An electromyographic analysis of theupper extremity in pitching

Nick M . DiGiovine, MD, Frank W . Jobe, MD, Marilyn Pink, MS, PT, andJacquelin Perry, MD, Inglewood, Calif.

The upper extremity is vulnerable to in;ury during the baseball pitch because ofthe repetitious nature of the action, the extremes in range of motion, and thehigh angular velocities and torques generated at the shoulder and elbow. Hencethis study was designed to describe the muscle-firing patterns through fine-wireelectromyography in 29 muscle bellies in the upper extremities of skilled pitchersduring the fastball pitch. The results demonstrated that the muscles functionedwith precise timing for [oint stabilization to prevent injury, [oint activation totransfer forces to the ball, and [oint deceleration to dissipate forces after ballrelease. The synchrony of reciprocal and sequential muscle contractionnecessary to accomplish these functions was clearly evident. This study providesa better understanding of the coordinated sequence of muscle activity during thethrowing motion; this understanding is crucial to the development of exerciseprotocols and surgical procedures used for treatment and prevention of shoulderand elbow iniuries in the throwing athlete. (J SHOULDER ELBOW SURG 7992; 7:75-25.)

Pitching is a complex sequence of body move­ments that result in the rapid propulsion of aprojectile, the ball. Four parameters determinethe effectiveness of a pitcher. These are the abil­ities to generate velocity, ma inta in accuracy,apply spin, and sustain endurance. Coordinat­ing the integration of these parameters is a highlevel of neuromuscular control. Synchrony ofmuscle contraction is vital to the motion. Effec­tiveness also requires performance at a levelthat maximally stresses the anatomic elementsinvolved. At work is a delicate balance betweenmobility and stability. The fine line that sepa­rates maximum performance and injury is oftencrossed, and shoulder and elbow injuries inpitchers are not uncommon.* Understanding thebiomechanics of pitching is important to preventand treat these injuries.

For the past 12 years the Biomechanics Lab­oratory at Centinela Hospital and the Kerlan­Jobe Clinic have collected electromyographicdata on muscular activity during the throwingmotion in athletes. The early reports were on

f rom the Biomechani cs Lab oratory, Centinela Ho spital Med­ical Center, Inglewo od , Colif.

Reprint requests: Marilyn Pink, MS, PT, Centinela HospitalMedical Center , Biomechanics Laboratory, 555 E. HardySt., Inglewood, CA 90301.

' References 7, 8, 10-12, 16, 23-26.32/1136032

selected muscles in a lim ited number of sub­[ects.": 13, 14 Also, the instrumentation to quan­tify the data as an integ rated signal was notinitially available. This study was designed todescribe a comprehensive, integrated, andquantified data base of electromyographic ac­tiv ity in 29 muscles of the shoulder girdle andupper extremity in a large sample of uninjured,skilled pitchers during the fastball pitchingmotion.

SUBJECTS AND METHODSFifty-six uninjured, skilled pitchers, who were

active at the college or professional level, wereevaluated at the Biomechanics Laboratory ofCentinela Hospital in Inglewood, California. In­dwelling electromyography was used to recordact ivity in 29 separate muscles or muscle belliesof the shoulder girdle and dominant upper ex­tremity during the fastball pitch. These includedmuscles responsible for scapular rotation (7muscles), glenohumeral motion (10 muscles),and elbow, forearm, wrist, and finger motion(12 muscles). Technical limitations of the elec­tromyography and telemetry unit allowed mon­itoring of only eight muscles for each pitcherduring a single pitching session . Occasionallypitchers volunteered to come back for a secondsession, but none came back for a third session.Different pitchers had different combinations ofthe eight muscles monitored. No pitcher had all

15

16 DiGiovine et 01. J. Shoulder Elbow Surg.January /February 1992

FINISHFOOT

DOWNMAXIMAL BALLEXTERNAL RELEASEROTATION

Figure 1 Six phases of pitching.

HANDSAPART

START

29 muscle bellies monitored. Recordings foreach muscle were obta ined in an average of 13pitchers (range, 10 to 18) (Table I).

Fifty-micran dual-wire electrodes were in­serted intramuscularly by means of the single­needle technique of Basmajian and Deluco."Manual muscle testing or electrical stimulationwas then performed to confirm placement,while muscle activity wos observed on an os­cilloscope. The electrodes were connected to abaHery-powered transmiHer belt pack to pre­vent restrictions in body movements. To corre­late electromyograph ic act ivity with specificmovements in the pitching sequence, a 16 mmmotion picture camera was used to film eachpitch at the rate of 400 or 450 frames persecond.

The electromyographic signals were tele­metered to a computer for conversion to digitalformat and quantified by integration of 2500samples every second . Excluding noise, thepeak 1-second signal during a maximal manualmuscle test (MMT) was selected as 100% activ­ity. All other muscular activity was assessedevery 20 msec and expressed as a relative per­centage of the MMT. To facilitate generalizedcomparisons a range of 0% to 20% was con­sidered low activity; 21% to 40%, moderate ac­tivity; 41% to 60%, high act ivity; and greaterthan 60%, very high activity.

The film was reviewed on a stop-action pro­jector. Single-frame viewing allowed each pitchto be accurately d ivided into six phases:windup, early cacking, late cocking, accelera­tion, deceleration, and follow-through (Fig. 1).As previously defined, follow-through beganwhen the ball left the fingers and continued untilmotian in the pitching arm ceosed.P- 14.26 De­celeration now describes the first one third ofthis phase, and follow-through is defined asthe second two thirds of this phase. Synchro­nization between the pitching phases and the

electromyographic data was achieved by useof electronic markers, wh ich were placed onboth the motion picture film and the electro­myographic recording . Electromyographic ac­tivity was averaged with in each phase of thepitch for each muscle and each pitcher. Re­corded measurements of the act iv ities for everyphase in each muscle were averaged amongpitchers and expressed as a mean and standarddeviation.

RESULTSScapular rotators. In the upper trapezius,

activity was very high during early cocking. Inthe middle trapezius, act ivity was high in latecocking . In all portions of the trapez ius, activitywas very high during acceleration. In the lowertrapezius, activity was high during follow­through. In both portions of the serratus ante­rior, activity was very high during late cocking,high during acceleration, and high during fol­low-through. In the rhomboids, activity was veryhigh during acceleration. In the levator scapu­lae, activity was very high during both late cock­ing and acceleration. In all of the scapular ro­tators, act ivity was moderate to very high duringdeceleration. During follow-through, activitywas moderate to high in the serratus anteriorand low to moderate in the rest of the scapularrotators (Fig. 2; Table I).

Deltoids. The three portions of the deltoidwere highly active during early cocking. In allportions of the deltoid, act ivity decreased tomoderate levels during late cocking and ac­celeration. The only deviation from this paHernwas the very high level of activity seen in theposterior deltoid during acceleration . All threeportions of the deltoid exhibited high levels ofact ivity during deceleration and low levels ofactivity during follow-through (Fig. 3; Table I).

Rotator cuff. The rotator cuff muscles as agroup demonstrated moderate activity during

Volume 7Number 7

Electromyography of upper extremity in pitching 17

Acceleration Deceleration Follow-through

Scapular musclesUpper trapezius 11 18:!: 16 64 ::!: 53 37 ::!: 29 69 ::!: 31 53::!: 22 14 ::!: 12Middle trapezius 11 7:!: 5 43 ::!: 22 51 ::!: 24 71 ::!: 32 35:!: 17 15 ::!: 14Lower trapezius 13 13 ± 12 39 ± 30 38 ::!: 29 76 ± 55 78 ± 33 25 ± 15Serratus anterior (sixth 11 14 ± 13 44 ::!: 35 69 ± 32 60 ± 53 51 ± 30 32 ± 18

rib)Serratus anterior (fourth 10 20 ± 20 40 ± 22 106 ::!: 56 50 ± 46 34 ± 7 41 ± 24

rib)Rhomboids 11 7 ± 8 35 ± 24 41 ± 26 71 ± 35 45 ± 28 14 ± 20Levator scapula 11 6 ± 5 35 ± 14 72 ± 54 77 ± 28 33 ± 16 14 ± 13

Glenohumeral musclesAnterior deltoid 16 15 ± 12 40 ± 20 28 ::!: 30 27 ± 19 47 ± 34 21 ± 16Middle deltoid 14 9 ± 8 44 ± 19 12 ± 17 36 ± 22 59 ± 19 16 ± 13Posterior deltoid 18 6 ± 5 42 ± 26 28 ± 27 68 ± 66 60 ± 28 13 ± 11Supraspinatus 16 13 ± 12 60 ± 31 49 ± 29 51 ± 46 39 ± 43 10 ± 9Infraspinatus 16 11 ::!: 9 30 ± 18 74 ± 34 31 ± 28 37 ± 20 20 ± 16Teres minor 12 5 ::!: 6 23 ± 15 71 ± 42 54 ± 50 84 ± 52 25 ± 21Subscapularis (lower 11 7 ± 9 26 ± 22 62 ::!: 19 56 ± 31 41 ± 23 25 ± 18

third)Subscapularis (upper 11 7 ± 8 37 ± 26 99 ± 55 115 ± 82 60 ± 36 16 ± 15

third)Pectoralis major 14 6 ± 6 11 ± 13 56 ± 27 54 ± 24 29 ± 18 31 ± 21Latissimus dorsi 13 12 ::!: 10 33 ± 33 50 ± 37 88 ± 53 59 ± 35 24 ± 18

Elbow and forearmmuscles

Triceps 13 4 ± 6 17 ± 17 37 ± 32 89 ::!: 40 54 ± 23 22 ± 18Biceps 18 8 ± 9 22 ± 14 26 ± 20 20 ± 16 44 ± 32 16:!: 14Brachialis 13 8 ± 5 17 ± 13 18 ± 26 20 ± 22 49 ± 29 13 ± 17Brachioradialis 13 5 ± 5 35 ± 20 31 ::!: 24 16 ± 12 46 ± 24 22 ± 29Pronator teres 14 14 ± 16 18 ± 15 39 ::!: 28 85 ± 39 51 :+: 21 21 ± 21Supinator 13 9 ± 7 38 ± 20 54 ± 38 55 ± 31 59 :+: 31 22 ± 19

Wrist and finger musclesExtensor carpi radialis 13 11 ± 8 53 ± 24 72 ± 37 30 ± 20 43 ± 24 22 ::!: 14

longusExtensor carpi radialis 15 17 ± 17 47 ± 26 75 ::!: 41 55 ± 35 43 :+: 28 24 ± 19

brevisExtensor digitorum com- 14 21 ± 17 37 ± 25 59 ± 27 35 ± 35 47 ::!: 25 24 ± 18

munisFlexor carpi radialis 12 13 ± 9 24 ± 35 47 ± 33 120 ± 66 79 :+: 36 35 ± 16Flexor digitorum superfi- 11 16 ± 6 20 ± 23 47 ± 52 80 ± 66 71 :+: 32 21 ± 11

cialisFlexor carpi ulnaris 10 8 ± 5 27 ± 18 41 ± 25 112 ± 60 77 ± 42 24 ± 18

'Means and standard deviations, expressed as a percentage of the maximal manual muscle test.

early cocking, very high activity during latecocking, high activity during acceleration, mod­erate to very high activity during deceleration,and low activity during follow-through. Thisgeneral pattern was interrupted by exceedinglyhigh activity in the upper third of the subscap­ularis during late cocking and acceleration. Ac­tivity was high in the supraspinatus in earlycocking, but the activity level was similar to thatof the rest of the rotator cuff in the other phasesof the pitch (Fig. 4; Table I).

Internal rotators. The activity of the hu­meral internal rotators was most notable in thelate cocking and acceleration phases-activitywas high to very high (Fig. 5; Table I).

Elbowflexors and extensors. Thetricepsexhibited very high activity during the acceler­ation phase but low to moderate activity in otherphases. In the elbow flexors, activity was low tomoderate in every phase of the pitch. The bra­chioradialis distinguished itself from the otherelbow flexors; its activity was moderate in early

18 DiGiovine et 01. J. Shoulder Elbow Surg.January / February 1992

OllOWTH ROUGH

LATECOCKING

12010080

60 ldJ~_~4020

0 -

_ UP TRAP

o SER 6T H

MU SCL ES

_ MID TRAP 0 LOW TRAP _ SER 4TH

o RHO MBO IDS _ LE VATOR SCAP UL A

Figure 2 EMG activity of scapular rotator muscles.

12010080604020

O --lL-- ......::;==-- ..L-- ---=;==--- ..L----=;==--- L..-- -=r==---- L.-- -=;==------/EARLY

COCKINGL AT E ACCEL ERATION DECE L-

COCKING ERAT IONFOL LOW

TH ROUGH

_ ANTER IOR DELTOID

o POSTERIOR DELTOI D

MUSCL ES

_ MIDDL E DELTO ID

_ SUPRASPINATUS

Figure 3 EMG activity of deltoids and supraspinatus.

and late cocking and high in deceleration (Fig.6; Table I).

Forearm pronator and supinator. Pro­nator activity was very high during acceleration.Both pronator and supinator activity was highduring deceleration (Fig. 7; Table I).

Wrist and fingerextensorsandflexors.In the wrist and finger extensors, activity washigh during early cocking and very high during

late cocking (Fig. 8; Table I). Activity of the wristand finger flexors was very high during accel­eration and deceleration (Fig. 9; Table I).

DISCUSSIONThe discussion will be approached through

the distinct phases of the baseball pitch. Muscleactivity above 40% MMT (high and very highactivity) will be addressed as key to understand-

Volume 7Number 7

Electromyography of upper extremity in pitching 19

12010080604020o J'-----=r=--"--.::y=--L_---=;=-_-L-_....:;::=-~C__..:::;=--/

EA RLYCOCKING

L ATE ACCELERATION DECEL-COCKIN G ERATI ON

FOLLOWTHROUGH

r--------- - - ----------- - - - - - - --,

MUS CLE S

_ INFRASPINATUS TERES MINOR 0 SUBSCAP LOW 1/3

_ SUBSCAP UP 1/3 0 SUPRASPINATUS

Figure 4 EMG activity of rotator cuff muscles.

12010080604020

O JL_ -.:;:.. L...-_-.-_-----L_-,,--_-L-_-,.--__c__--,--_-----'

EARLYCOCK ING

_ PECTOR ALIS MA J OR

o SUBSCAP LO WER 1/3

MUS CL ES

_ L ATISSIMUS DORSI

_ SUBSCAP UPPER 1/ 3

Figure 5 EMG activity of internal humeral rotator muscles.

ing the muscle mechanics during the fastballpitch.

Windup. During windup the activity of themuscles was below 21% MMT in all cases. Thelow activity in the upper extremity reflected the

lack of critical events related to performanceor to injury potential.

Early cocking. During this phase the tra­pezius and serratus anterior actively positionedthe glenoid for the humeral head as the arm

20 DiGiovine et 01. 1. Shoulder Elbow Surg.January / February 1992

12010080604020

O --"-- ---,- - "--- -.--- --L- ----,r-- -L-- ---,- - "--- -.--- -J

MUSCL ES

_ TRICEPS _ BI CEPS 0 BRACH IALI S _ BRA CHIORADI ALIS

Figure 6 EMG activity of elbow flexor and extensor muscles.

120100

80604020

O --"-- -,-- --'::....-- ,-- ...L.-- -,-- --''--- .-_ -L.._ ---.-_ -----'EARLY

COC KINGLAT E ACCELERATION DECEL-

COC KING ERATIONFOLL OW

THROUGH

MUSCL ES

- PRONATOR TERES _ SUPINATOR

Figure 7 EMG activity of forearm pronator and supinator muscles.

moved into 104° obduction." These scapularmuscles formed a force couple to upwardly ro­tate and protract the scapula, and the deltoidsand supraspinatus abducted the arm. If thescapula is not positioned for the abducting androtating humerus, impingement can occur.

The synergism of the deltoids and supraspi­natus has been noted in other overhandsportS.1 8

-2 0 The supraspinatus, which inserts

closer to the joint axis than does the deltoid,assisted with humeral abduction while it keptthe humeral head congruent with the glenoidand stabilized or protected the joint. If the del­toids were to abduct the humerus without thesupraspinatus, they would be in a mechanicalposition to lever the head out of the joint.Saha2 2 described the humeral muscles as driv­ers or steerers. In this case the deltoids were

Volume 7Number 7

Electromyography of upper extremity in pitching 21

LATECOCKING

--If-+----.

EMG ACf~VrrY'( ¥oMMf )---------------,

EARLYCOCKING

120 ....100 ­80604020O -"'---,---L.---,-----L.--r--L-_,--_.L-_-:;=~_/

- ECRL

MUSCLES

ECRB 0 EDC

Figure 8 EMG act ivity of wrist and finger extensor muscles. fCRL, Extensorcarpi radialis longus ; fCRB, extensor carpi rad ial is brevis; f OC, extensordig itorum commun is.

12010080604020o -¥----r--..L.----r--L----,---~e.--.---_----L:....__ _._---./

FOLLOWTHROUGH

_ FCR

MUSCLES

FDS DFCU

Figure 9 EMG activity of wrist and finger flexor muscles. FCR, Flexor carpiradial is; F05, flexor digitorum superficialis; FCU, flexor carpi ulnarus.

the drivers that positioned the arm in space andthe supraspinatus was the steerer that delicatelyfine-tuned the position of the head in the gle­no id. This relationship of the deltoids and su­praspinatus separated the supraspinatus fromthe rotators of the rotator cuff and functionally

aligned the supraspinatus with the deltoids forhumeral abduction.

The extensor carpi radial is longus and exten­sor carpi radial is brevis were the only othermuscles in the upper extremity that revealedhigh activity during early cocking . During this

22 DiGiovine et 01.

Supr••pl atus

T. r••minor

T.r ••major

SGHl

Su bacapul.r I.

J. Shoulder Elbow Surg.January / February 1992

IGHl

Peel.m.jor

Figure 10 Anatomy of glenohumeral joint and relative position of sta­bilizing structures with arm at 90° abduction and 90° external rotation.SGHL, superior glenohumeral ligament; IGHL, inferior glenohumeral lig­ament.

time the wrist moved from a position of slightflexion to a position of extension. The wrist ex­tension occurred as the arm was abducting andthe palm was facing the ground. Hence the wristextension was done against gravity with theweight of the ball.

Late cocking. During late cocking the hu­merus maintained its level of abduction butmoved from 18° to 11° horizontal adduction and46° to 170° external rototion.? In that there wasno further humeral abduction, the scapula didnot need additional elevation. Yet it did needto provide a stable base for the rapid externalrotation of the humerus. The middle trapezius,rhomboids, and levator scapula have all beenshown to be scapular retractors that functionisometrically at the end of the rcnqe.!? Thusthese are key muscles in providing scapular sta­bilization. The serratus anterior was the keymuscle opposing the retractors while stabilizingand protracting the scapula. In addition to form­ing a force couple for stabilization, these mus­cles may have helped to "tip" the scapula sothat the glenoid offered maximum congruencyfor the externally rotating humeral head. An un­stable or improperly positioned scapula couldbe the source of a potential injury, because the

scapuloglenohumeral rhythm and synchronywould be offset and impingement could occur.

As the humerus ceased to increasingly ab­duct, the deltoids were less active during latecocking. The active glenohumeral muscles in­cluded the rotator cuff along with the pectoralismajor and latissimus dorsi.

Turkel et al. 2 7 described the relative positionof the muscles and ligaments around the gle­nohumeral joint in a position of 90° abduction,neutral horizontal adduction/abduction, and90° external rotation (Fig. 10). As seen in Fig.10, the subscapularis, pectoralis major, and la­tissimus dorsi are all positioned anterior to theglenohumeral joint. These muscles form whatwe have named the"anterior wall." The ante­rior wall provides stability to the anterior aspectof the joint. At the end of late cocking when thehumerus is externally rotated to 170°, the an­terior aspect of the joint is vulnerable to insta­bility and subluxation. It is now understood thatif subtle instability and subluxation are left un­heeded, they can lead to impingement and ro­tator cuff tearing.' 0 Thus, if the anterior wallmuscles are not firing, the possibility of injuryis increased. In addition, during this phase thehumerus was horizontally adducted 29°.6 The

Volume 7Number 7

Electromyography of upper extremity in pitching 23

pectoralis major undoubtedly contributed tothat motion.

The posterior ratator cuff muscles were alsoquite active during late cocking. The infraspi­natus and teres minor were actively externallyrotating the humerus. Their posterior placementalso offered a posterior restraint to the anteriorsubluxation. A weakness of either of these mus­cles would diminish the posterior restraint.

The supraspinatus was the least active of therotator cuff muscles. As mentioned previously,the supraspinatus has demonstrated a synergywith the deltoid. During this phase the armmaintained rather than increased its level of el­evation. Hence activity of the supraspinatus waslower in late cocking than in early cocking butstill high. Also, in the position of extreme ex­ternal rotation, the supraspinatus was rotatedposteriorly. At this point the supraspinatuswould be in a relatively ineffective position withless superior compressive force.

The superior compressive force, which wasabdicated by the supraspinatus, may have beenin part provided by the subscapularis. The up­per portion of the subscapularis was more ac­tive than the lower portion. At the point of 170°humeral external rotation with 102° abduction,the upper portion of the subscapularis was ro­tated superiorly and was able to offer somecompression and support for the superior por­tion of the anterior wall.

The only elbow/ forearm muscle that was ac­tive during late cocking was the supinator. Theliterature has shown that during this phase theforearm is in supination. Pronation does not be­gin until 10 msec before ball releose." Thus thefunction of the supinator is to appropriately po­sition the forearm.

All of the wrist and finger muscles demon­strated high or very high activity at this time. Bycocontracting these muscles the pitchers wereable to have a stable base from which they sub­sequently launched the ball.

Acceleration. During acceleration the hu­merus internally rotated approximately 100° in0.05 second and the elbow extended an aver­age of 54°.6 Angular velocities of 6100° persecond'v 17 created a humeral internal rotationtorque of 14,000 inch-pounds." The highestshoulder joint compression force (860 Newton­meters}? was observed at this time.

A stable scapula is needed as a fulcrum forthe high angular velocities and torques. All ofthe scapular muscles demonstrated very highactivity as they filled this function.

As the humerus was internally rotating, theposterior deltoid was optimally positioned to bethe primary humeral horizontal abductor, as

noted by its very high electrical activity. The su­praspinatus was also highly active as it againdemonstrated its relationship to the deltoid.

At this point the teres minor and the infra­spinatus had differing levels of activity-highin the teres minor and moderate in the infra­spinatus. This may have clinical relevance, be­cause in our experience posterior cuff tender­ness in baseball pitchers can frequently be iso­lated to the teres minor. Also, our experiencewith the electromyographic manual muscle test­ing has revealed higher levels of activity in theteres minor when the humerus is abducted orextended. In addition, electromyographic in­vestigations in other sports have shown sepa­rate functions for the infraspinatus and teresminor. 1 8 - 2 o

The activity in the teres minor is similar to theactivity in the pectoralis major. These two mus­cles appear to form a force couple. As the pec­toralis major forcefully contracts at the rela­tively high elevations for adduction and internalrotation, the teres minor provides a stabilizingposterior restraint. The direction of the fibers ofthe teres minor give it an extension component,which may be the reason it performs thisfunction.

The large angular velocity during accelera­tion is the result of energy that is transferredfrom the trunk, with augmentation by the latis­simus dorsi and pectoralis major. Bassett et ol."have shown that although both the latissimusdorsi and the pectoralis major can potentiallygenerate large internal rotation torques aboutthe shoulder, the latissimus dorsi is anatomi­cally positioned to generate the greater torque.This is consistent with the relative electro­myographic activities in these two muscles dur­ing this phase; the latissimus dorsi has thegreater activity. The pectoralis major and latis­simus dorsi were the main upper extremity mus­cles that actively contributed velocity to the ball.A clinical study indicated that these muscleswere the only ones in the upper extremity tohave a positive correlation between peaktorque developed in isokinetic testing and pitch­ing velocity."

The subscapularis, especially the upper por­tion, also exhibited very high activity during thisphase and functioned with the latissimus dorsiand the pectoralis major. The subscapularis ap­parently functioned as the steering muscle toprecisely position the humeral head in the gle­noid; this is similar to the relationship of thesupraspinatus with the deltoid. This alignmentof the subscapularis with the latissimus dorsihas been observed in other sports ac­tivities. 1 8 - 2 o

24 DiGiovine et 01.

Elbow extension, which began in late cock­ing, continued into the acceleration phase andreached an angular velocity of 22000 per sec­ond."- 17 Extension of the elbow took place bymeans of two mechanisms, contraction of thetriceps initially and forward momentum of theforearm secondarily.": 6, 21 In a pitcher with acomplete radial nerve block, the elbow will ini­tially flex to 1550 in the late cocking phase. 2 1

This occurs as the torque generated by the ro­tating body and arm exerts a centripetal forceon the inertia of the forearm, hand, and ball,causing them to collapse toward the body asthe late cocking phase ends. Very high tricepsactivity is mainly responsible for resisting thiscentripetal flexion torque at the elbow,"- 6, 21

Although triceps-activated elbow extensionmay not be directly responsible for increasedball velocity, it maintains elbow position andthereby provides the most effective moment armfor other more forceful body rotations to propelthe ball.

During acceleration there is a large valgusstress at the elbow. The only muscles of thelower arm with very high activity were those thatoriginate on the medial epicondyle of the elbow(pronator teres, flexor carpi radialis, flexor dig­itorum superficialis, and flexor carpi ulnaris).Their common site of origin allows them to dy­namically assist with the medial joint stabili­zation against the large valgus stress.

In addition, the forearm pronated during ac­celeration. The pronator teres was active, butthe supinator also needed to be active to controlthe degree of rapid pronation.

Motion of the wrist during acceleration tookplace from hyperextension to slight extensionjust before ball relecse.!? The extensor carpiradialis brevis was mainly responsible for theslight extension as it extended the wrist in aneutral position, as opposed to the radial de­viation component of the extensor carpi radialislongus.

Deceleration. During the decelerationphase the excess kinetic energy that was nottransferred to the ball was safely dissipated bycontrolled deceleration of the upper extremity.Deceleration has been measured to be a neg­ative 500,000 0

/ sec/ sec at the shoulder and el­bow."" An external rotation torque of 15,000inch-pounds is experienced by the humerus."

In general, opposing muscles around theshoulder, elbow, and wrist fired simultaneouslyto control the deceleration.

In the scapula the trapezius, serratus anterior,and rhomboids all demonstrated high or veryhigh activity. All three heads of the deltoid were

J. Shoulder Elbow Surg.January / February 7992

active, most notably, the middle and posteriorheads because they were positioned more an­tagonistically to the motion than was the ante­rior head.

The teres minor demonstrated the highestlevel of activity of all the glenohumeral muscles.This is a carryover of its activity as describedin the acceleration phase and is clinically sub­stantiated by the posterior rotator cuff pain,which can be isolated to the teres minor andreproduced in a deceleration motion.

The latissimus dorsi was more active than thepectoralis major during deceleration. This islogical, because the pectoralis major loses itsmechanical advantage once the humerus dropsbelow 900 elevation. The subscapularis alsohad high activity; in its role as companion mus­cle to the latissimus dorsi, it prevented the hu­meral head from subluxating during rapid in­ternal rotation (see Discussion, Acceleration).

All of the elbow, forearm, wrist, and fingermuscles showed high activity, and the wristflex­ors had very high activity. This is reasonable,since these smaller, more distal joints wouldhave much kinetic energy to dissipate.

Follow-through. Follow-through seems tobe a noncritical motion, since all shoulder girdleand upper extremity muscles exhibited activitybelow 42% MMT. During this phase of the pitch,all of the kinetic energy has dissipated, and thetrunk is beginning to extend, allowing thepitcher to field his position.

CONCLUSIONSThe muscles of the upper extremity during the

baseball pitch contract in a highly coordinatedmanner, balancing the requirements of stabili­zation with rapid motion. The purpose of sta­bilization is to protect the musculoskeletal ele­ments and maximize ball velocity by providingthe most effective moment arms for the transferof torques. The purpose of rapid motion isreadily apparent; it influences ball velocity, ac­curacy, and spin.

Every muscle group, and indeed every mus­cle, had a unique role. As a group the scapularmuscles were important, because they first func­tioned to optimally position the glenoid againstthe humeral head, then provided a stable basefor the rapid humeral motion, and finally con­trolled the deceleration of the scapula after ballrelease. The three components of the deltoidwere active to position the arm. The supraspi­natus functioned with the deltoid as it fine-tunedthe position of the humeral head against theglenoid. The infraspinatus and teres minor bothexternally rotated the humerus in late cocking.

Volume 7Number 7

Electromyography of upper extremity in pitching 25

During acceleration and deceleration they haddiffering activities. The teres minor, unlike theinfraspinatus, maintained a high level of activ­ity, providing a posterior restraint to limit hu­meral head translation. This may have clinicalrelevance, because posterior cuff pain in base­ball pitchers can frequently be isolated to theteres minor. The subscapularis, pectoralis ma­jor, and latissimus dorsi were part of the com­ponents of the" anterior wall," which affordedanterior stability during the time of maximal hu­meral external rotation. All musclesof the lowerarm, which originate on the medial epicondyleof the humerus, demonstrated very high activityduring the time of valgus stress to the elbow.They were likely adding a dynamic componentto elbow stability.

The critical role of the deceleration phase tohelp dissipate the kinetic energy that was notimparted to the ball is now evident; a lack ofmuscular control at this time would undoubtedlylead to injury. On the other hand, the windupand follow-through phases demonstrated alack of consequential events in the upper ex­tremity.

This study has provided information on boththe coordinated sequence of muscular eventsand their relative magnitudes in the upper ex­tremity during pitching. This knowledge is crit­ical to the development of exercise protocolsand surgical procedures that are used for theprevention and treatment of shoulder and el­bow injuries resulting from throwing motions inathletes.

REFERENCES1. Atwater AE. Biomechanics of overarm throwing move­

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