MDF and the data were nor-malized by the mean.

The preparation of the experiment in-cluded shaving and cleaning the volunteer’sskin at the positions of the electrodes. Also,palpation of the muscle belly with the sub-ject in the testing position was used to deter-mine electrode placement. The electrodeswere fixed with micropore adhesive tape atthe midline of the muscle belly with theirdetection surface perpendicular to the musclefibers (11). The reference electrode was fixedover the proximal anterior tibia shaft to elimi-nate possible external interferences. To en-sure the reproducibility of the different meas-urements along the one-week duration of thestudy, a plastic mold of the quadriceps waselaborated, on which the positions of everyelectrode were identified.

In a separate experiment but under iden-tical conditions, the contralateral femoralquadriceps of the same volunteer was stud-ied by MRI for visual determination of theeffectiveness of eccentric exercise in induc-ing muscle injury. The MRI of the contralat-eral quadriceps femoral muscle was evalu-ated before and during the first 7 days afterinjury induced by eccentric exercise. Theexperiments were executed using a 0.5 TeslaNMR scanner developed by the MagneticResonance Group of the Physics Institute,São Paulo University, São Carlos, SP, Bra-zil. Axial images of the thigh were obtained

using the inversion-recovery technique with2000-ms repetition time, 140-ms inversiontime and 70-ms echo time. The field of viewof the images was 256 x 256 mm with a 150x 250 point acquisition matrix, slice thick-ness of 15 mm and 4 averages. Second orderflow compensation was used in all threedirections.

The two complementary techniques usedto investigate muscle injury, torque meas-urement and EMG were applied once beforethe eccentric exercise for reference purposes,immediately after the exercise and then on adaily basis to monitor the recovery processover a period of 7 days. In addition to con-stant monitoring, the serum CK levels weredetermined at three time points during thestudy, i.e., immediately before the exerciseand on the 2nd and 7th day, in order todetermine the possible presence of muscleinjury. The CK measurements were carriedout using the CK-NAC UV kit from UnitestWiener Laboratory (Rosario, Argentina).

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Figure 1. Effect of eccentric exercise on maximal isometric torque (A), root mean square(B), median frequency (C), and creatine kinase (D) before (Pre) and immediately after (Post)exercise, and on the 7 days following exercise. Results are reported as means ± SD for onefemale subject. VL = vastus lateralis; VMO = vastus medialis oblique.

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Figure 1A-C shows the results of torqueand EMG measurements. Immediately afterthe eccentric exercise, there was a decreasein maximal isometric torque, as well as in theRMS and MDF values of both VL and VMOmuscles. The maximal isometric torque ofthe quadriceps muscle decreased by 52%compared to its pre-exercise level and quicklystarted to recover, continually improving untilreaching its pre-exercise level between the4th and 5th day (Figure 1A). The MDF val-ues showed a similar behavior (Figure 1C)demonstrating a small initial decrease of 3and 5% in VL and VMO, respectively. TheRMS measurements also showed a 48 and30% decrease of VL and VMO muscles,respectively, but, in contrast to the otherexperiments, they continued to decrease upto the 2nd day (Figure 1B).

When the recovery period was analyzed(days 1 to 7), a different behavior of maxi-mal isometric torque, RMS and MDF wasobserved. The maximal isometric torque ofthe quadriceps gradually recovered to pre-injury levels by the 5th day (Figure 1A).Although the largest reduction in RMS forboth VMO and VL muscles occurred on the2nd day after injury (63 and 66%, respec-tively), the VL muscle recovered the pre-injury values of RMS by the 7th day, whileVMO did not recover them until the 7th day(Figure 1B).

The decrease of isometric torque, RMSand MDF, identified in the quadriceps muscleimmediately after the eccentric exercise, in-dicates a failure in the mechanism of musclefiber contraction, probably associated withmuscle fiber fatigue. The difference in RMSbetween VMO and VL during the regenera-tion period cannot be easily explained, butone possibility that could be considered isthat VMO muscle fibers were injured by theeccentric exercise more than VL fibers. Un-fortunately, it was not possible to comparethe extent of injury between VMO and VL.

It has been reported that RMS representsthe number of active motor units during

muscle contraction, and is usually describedas a measurement of skeletal muscle activity(12). As the maximal isometric torque is alsoa measurement of muscle activity, reductionin both isometric torque and RMS, as dem-onstrated in the present study, may be asso-ciated with muscle injury. Previous studiesusing eccentric exercise to induce injury inthe quadriceps muscle also reported a de-crease in both muscle force (4) and RMS(13).

Immediately after injury, MDF decreasedfor both muscles (Figure 1C). A decrease inMDF after eccentric exercise was also ob-served in other studies (14,15). The decreasein MDF could be attributed to decreasedconduction velocity of the active musclefibers (16,17). Decreased muscle fiber con-duction velocity has been associated withproton accumulation (16). Generally, MDFmay decrease more if the blood lactate con-centration is high, although changes werealso observed in the absence of blood lactate(18). Other factors could also be associatedwith the decrease in MDF, such as impair-ment of the excitation-contraction coupling(18) and the selective injury of the fast twitchfibers after eccentric exercise (2), which iscompensating for an increased activity of theslow twitch fibers trying to maintain forceoutput (14). However, during the recoveryperiod, an increase of both RMS and MDFwas observed suggesting that after fatigueassociated to muscle fiber injury, moremuscle activation was needed to achieve thesame relative torque level as pre-exerciselevel.

The serum level of CK was analyzed onlyat three times during the study just to con-firm the possible presence of muscle injury.Relative to the pre-exercise level, on the 2ndday a pronounced increase of 109% wasobserved (Figure 1D), and even by the 7thday CK remained 32% higher than the firstmeasurement. An increased CK level aftereccentric exercise associated with muscleinjury has been well documented (7).

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Figure 2. Images of the contralateralquadriceps muscle obtained by mag-netic resonance imaging (MRI) beforeeccentric exercise (Pre) and on the 7days following exercise. Large areas ofmuscle injury (asterisk) are observed inthe quadriceps muscle from the first(day 1) to the seventh (day 7) day aftereccentric exercise. Femoral bone (Pre,circle), vastus medialis (VM) and vastuslateralis (VL). See text for MRI param-eters.

MRI showed large areas of muscle in-jury, predominantly located in the deep re-gions of the quadriceps. Observe the hyper-intense regions in the images from the 1st tothe 7th day after injury (Figure 2) comparedto the reference image obtained before exer-cise. The lesions formed diffuse areas of

muscle injury in all regions of the quadricepsmuscle, although the deep regions of themuscle appeared to be more affected. Due tothe diffuse character of the lesions and thelow contrast of the images obtained it wasnot possible to compare the incidence ofinjury between VL and VMO, although it

Pre Day 1 Day 2

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could be seen that both muscles were af-fected.

In conclusion, the results of this studydemonstrate that bouts of eccentric exercisedecrease the isometric torque of the quadri-

ceps muscle, as well as the EMG signals ofVMO and VL muscles and these changes arerecovered within one week, despite the pres-ence of signals of muscle regeneration.

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