Clinical Perspectives Regarding Eccentric Muscle Injury

8/8/2019 Clinical Perspectives Regarding Eccentric Muscle Injury

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CLINICAL ORTHOPAEDICS AND RELATED RESEARCHNumber 403S, pp. S81S89

2002 Lippincott Williams & Wilkins, Inc.

S81

Muscle strain injuries occur to predictable mus-

cles at consistent locations during expected sport-

ing maneuvers when a muscle is stretched and

then activated, particularly during high intensity

bursts of activity. More than 30% of the injuries

seen in the clinicians office are injuries to skeletal

muscle. The typical location of the injury is just

proximal to the distal muscle tendon junction re-

gardless of strain rate or architecture of the mus-

cle. After the injury, the muscle is weaker, con-

tinues to weaken, then recovers during the next

week. An inflammatory response is seen in the fol-

lowing 1 to 2 days. By the seventh day, fibrous tis-sue replaces the inflammatory reaction and a scar

forms. When a muscle is stretched, its tension still

is reduced making the healing muscle more sus-

ceptible to a repeat injury. Viscoelastic properties

of muscle also can help explain how muscle can be

protected against strain injury. A 1 C increase

in muscle temperature (warm-up) increases the

muscle length to failure and a fatigued muscle is

more susceptible to strain injury. It probably is

impossible to prevent muscle strain injury; how-

ever, preventive measures can make muscle more

resistant to these stretch-induced injuries.

Common acute injuries to skeletal muscle such

as contusions, lacerations, strains, ischemia,and complete ruptures can lead to significantpain and disability with time lost to occupa-

tional and leisure activity participation. Theimportance of strains, a stretch injury, is clear

to the occupational or sports medicine physi-cian because stretch-induced injuries can ac-

count for as much as 30% of the typical sportsmedicine practice.13,17

A muscle strain injury is characterized by a

disruption of the muscle-tendon unit6 leadingto localized pain and general weakness of the

muscle when activity is attempted. Improperrest and rehabilitation of a minor strain of

skeletal muscle frequently precedes a far moredisabling injury that additionally increases thetime lost to work and athletics.

Despite the frequency of these injuries, the

understanding of the pathophysiology, treat-ment, and recovery is limited especially whenone compares the understanding of damage to

SESSION 2: ECCENTRIC MUSCLE

INJURY

Clinical Perspectives RegardingEccentric Muscle Injury

Donald T. Kirkendall, PhD; and William E. Garrett, Jr., MD, PhD

From the Department of Orthopaedics, University ofNorth Carolina-Chapel Hill, Chapel Hill, NC.

Reprint requests to Donald T. Kirkendall, PhD, Dept ofOrthopaedics, CB# 7055, University of North Carolina,Chapel Hill, NC 27599-7055.

DOI: 10.1097/01.blo.0000031989.92980.2b


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ligament, tendon, and bone. The natural history,

self-limiting nature, and minimal surgical re-quirements may have made stretch-induced

injuries of less interest to clinicians. The pur-pose of the current review is to discuss stretch-induced injuries, the mechanism of injury, lo-

cation of injury, treatment, and some pertinentobservations from the clinic.

Mechanism of Injury

To reproduce the injury in the laboratory, a ba-sic understanding of how muscles are injured

in sport or occupational settings needs to beappreciated. It is well-accepted that musclestrain injuries occur when the muscle is acti-

vated while being stretched.13,18,28 In addition,eccentric contraction of the muscle is a fre-

quent occurrence.9,17,28 Eccentric contractionis an important factor contributing to the in-

jury because muscle forces can be higher dur-ing lengthening23 that adds to the forces trans-mitted to muscle by noncontractile connective

tissue.3 On the athletic field, muscle strain in-juries are common in speed athletes such as

sprinters and participants in American foot-ball, basketball, soccer, rugby, and others sports.

Certain muscles also are most susceptible toinjury than others as shall be shown.

Muscle injury constitutes a spectrum of

problems from the self-limiting delayed onsetmuscle soreness2,4,11,21 to muscle strains to

complete disruptions or avulsions from bony at-

tachments. To study this, standard laboratorytechniques for muscle mechanics and electro-physiology on hindlimb muscles in rabbits,mainly, the tibialis anterior and the extensor

digitorum longus were used. A model was de-veloped to produce a strain injury. The first

question was how strong a contraction wasneeded to produce a strain injury and it was

found that activation alone failed to induce astrain injury.5 To obtain an injury, stretch wasnecessary. The forces needed to cause muscle

failure were several times the force normallyproduced actively during a maximal isometric

contraction,7 suggesting that passive forcesmust be considered. Therefore, intact muscle of

the hindlimb in rabbits, with intact neural and

vascular supply, could be passively stretched to

failure or activated during stretch.

Passive Stretch Injuries

Muscles stretched to fail do so at the proximalor distal tendon. Factors that might influence

injury are the rate of strain (1, 10, and 100cm/second), muscle architecture (pennation),

or mechanical properties of the muscle. Fail-ure was independent of strain rate or architec-

ture and failed at the (most frequently distal)muscle-tendon junction (Fig 1), leaving a

small variable amount of muscle tissue still at-tached to the tendon.5 Therefore, the site ofstretched-induced injury was predictably near

the muscle-tendon junction, but most oftenwas not a complete avulsion because a small

variable amount of muscle remained connectedwith the tendon. Another factor in muscle me-

chanics, muscle length, has no consistent effecton muscle strain.

Active Stretch Injuries

Most clinicians would agree that strain injuries

occur during powerful eccentric contractions,so a laboratory condition to mimic that seen

clinically was devised. Hindlimb muscles ofrabbits again were isolated and stretched to

Clinical OrthopaedicsS82 Kirkendall and Garrett and Related Research

Fig 1. The gross appearance of the tibialis ante-rior muscle of the rabbit after controlled strain in-

jury is shown. A small hemorrhage is evident at

the distal tip of the injured muscle (arrow) at 24hours. (Reprinted with permission from Garrett JrWE: Muscle strain injuries. Am J Sports Med24(Suppl):S2S8, 1996.)


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failure. However, during stretch one of three

conditions of activation was applied: tetanicstimulation, submaximal stimulation, or no

stimulation.15 The location of failure was pre-dictable, the muscle-tendon junction, and thetotal strain at failure were similar among the

three conditions. Interestingly, the force gen-erated at failure only was 15% greater in the

activated muscles. However, the energy ab-sorbed (the difference in strain energy between

passive and active conditions) was approxi-mately 100% greater in the activated condition

(Fig 2), suggesting that passive elements ofmuscle can absorb energy, but that their abil-ity to absorb energy is enhanced greatly when

the muscle is activated.This may suggest that muscles are able to

protect themselves and joint structures from in-jury; the more energy that the muscle can ab-

sorb, the more resistant the muscle is to injury.The passive and contractile elements of muscleboth contribute to the ability of the muscle to ab-

sorb energy. The passive elements (not depen-dent on activation) include connective tissue and

the fibers. The contractile element of the musclealso participates because activation of the mus-

cle increases the ability to absorb energy (Fig 2).The increase in energy absorbed attribut-

able to contraction was approximately 100%.

Any condition that diminishes the ability ofthe muscle to contract would reduce the abil-

ity of the muscle to absorb energy making themuscle more susceptible to injury. Two vari-

ables that seem to be factors in muscle strain

injuries are fatigue and weakness.Nondisruptive Injuries

So far, the discussion has been directed at in-

juries leading to a complete disruption of themuscle-tendon unit. A change in linearity of

the force-displacement curve of a stretched,inactivated muscle indicates a plastic defor-mation has occurred, indicating alteration to

the material structure. Using this model, phys-iologic, mechanical, and histologic character-

istics of muscle can be observed.Although the model may cause a nondis-

ruptive injury, ultrastructural damage still oc-curs. Histologic sections of these injuries show

damage near the muscle-tendon junction witha variable amount of muscle tissue still at-tached to the tendon with some hemorrhaging.

A pronounced inflammatory response is seenshortly after the injury. By the seventh day, fi-

brous tissue starts to replace the inflammatoryreaction leading to scar tissue.15

This type of damage to the tissue would af-

fect the ability of the muscle to develop tension.Immediately after injury the muscle is weaker,

developing only approximately 70% of the

normal tension. The weakness progresses andwithin 24 hours, the muscles ability to developtension declines further to 50% of the con-tralateral control muscle. With time, tension de-

velopment improves and by the seventh day,the muscle can develop 90% of the tension pro-

duced by its contralateral control muscle.In contrast, when muscle with a 7-day-old

nondisruptive injury is stretched and the ten-sile strength is recorded, this tensile strength is77% of the control muscle.16 This is well be-

low the 90% tension developed that was justmentioned. As stretch is a factor in strains, this

loss of tensile strength may make the musclemore susceptible to a second injury, a scenario

frequently seen by clinicians.

Number 403SOctober, 2002 Eccentric Muscle Injury S83

Fig 2. The energy absorbed is shown as the areaunder each length-tension deformation curve.The lower curve is a passive preparation. The fig-ure shows the relative differences in energy ab-

sorbed to failure in stimulated versus passivemuscle preparations. (Reprinted with permissionfrom Garrett Jr WE: Muscle strain injuries. Am JSports Med 24(Suppl):S2S8, 1996.)


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The Visoelasticity of Skeletal Muscle

Important factors in preventing muscle strain in-juries include flexibility, warm-up, and stretch-

ing before exercise. The beneficial adaptationbecause of stretching frequently is attributed to

stretch reflex mechanisms. An additional fea-

ture of muscle, viscoelasticity, must be con-sidered. Viscoelasticity should be observed byhanging a weight on a muscle and observing itsnew length, then watching the muscle slowly

continue to increase in length with time. For ten-dons and ligaments, stretching the tissue to a

constant length leads to a gradual reduction intension with time called stress-relaxation. When

done cyclically, a gradual decrease in tensionoccurs with each successive stretch.1

This gradual decrease in tension can be

seen experimentally. Hindlimb muscle fromrabbits was stretched from an initial force of

1.96 N to 78.4 N, held for 30 seconds then re-turned to the initial force and repeated 10

times (Fig 3). The length necessary to reachthe predetermined tension increased 3.45%during the 10 cycles and 80% of this change in

length occurring in the first four stretches.27

Another way to look at the same feature is

to stretch the muscle to 10% above its restinglength and return it to its resting length and re-

peat 10 times (Fig 4). Tension is reduced byapproximately 17% during the 10 cycles withmost of the reduction occurring in the first four

cycles.25

It is obvious that repetitive stretching re-

duces the load on the muscle-tendon unit at anygiven length in the absence of reflex effects or

other mediation by the central nervous system.No differences were apparent for the two con-ditions when repeated on innervated or dener-

vated muscle. These data clearly show a large

component of the changes in muscle causedby stretching are a result of inherent muscle-tendon viscoelasticity. Certainly, there are ad-

ditional reflex and central nervous system ef-fects affecting muscle that is being stretchedespecially during physiologic movements.

Clinical Applications

In the rabbit, muscle strain injuries occur at themuscle-tendon junction. Is this the same finding

seen in the clinic? Acute hamstring strain in-juries in 10 college athletes were evaluated clin-ically and imaged with computed tomography

(CT) scans within 48 hours of the injury to de-termine injury mechanism and location.8 All in-

juries occurred while sprinting or kicking a


Fig 3. The percent increase in length of the exten-

sor digitorum longus when repeatedly stretched toa constant tension is shown. (Reprinted with per-mission from Garrett Jr WE: Muscle strain injuries.Am J Sports Med 24(Suppl):S2S8, 1996.)

Fig 4. Muscle tension of the extensor digitorumlongus when repeatedly stretched to the samelength (10% beyond resting length) is shown.(Reprinted with permission from Garrett JrWE: Muscle strain injuries. Am J Sports Med24(Suppl):S2S8, 1996.)


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soccer ball. The injury mostly was proximal

and lateral, typically to the biceps femoris. Thecommon mechanism involved ballistic hip flex-

ion and knee extension. By CT scanning, the in-jured area appeared as a region of hypodensitysuggesting inflammation and edema, not local-

ized bleeding. To understand the anatomy at thelocation of injury, cadaveric dissections were

done. In this sample, nine of 10 injuries seemedto be to the long head of the biceps femoris, lo-

calized to the muscle tendon junction of thecommon tendon of the hamstrings. The tenth pa-

tient (a soccer player) injured his semimembra-neous while kicking overhead suggesting a dif-ferent mechanism than that seen in the sprinters.

Additional imaging studies were done on 50patients who had CT scans (n 27) or magnetic

resonance imaging (MRI) (n 23) to better ob-serve the location of muscle strain injuries.22

Injuries were specific to the quadriceps, ham-strings, adductors, and triceps surae groups. T2-weighted images were better than T1-weighted

images for observing the edema, inflammation,and possible hemorrhage. Computed tomogra-

phy scanning showed the expected areas of lowdensity. Quadriceps strains were to the rectus

femoris whereas adductor strains were to the ad-ductor longus. Of the 17 hamstring strain in-

juries, 11 were to the biceps femoris, four were

to the semimembraneous, and two were to thesemitendinosus. All injuries to the triceps surae

group were at the distal muscle tendon junction

of the medial head of the gastrocnemius. Theeffectiveness of CT scanning and MRI ofstrain injuries was shown and particular mus-cles susceptible to strain injuries were identi-

fied. The muscles were two-joint muscles (bi-ceps femoris, rectus femoris, gastrocnemius), of

a complex architecture (adductor longus) andoccurred, as can be determined best by CT scan-

ning and MRI, at the muscle-tendon junction.Clinically, there are curious, unexplained

muscle injuries, particularly a persistent intra-substance strain of the rectus femoris. An un-derstanding of the nature of the strain injury

was inconsistent with an understanding of theanatomy of the rectus femoris. Dissection of the

rectus femoris muscle from cadavers shows a

direct head originating from the anterior infe-

rior iliac spine and an indirect head originatingfrom the superior acetabular ridge.12 The ten-

don of the indirect head extended well into themass of the rectus femoris. Although prior lab-oratory work showed that most strain injuries

occur superficially at the muscle tendon junc-tion, clinical evidence pointed to a strain at the

muscle tendon junction of the deep, indirecthead giving the appearance of an intrasubstance

injury. These are different from the classicallyseen injury near the distal tendon because of the

thigh asymmetry, chronic pain, and the devel-opment of anterior thigh masses. Ten patientswith an incomplete intrasubstance strain of the

proximal, deep tendon of the rectus femoriswere evaluated with physical examination and

imaging studies. Patients presented anywherefrom 4 to 156 weeks after injury. Eight of the

10 injuries involved sprinting or kicking (twopatients could not recall the mechanism) and allbut one patient had pain when running. Imag-

ing studies detected the strain to be in the areaof the tendon of the indirect head of the rectus

femoris. Surgical exploration was done on twopatients leading to removal of the muscle in one

patient and the excision of a fibrotic mass in theother patient. After surgery, both patients wereasymptomatic and returned to full activity. The

reason for chronic pain in these subjects wasunknown, but may be attributable to differential

activation of the superficial and deep portions

of the muscle.Because so many strain injuries seem to be

dependent on the architecture of the muscle

and after clinical experience with anterior thighstrains12 suggests a detailed study of the archi-tecture of the rectus femoris seemed appropri-

ate to determine whether these persistent strainswere related to some curious architectural fea-

ture.10 The rectus femoris of fresh or embalmedcadavers was dissected with the superficial anddeep tendons confirmed. The tendon of the

deep component extended nearly the musclesentire length. It arose from the superior ac-

etabular ridge and traversed somewhat medi-ally over the course of the muscle. It began

rounded, flattened out, and migrated laterally



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and was nearly vertical in the distal third of the

muscle (Fig 5). The pennation of the rectusfemoris was more complex than the typical

bipennate arrangement normally attributed tothe muscle. The proximal 13 seemed to beunipennate whereas the distal 23 was bipennate.

The deep tendon and the bipennate arrangementof the distal portion of the muscle created a

muscle within a muscle. Exploration of threechronic strain injuries showed a pseudocyst

consisting of vascular, fibrotic loose connec-tive tissue that surrounded the deep tendon.

Serous fluid collected between the connectivetissue and the tendon. This anatomic finding isconsistent with CT or MRI scans of vascular

fibrotic processes of the deep tendon of this in-direct head.

The most common hamstring strain seen in-

volves one muscle, usually the biceps femoris.More extensive injuries involve more than one

muscle, typically at the common tendon of ori-gin of the hamstrings. A unique mechanism of

severe hamstring strain injury involves water

skiers.

20

Novice skiers assume a crouched posi-tion before being pulled by the boat into a

standing position. If the skier extends the kneestoo soon, the ski is forced down into the water.

Forward momentum of the boat pulls the skierforward leading to excessive hip flexion while

the knees are extended. This powerful stretchleads to a muscle-tendon junction injury or to amore disabling injury involving avulsion from

the ischial tuberosity. Hamstring strains alsooccurred in experienced skiers when falling

forward on a slalom ski. Twelve water skierswith a history of skiing-induced hamstring in-

juries were followed up between .5 and 18years after the injury. All patients knew theyhad a significant injury when the accident oc-

curred. Complete or partial avulsion occurred atthe proximal tendon. The extent of the injury

was obvious on the physical examination, re-vealing distal tendon retraction of the ham-

string muscles and obvious thigh asymmetry.Conservative treatment leads to a poor progno-sis whereas surgical repair is an alternative.

Seven of the 12 patients returned to prior sportsat a lower level and the rest of the patients, all

with complete disruptions, were hampered in

sports involving running or requiring agility.Acute groin injuries also are common in ath-

letes, especially those who play soccer26 and icehockey. Frequently, the adductor longus is in-

jured during hip abduction. Direct and indirecthernias also may occur. There is another ab-

normality in the lower abdominal wall muscu-lature that causes a vague and poorly localized

groin pain. This pain happens during high in-tensity, ballistic motions such as kicking orsprinting. This injury most often is seen in very

high caliber athletes during intense trainingand competition. This athletic pubalgia is as-

sociated with pain and muscle-tendon injuryin the inguinal area near the attachment of the

rectus abdominis to the pubis and in the adja-


Fig 5. The architecture of the indirect head of therectus femoris muscle is shown. (Reprinted withpermission from Garrett Jr WE: Muscle strain in-

juries. Am J Sports Med 24(Suppl):S2S8, 1996.)


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cent internal oblique muscles, near the region

of abdominal wall weakness seen with directinguinal hernias. However, this pain may ex-

ist without any evidence of herniation. Whenconservative measures fail, a herniorrhaphyprocedure reinforcing the abdominal wall mus-

culature can provide relief.

Prevention Strategies

Repetitive Stretch and Failure Properties

Viscoelastic properties contribute to changesin muscle length and increased length can be

seen to decrease strain in a muscle. Therefore,does stretching prevent muscle strains? To

study stretching, repeated stretch-release cy-cles were studied with the rabbit model.24

First, the force to failure of the hindlimb mus-

cle was determined. The contralateral musclesthen were stretched cyclically to 50% or 70%

of the force to failure. Ten cycles to 50% offailure force resulted in an increase in muscle

length at failure with no change in the force atfailure or energy absorbed. When muscles

were stretched to 70% of failure force, macro-scopic evidence of failure was seen even be-fore the 10 cycles were completed. Therefore,

cyclic stretching seems to be beneficial in thatstretching leading to forces in excess of 70%

may make the muscle more, rather than less,likely for injury.

Warm-upViscoelasticity is known to be temperature de-

pendent and warm-up is considered to be pro-tective against muscle strains. Hindlimb mus-

cle from the rabbit was held isometrically andtetanically stimulated for 10 to 15 secondswhich resulted in a 1 C rise in muscle tem-

perature.19 Before failure, the muscle was ableto stretch more, and more force production

was capable. Although the changes might becaused by temperature elevation, the effects of

stretch cannot be discounted despite the mus-cle being held isometrically. A constant lengthstill must allow for some stretch of the muscle-

tendon unit as the fibers contract and elasticcomponents become stretched.

Prior Injury

Clinically, physicians see a minor strain pre-

ceding a more major injury. This would sug-gest that after a minor injury, the mechanical

characteristics of the muscle somehow are al-tered, precipitating a more significant injury.

To determine the mechanical characteristicsof a muscle with a minor strain, the extensordigitorum longus of rabbits had a nondisrup-

tive strain by stretching the muscle just shortof tissue rupture.27 Isometric and isotonic con-

tractile properties of the control muscle wereused for comparison. The muscle then was

stretched passively to failure at a rate of 10 cmper minute. The peak tensile load and length atthat load were derived for use on the experi-

mental contralateral limb. The length changeto peak load (of the control limb) was dupli-

cated in the experimental muscle, just short of

a disruptive injury. The injured muscle thenwas subjected to passive stretch to failure.Histologic evaluation was done on the minor

injuries in a subset of rabbits. In the experi-mental muscles, the peak load to rupture was63% of control and the length at rupture was

79% of control. Isotonic shortening was re-duced by 51% and 6% for 100 g and 1000 g

weights, respectively. The minor strain injurycaused incomplete disruptions along the mus-

cle tendon junction making a muscle moresusceptible to another injury when precededby a prior minor injury. Therefore, early return

to activity before complete healing is a risk formore severe injury. In addition, aggressive re-

habilitation designed to return an athlete tocompetition may be too stressful for the mus-

cle risking additional injury. Injection for lo-cal pain relief while the muscle still is injuredalso may not be appropriate because the lack

of inhibition from pain could result in exces-sive stress on the muscle, also increasing the

risk of additional injury.

Fatigue

Clinical observation and the literature suggest

that muscle strain injuries occur late in trainingsessions or competitive settings. This leads one

to conclude that fatigue must play some role in



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the risk of muscle injury. Mair et al14 fatigued

the extensor digitorum longus of rabbits to 25%or 50% of the force of the contralateral control

by cycles of 5-second isometric tetanic con-tractions followed by 1-second rest. The mus-

cle was activated while being pulled (at 1, 10,

or 50 cm/second) to failure. Similar data werecollected on the nonfatigued, contralateral con-

trol muscle. The force and length at failure andthe energy absorbed before failure were deter-

mined. There was a trend toward a reduction inforce for all strain rates tested. The rate of strain

did not influence force at failure. There was nochange in muscle length at any of the strainrates and significantly less energy absorbed in

both fatigue conditions with the greatest lossoccurring in the most fatigued muscle. The re-

duction in absorbed energy was the greatestwhen the muscle was pulled at 1 cm per second;

the slower the rate that the muscle is stretched,the greater the energy that is absorbed. These

data indicate that muscles become damaged atthe same length regardless of fatigue. In con-trast, fatigued muscle is unable to absorb en-

ergy before reaching the amount of stretch thatcauses injuries. Proper conditioning to reduce

or to delay fatigue is seen as a part of a rationalefor prevention of muscle strain injury.

Treatment of Muscle Strain Injuries

The pain of a muscle strain may prompt physi-

cians to prescribe antiinflammatory medica-

tion in response to the inflammatory responsesknown to occur after an injury. This treatmentlargely is empirical. Before wide use of anti-

inflammatory drugs can be accepted, the ef-fects of such medication on muscle recoveryneed to be evaluated. Obremsky et al16 in-

duced a strain injury to the tibialis anterior ofrabbits (strain rate of 10 cm/minute) that sub-

sequently were administered piroxicam (16mg/kg) within 6 hours plus 13 mg/kg every 6

hours. Contractile properties and histologicfeatures were determined at 1, 2, 4, or 7 daysafter the injury. Data were compared with the

data from control animals that did not receivemedication. On Day 1, there was a signifi-

cantly greater force in the treated animals.

There was no difference between the treated

and untreated animals on Days 2, 4, or 7. Thetreated animals had a delay in the histologic

repair process. These muscles showed delayedinflammatory cell infiltration, necrosis, myo-

tube regeneration, and collagen deposition.

Based on these results, nonsteroidal antiin-flammatory agents may be of some benefit for

the early treatment of pain control and func-tional improvement. However, the delay in the

repair process seen histologically raised con-cern regarding long-term treatment.

One of the most common injuries seen in theoffice of the practicing physician is the musclestrain. Until recently, few data were available

on the basic science of and clinical applicationof this basic science to treatment and preven-

tion of muscle strains. Studies in the last 10 to15 years represent action taken on the direction

of investigation into muscle strain injuriesfrom the laboratory and clinical fronts.

Findings from the laboratory indicate that

certain muscles are susceptible to strain injury(muscles that cross multiple joints or have

complex architecture) and have a strain thresh-old for passive and active injury. Strains are

the result of excessive stretch or stretch whilethe muscle is being activated. When the mus-

cle tears, the damage is localized very near themuscle tendon junction. After injury, the mus-cle is weaker and at risk for additional injury.

The force output of the muscle recovers dur-

ing the following days while the muscle un-dertakes a predictable progression toward tis-sue healing.

Imaging has been used to document the siteof injury as the muscle-tendon junction. Thecommonly injured muscles are the hamstrings,

the rectus femoris, the gastrocnemius and theadductor longus although injuries inconsistent

with involvement of one muscle tendon junctionstill proved to be at a tendinous origin within the

substance of the muscle. Some injuries have apoor prognosis and may be helped with surgeryincluding injuries to the rectus femoris, the ham-

string origin, and the abdominal wall.The risk of reinjury is increased with an in-

completely healed strain injury. Early use of



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nonsteroidal antiinflammatory agents may be

helpful, but longer-term use may not be helpful.Warm-up, temperature, and stretching may

be beneficial in reducing the risk of strain injury.Many of the factors protecting muscle suchas strength, endurance, and flexibility also are

essential for maximum performance. Futurestudies should clarify the repair and recovery

process emphasizing not only the recovery offunction, but also the susceptibility to reinjury

during the recovery phase.

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Clinical Perspectives Regarding Eccentric Muscle Injury

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