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An electromyographic analysis of the upper extremity in pitching
An electromyographic analysis of the upper extremity in pitching Nick M . DiGiovine, MD, Frank W . Jobe, MD, Marilyn Pink, MS, PT, and Jacquelin Perry, MD, Inglewood, Calif.
The upper extremity is vulnerable to in;ury during the baseball pitch because of the repetitious nature of the action, the extremes in range of motion, and the high angular velocities and torques generated at the shoulder and elbow. Hence this study was designed to describe the muscle-firing patterns through fine-wire electromyography in 29 muscle bellies in the upper extremities of skilled pitchers during the fastball pitch. The results demonstrated that the muscles functioned with precise timing for [oint stabilization to prevent injury, [oint activation to transfer forces to the ball, and [oint deceleration to dissipate forces after ball release. The synchrony of reciprocal and sequential muscle contraction necessary to accomplish these functions was clearly evident. This study provides a better understanding of the coordinated sequence of muscle activity during the throwing motion; this understanding is crucial to the development of exercise protocols and surgical procedures used for treatment and prevention of shoulder and 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 a projectile, the ball. Four parameters determine the 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 high level of neuromuscular control. Synchrony of muscle contraction is vital to the motion. Effectiveness also requires performance at a level that maximally stresses the anatomic elements involved. At work is a delicate balance between mobility and stability. The fine line that separates maximum performance and injury is often crossed, and shoulder and elbow injuries in pitchers are not uncommon.* Understanding the biomechanics of pitching is impo rtant to prevent and treat these injuries. For the past 12 years the Biomechanics Laboratory at Centinela Hospital and the KerlanJobe Clinic have collected electromyographic data on muscular activity during the throwing motion in athletes. The early reports were on f rom the Biomechani cs Lab oratory, Centinela Ho spital M edical Center, Inglewo od , Colif. Reprint requests: Marilyn Pink, MS, PT, Centinela Hospital M edical Center , Biomechanics Labo ratory, 555 E. Hardy St., Inglewood, CA 90301. ' References 7, 8, 10- 12, 16, 23-26.
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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 not initially available. This study was designed to describe a comprehens ive, integrated, and quantified data base of electromyographic activ ity in 29 muscles of the shoulder girdle and upper extremity in a large sample of uninjured, skilled pitchers during the fastball pitching motion.
SUBJECTS AND METHODS Fifty-six uninjured, skilled pitchers, who were active at the college or professional level, were evaluated at the Biomechanics Laboratory of Centinela Hospital in Inglewood, California. Indwelling electromyography was used to record act ivity in 29 separate muscles or muscle bellies of the shoulder girdle and dominant upper extremity during the fastball pitch. These included muscles responsible for scapular rotation (7 muscles), 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 pitcher during a single pitching session . Occasionally pitchers volunteered to come back for a second session, but none came back for a third session . Different pitchers had different combinations of the eight muscles monitored . No pitcher had all
15
16 DiGiovine et 01.
START
J. Shoulder Elbow Surg. January /February 1992
HANDS APART
FOOT DOWN
MAXIMAL EXTERNAL ROTATION
BALL RELEASE
FINISH
Figure 1 Six phases of pitching. 29 muscle bellies monitored. Recordings for each muscle were obta ined in an average of 13 pitchers (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 stimulation was then performed to confirm placement, while muscle activity wos observed on an oscilloscope. The electrodes were connected to a baHery-powered transmiHer belt pack to prevent restrictions in body movements. To correlate electromyograph ic act ivity with specific movements in the pitching sequence, a 16 mm motion picture camera was used to film each pitch at the rate of 400 or 450 frames per second. The electromyographic signals were telemetered to a computer for conversion to digital format and quantified by integration of 2500 samples every second . Excluding noise, the peak 1-second signal during a maximal manual muscle test (MMT) was selected as 100% activity. All other muscular activity was assessed every 20 msec and expressed as a relative percentage of the MMT. To facilitate generalized comparisons a range of 0% to 20% was considered low activity; 21% to 40%, moderate activity; 41% to 60%, high act ivity; and greater than 60%, very high activity. The film was reviewed on a stop-action projector. Single-frame viewing allowed each pitch to be accurately d ivided into six phases: windup, early cacking, late cocking, acceleration, deceleration, and follow-through (Fig. 1). As previously defined, follow-through began when the ball left the fingers and continued until motian in the pitching arm ceosed .P- 14.26 Deceleration now describes the first one third of this phase, and follow-through is defined as the second two thirds of this phase. Synchronization between the pitching phases and the
electromyographic data was achieved by use of electronic markers, wh ich were placed on both the motion picture film and the electromyographic recording . Electromyographic activity was averaged with in each phase of the pitch for each muscle and each pitcher. Recorded measurements of the act iv ities for every phase in each muscle were averaged among pitchers and expressed as a mean and standard deviation.
RESULTS Scapular rotators. In the upper trapezius, activity was very high during early cocking. In the middle trapezius, act ivity was high in late cock ing . In all portions of the trapez ius, activity was very high during acceleration. In the lower trapezius, 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 very high 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 during deceleration. During follow-through, activity was moderate to high in the serratus anterior and low to moderate in the rest of the scapular rotators (Fig. 2; Table I). Deltoids. The three portions of the deltoid were highly active during early cocking. In all portions of the deltoid, act ivity decreased to moderate levels during late cocking and acceleration. The only deviation from this paHern was the very high level of activity seen in the posterior deltoid during acceleration . All three portions of the deltoid exhibited high levels of act ivity during deceleration and low levels of activity during follow-through (Fig. 3; Table I). Rotator cuff. The rotator cuff muscles as a group demonstrated moderate activity during
Electromyography of upper extremity in pitching
Volume 7 Number 7
17
Acceleration Deceleration Follow-through Scapular muscles Upper trapezius Middle trapezius Lower trapezius Serratus anterior (sixth rib) Serratus anterior (fourth rib) Rhomboids Levator scapula Glenohumeral muscles Anterior deltoid Middle deltoid Posterior deltoid Supraspinatus Infraspinatus Teres minor Subscapularis (lower third) Subscapularis (upper third) Pectoralis major Latissimus dorsi Elbow and forearm muscles Triceps Biceps Brachialis Brachioradialis Pronator teres Supinator Wrist and finger muscles Extensor carpi radialis longus Extensor carpi radialis brevis Extensor digitorum communis Flexor carpi radialis Flexor digitorum superficialis Flexor carpi ulnaris
64 ::!: 43 ::!: 39 ± 44 ::!:
53 22 30 35
11 11 13 11
18:!: 16 7:!: 5 13 ± 12 14 ± 13
10
20 ± 20 40 ± 22 106 ::!: 56
50 ± 46
35 ± 24 41 ± 26 35 ± 14 72 ± 54
11 11
7± 8 6± 5
37 ::!: 51 ::!: 38 ::!: 69 ±
29 24 29 32
69 ::!: 71 ::!: 76 ± 60 ±
31 32 55 53
53::!: 35:!: 78 ± 51 ±
22 17 33 30
14 ::!: 15 ::!: 25 ± 32 ±
12 14 15 18
34 ± 7
41
24
71 ± 35 77 ± 28
45 ± 28 33 ± 16
14 ± 20 14 ± 13
27 ± 36 ± 68 ± 51 ± 31 ± 54 ± 56 ±
47 ± 59 ± 60 ± 39 ± 37 ± 84 ± 41 ±
±
16 14 18 16 16 12 11
15 ± 9± 6± 13 ± 11 ::!: 5 ::!: 7±
12 8 5 12 9 6 9
40 ± 44 ± 42 ± 60 ± 30 ± 23 ± 26 ±
11
7
8
37 ± 26 99 ± 55
115 ± 82
60 ± 36
16 ± 15
14 13
6 ± 6 11 ± 13 56 ± 27 12 ::!: 10 33 ± 33 50 ± 37
54 ± 24 88 ± 53
29 ± 18 59 ± 35
31 ± 21 24 ± 18
13 18 13 13 14 13
4± 8± 8± 5± 14 ± 9±
6 9 5 5 16 7
17 ± 22 ± 17 ± 35 ± 18 ± 38 ±
89 ::!: 20 ± 20 ± 16 ± 85 ± 55 ±
54 ± 44 ± 49 ± 46 ± 51 :+: 59 :+:
23 32 29 24 21 31
22 ± 18 16:!: 14 13 ± 17 22 ± 29 21 ± 21 22 ± 19
13
11
8
53 ± 24 72 ± 37
30 ± 20
43 ± 24
22 ::!: 14
15
17 ± 17 47 ± 26 75 ::!: 41
55
35
43 :+: 28
24 ± 19
14
21 ± 17 37 ± 25 59 ± 27
35 ± 35
47 ::!: 25
24 ± 18
12 11
13 ± 9 16 ± 6
24 ± 35 47 ± 33 20 ± 23 47 ± 52
120 ± 66 80 ± 66
79 :+: 36 71 :+: 32
35 ± 16 21 ± 11
27 ± 18 41
112 ± 60
77 ± 42
24 ± 18
10
8
±
±
±
5
20 19 26 31 18 15 22
17 14 13 20 15 20
28 ::!: 30 12 ± 17 28 ± 27 49 ± 29 74 ± 34 71 ± 42 62 ::!: 19
37 ± 32 26 ± 20 18 ± 26 31 ::!: 24 39 ::!: 28 54 ± 38
±
25
±
19 22 66 46 28 50 31
40 16 22 12 39 31
34 19 28 43 20 52 23
21 ± 16 ± 13 ± 10 ± 20 ± 25 ± 25 ±
16 13 11 9 16 21 18
'Means and standard deviations, expressed as a percentage of the maximal manual muscle test.
early cocking, very high activity during late cocking, high activity during acceleration, moderate to very high activity during deceleration, and low activity during follow-through. This general pattern was interrupted by exceedingly high activity in the upper third of the subscapularis during late cocking and acceleration. Activity was high in the supraspinatus in early cocking, but the activity level was similar to that of the rest of the rotator cuff in the other phases of the pitch (Fig. 4; Table I).
Internal rotators. The activity of the humeral internal rotators was most notable in the late cocking and acceleration phases-activity was high to very high (Fig. 5; Table I). Elbow flexors and extensors. The triceps exhibited very high activity during the acceleration phase but low to moderate activity in other phases. In the elbow flexors, activity was low to moderate in every phase of the pitch. The brachioradialis distinguished itself from the other elbow flexors; its activity was moderate in early
18 DiGiovine et 01.
J. Shoulder Elbow Surg. January / February 1992
120 100 80
60 ldJ~_~
40
20 0-
OllOW TH RO UG H
L ATE COCKING
MU SCL ES
0
_
UP TRAP
_
MID TRAP
o
SER 6T H
o
RHO M BO IDS _
LOW TRAP
_
SER 4TH
LE VATO R SCAP UL A
Figure 2 EMG activity of scapular rotator muscles.
60
120 100 80 40
20 O --lL-- . . . ::;==-- ..L-- ---=;==--- ..L----=;==--- L..-- -=r==---- L.-- -=;==------/ EARLY CO CKING
L AT E COC KING
ACCEL ERATION
DECE LERAT ION
FO L LOW TH ROU G H
MU SCL ES _
ANTER IOR DELTOID
_
M ID DL E DELTO ID
o
POSTERIOR DELTOI D
_
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 high during deceleration (Fig. 7; Table I).
Wrist and finger extensors and flexors. In the wrist and finger extensors, activity was high during early cocking and very high during
late cocking (Fig. 8; Table I). Activity of the wrist and finger flexors was very high during acceleration and deceleration (Fig. 9; Table I).
DISCUSSION The discussion will be approached through the distinct phases of the baseball pitch. Muscle activity above 40% MMT (high and very high activity) will be addressed as key to understand-
Electromyography of upper extremity in pitching
Volume 7 Number 7
19
120 100 80 60 40 20
o
J'-----=r=--"--.::y=--L_---=;=-_-L-_....:;::=-~C__..:::;=--/
EA RLY COC K I NG
L ATE ACCELERATION COCKIN G
r--------- -
-
DECEL ERATI ON
- - - - - - - - - --
F O L LOW THR O UGH
-
-
-
-
-
-
--,
MUS CLE S _
INFRASPINATUS
_
SUBSCAP UP 1/3
TERES MINOR
0
0
SUBSCAP LOW 1/3
SUPRASPINATUS
Figure 4 EMG activity of rotator cuff muscles.
12 0 100 80 60 40 20
O JL_ -.:;:..
L...-_-.-_-----L_-,,--_-L-_-,.--_ _c __--,--_-----'
EARLY CO C K ING
MUS CL ES _
o
PECTOR ALIS MA J OR
_
L AT IS SIM US DORSI
SUBSCAP LO WER 1/3
_
SUBS CAP UPPER 1/ 3
Figure 5 EMG activity of internal humeral rotator muscles.
ing the muscle mechanics during the fastball pitch. Windup. During windup the activity of the muscles was below 21 % MMT in all cases. The low activity in the upper extremity reflected the
lack of critical events related to performance or to injury potential. Early cocking. During this phase the trapezius and serratus anterior actively positioned the glenoid for the humeral head as the arm
20
DiGiovine et 01.
1. Shoulder Elbow Surg. January / February 1992
120 100 80 60 40 20 O --"--
---,-
-
"---
-.---
--L-
----,r --
-L--
---,-
-
"---
-.---
-J
MUSC L ES _
TRICEPS
_
BI C E PS
0
BRA CH IALI S
_
BRA CHIORADI ALIS
Figure 6 EMG activity of elbow flexor and extensor muscles.
120 100 80 60 40 20 O --"--
-,--
--'::....--
EARLY COC KING
, --
...L.--
-,--
--''---
LAT E ACCELERATION COC KIN G
. -_ -L.._ ---.-_ -----'
DECELERATION
F O LL OW THR O UGH
MU SCL ES -
PRONATOR TERES
_
SUP INATOR
Figure 7 EMG activity of forearm pronator and supinator muscles.
moved into 104° obduction." These scapular muscles formed a force couple to upwardly rotate and protract the scapula, and the deltoids and supraspinatus abducted the arm. If the scapula is not positioned for the abducting and rotating humerus, impingement can occur. The synergism of the deltoids and supraspinatus has been noted in other overhand sportS. 1 8 - 2 0 The supraspinatus, which inserts
closer to the joint axis than does the deltoid, assisted with humeral abduction while it kept the humeral head congruent with the glenoid and stabilized or protected the joint. If the deltoids were to abduct the humerus without the supraspinatus, they would be in a mechanical position to lever the head out of the joint. Saha 2 2 described the humeral muscles as drivers or steerers. In this case the deltoids were
Volume 7 Number 7
Electromyography of upper extremity in pitching
EMG 120 .... 100 80 60
40 20
ACf~VrrY'(
21
¥oMMf )
- - - - - - - - - - - - ---,
--If-+----.
O -"'---,---L.---,-----L.--r--L-_,--_.L-_-:;=~_/
EARLY COC KING
LATE COCKING
MUSCLES -
ECRL
ECRB
0
EDC
Figure 8 EMG act ivity of wrist a nd finger extensor muscles. fCRL, Extensor car pi radialis longus ; fCRB, extensor carpi rad ialis brevis; f OC, extensor dig itorum commun is.
120 100 80 60
40 20
o
-¥----r--..L.----r--L----,---~e.--.---_----L:....__
_._---./
FOLLOW THR O UGH
MUSCL ES _
FCR
FDS
DFCU
Figure 9 EMG activity of wrist and finger flexor muscles. FCR, Flexor carpi radial is; F05, flexor digitorum superficialis; FCU, flexor carpi ulna rus.
the drivers that positioned the arm in space and the supraspinatus was the steerer that delicately fine -tuned the position of the head in the gleno id. This relationship of the deltoids and supraspinatus separated the supraspinatus from the rotators of the rotator cuff and functionally
aligned the supraspinatus with the deltoids for humeral abduction . The extensor carp i radial is longus and extensor carpi radial is brevis were the only other muscles in the upper extremity that revealed high activity during early cocking . During this
22 DiGiovine et 01.
J. Shoulder Elbow Surg. January / February 1992
SGHl
Su bac apul.r I.
Supr•• pl atus
IGHl
Peel.
T. r••
m.jor
mino r
T.r •• ma 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 slight flexion to a position of extension. The wrist extension occurred as the arm was abducting and the palm was facing the ground. Hence the wrist extension was done against gravity with the weight of the ball. Late cocking. During late cocking the humerus maintained its level of abduction but moved from 18° to 11° horizontal adduction and 46° to 170° external rototion.? In that there was no further humeral abduction, the scapula did not need additional elevation. Yet it did need to provide a stable base for the rapid external rotation of the humerus. The middle trapezius, rhomboids, and levator scapula have all been shown to be scapular retractors that function isometrically at the end of the rcnqe.!? Thus these are key muscles in providing scapular stabilization. The serratus anterior was the key muscle opposing the retractors while stabilizing and protracting the scapula. In addition to forming a force couple for stabilization, these muscles may have helped to "tip" the scapula so that the glenoid offered maximum congruency for the externally rotating humeral head. An unstable or improperly positioned scapula could be the source of a potential injury, because the
scapuloglenohumeral rhythm and synchrony would be offset and impingement could occur. As the humerus ceased to increasingly abduct, the deltoids were less active during late cocking. The active glenohumeral muscles included the rotator cuff along with the pectoralis major and latissimus dorsi. Turkel et al. 2 7 described the relative position of the muscles and ligaments around the glenohumeral joint in a position of 90° abduction, neutral horizontal adduction/abduction, and 90° external rotation (Fig. 10). As seen in Fig. 10, the subscapularis, pectoralis major, and latissimus dorsi are all positioned anterior to the glenohumeral joint. These muscles form what we have named the" anterior wall." The anterior wall provides stability to the anterior aspect of the joint. At the end of late cocking when the humerus is externally rotated to 170°, the anterior aspect of the joint is vulnerable to instability and subluxation. It is now understood that if subtle instability and subluxation are left unheeded, they can lead to impingement and rotator cuff tearing.' 0 Thus, if the anterior wall muscles are not firing, the possibility of injury is increased. In addition, during this phase the humerus was horizontally adducted 29°.6 The
Volume 7 Number 7
Electromyography of upper extremity in pitching
pectoralis major undoubtedly contributed to that motion. The posterior ratator cuff muscles were also quite active during late cocking. The infraspinatus and teres minor were actively externally rotating the humerus. Their posterior placement also offered a posterior restraint to the anterior subluxation. A weakness of either of these muscles would diminish the posterior restraint. The supraspinatus was the least active of the rotator cuff muscles. As mentioned previously, the supraspinatus has demonstrated a synergy with the deltoid. During this phase the arm maintained rather than increased its level of elevation. Hence activity of the supraspinatus was lower in late cocking than in early cocking but still high. Also, in the position of extreme external rotation, the supraspinatus was rotated posteriorly. At this point the supraspinatus would be in a relatively ineffective position with less superior compressive force. The superior compressive force, which was abdicated by the supraspinatus, may have been in 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 some compression and support for the superior portion of the anterior wall. The only elbow / forearm muscle that was active during late cocking was the supinator. The literature has shown that during this phase the forearm is in supination. Pronation does not begin until 10 msec before ball releose." Thus the function 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. By cocontracting these muscles the pitchers were able to have a stable base from which they subsequently launched the ball. Acceleration. During acceleration the humerus internally rotated approximately 100° in 0.05 second and the elbow extended an average of 54°.6 Angular velocities of 6100° per second'v 17 created a humeral internal rotation torque of 14,000 inch-pounds." The highest shoulder joint compression force (860 Newtonmeters}? was observed at this time. A stable scapula is needed as a fulcrum for the high angular velocities and torques. All of the scapular muscles demonstrated very high activity as they filled this function. As the humerus was internally rotating, the posterior deltoid was optimally positioned to be the primary humeral horizontal abductor, as
23
noted by its very high electrical activity. The supraspinatus was also highly active as it again demonstrated its relationship to the deltoid. At this point the teres minor and the infraspinatus had differing levels of activity-high in 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 experience with the electromyographic manual muscle testing has revealed higher levels of activity in the teres minor when the humerus is abducted or extended. In addition, electromyographic investigations in other sports have shown separate functions for the infraspinatus and teres minor. 1 8 - 2 o The activity in the teres minor is similar to the activity 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 internal rotation, the teres minor provides a stabilizing posterior restraint. The direction of the fibers of the teres minor give it an extension component, which may be the reason it performs this function. The large angular velocity during acceleration is the result of energy that is transferred from the trunk, with augmentation by the latissimus dorsi and pectoralis major. Bassett et ol." have shown that although both the latissimus dorsi and the pectoralis major can potentially generate large internal rotation torques about the 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 the greater 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 muscles were the only ones in the upper extremity to have a positive correlation between peak torque developed in isokinetic testing and pitching velocity." The subscapularis, especially the upper portion, also exhibited very high activity during this phase and functioned with the latissimus dorsi and the pectoralis major. The subscapularis apparently functioned as the steering muscle to precisely position the humeral head in the glenoid; this is similar to the relationship of the supraspinatus with the deltoid. This alignment of the subscapularis with the latissimus dorsi has been observed in other sports activities. 1 8 - 2 o
24 DiGiovine et 01.
J. Shoulder Elbow Surg. January / February 7992
Elbow extension, which began in late cocking, continued into the acceleration phase and reached an angular velocity of 2200 per second."- 17 Extension of the elbow took place by means of two mechanisms, contraction of the triceps initially and forward momentum of the forearm secondarily.": 6, 21 In a pitcher with a complete radial nerve block, the elbow will initially flex to 155 in the late cocking phase. 2 1 This occurs as the torque generated by the rotating body and arm exerts a centripetal force on the inertia of the forearm, hand, and ball, causing them to collapse toward the body as the late cocking phase ends. Very high triceps activity is mainly responsible for resisting this centripetal flexion torque at the elbow,"- 6, 21 Although triceps-activated elbow extension may not be directly responsible for increased ball velocity, it maintains elbow position and thereby provides the most effective moment arm for other more forceful body rotations to propel the ball. During acceleration there is a large valgus stress at the elbow. The only muscles of the lower arm with very high activity were those that originate 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, but the supinator also needed to be active to control the degree of rapid pronation. Motion of the wrist during acceleration took place from hyperextension to slight extension just before ball relecse.!? The extensor carpi radialis brevis was mainly responsible for the slight extension as it extended the wrist in a neutral position, as opposed to the radial deviation component of the extensor carpi radialis longus. Deceleration. During the deceleration phase the excess kinetic energy that was not transferred to the ball was safely dissipated by controlled deceleration of the upper extremity. Deceleration has been measured to be a negative 500,000 sec/ sec at the shoulder and elbow."" An external rotation torque of 15,000 inch-pounds is experienced by the humerus." In general, opposing muscles around the shoulder, elbow, and wrist fired simultaneously to control the deceleration. In the scapula the trapezius, serratus anterior, and rhomboids all demonstrated high or very high activity. All three heads of the deltoid were 0
0
0
/
active, most notably, the middle and posterior heads because they were positioned more antagonistically to the motion than was the anterior head. The teres minor demonstrated the highest level of activity of all the glenohumeral muscles. This is a carryover of its activity as described in the acceleration phase and is clinically substantiated by the posterior rotator cuff pain, which can be isolated to the teres minor and reproduced in a deceleration motion. The latissimus dorsi was more active than the pectoralis major during deceleration. This is logical, because the pectoralis major loses its mechanical advantage once the humerus drops below 90 0 elevation. The subscapularis also had 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 finger muscles showed high activity, and the wristflexors had very high activity. This is reasonable, since these smaller, more distal joints would have much kinetic energy to dissipate. Follow-through. Follow-through seems to be a noncritical motion, since all shoulder girdle and upper extremity muscles exhibited activity below 42% MMT. During this phase of the pitch, all of the kinetic energy has dissipated, and the trunk is beginning to extend, allowing the pitcher to field his position.
CONCLUSIONS The muscles of the upper extremity during the baseball pitch contract in a highly coordinated manner, balancing the requirements of stabilization with rapid motion. The purpose of stabilization is to protect the musculoskeletal elements and maximize ball velocity by providing the most effective moment arms for the transfer of torques. The purpose of rapid motion is readily apparent; it influences ball velocity, accuracy, and spin. Every muscle group, and indeed every muscle, had a unique role. As a group the scapular muscles were important, because they first functioned to optimally position the glenoid against the humeral head, then provided a stable base for the rapid humeral motion, and finally controlled the deceleration of the scapula after ball release. The three components of the deltoid were active to position the arm. The supraspinatus functioned with the deltoid as it fine-tuned the position of the humeral head against the glenoid. The infraspinatus and teres minor both externally rotated the humerus in late cocking.
Volume 7 Number 7
Electromyography of upper extremity in pitching
During acceleration and deceleration they had differing activities. The teres minor, unlike the infraspinatus, maintained a high level of activity, providing a posterior restraint to limit humeral head translation. This may have clinical relevance, because posterior cuff pain in baseball pitchers can frequently be isolated to the teres minor. The subscapularis, pectoralis major, and latissimus dorsi were part of the components of the" anterior wall," which afforded anterior stability during the time of maximal humeral external rotation. All muscles of the lower arm, which originate on the medial epicondyle of the humerus, demonstrated very high activity during the time of valgus stress to the elbow. They were likely adding a dynamic component to elbow stability. The critical role of the deceleration phase to help dissipate the kinetic energy that was not imparted to the ball is now evident; a lack of muscular control at this time would undoubtedly lead to injury. On the other hand, the windup and follow-through phases demonstrated a lack of consequential events in the upper extremity. This study has provided information on both the coordinated sequence of muscular events and their relative magnitudes in the upper extremity during pitching. This knowledge is critical to the development of exercise protocols and surgical procedures that are used for the prevention and treatment of shoulder and elbow injuries resulting from throwing motions in athletes.
REFERENCES
25
8. Glousman R, Jobe FW, Tibone JE, Moynes DR,Antonelli D, Perry J. Dynamic electromyographic analysis of the throwing shoulder with glenohumeral instability. J Bone Joint Surg [Am] 1988;70A:220-6. 9. Gowan ID, Jobe FW, Tibone JE, Perry J, Maynes DR. A comparative electramyagraphic analysis of the shoulder during pitching: professionol versus amateur pitchers. Am J Sports Med 1987;15:586-90. 10. Jobe FW, Bradley JP. Rotator cuff injuries in baseball: prevention and rehabilitation. Sports Med 1988;6:37887. 11. Jobe FW, Maynes DR. Delineation of diagnostic criteria and a rehabilitation program for rotator cuff injuries. Am J Sports Med 1982;10:336-9. 12. Jobe FW, Moynes DR, Brewster CEo Rehabilitation of shoulder joint instabilities. Orthop Clin North Am 1987; 18:473-82. 13. Jobe FW, Moynes DR,Tibone JE, Perry J. An EMG analysis of the shoulder in pitching: a second report. Am J Sports Med 1984;12:218-20. 14. Jobe FW, Tibone JE, Maynes DR, Perry J. An EMG analysis of the shoulder in throwing and pitching: a preliminary report. Am J Sports Med 1983;11:3-5. 15. Moseley JB, Jobe FW, Pink M, Perry J, Tibone JE. EMG analysis of the scapular muscles during a shoulder rehabilitation program. Am J Sports Med (in press). 16. Pappas AM, Zawacki RM, McCarthy CF. Rehabilitation of the pitching shoulder. Am J Sports Med 1985;13:22335. 17. Pappas AM, Zawacki RM, Sullivan TJ. Biomechanics of baseball pitching: a preliminary report. Am J Sports Med 1985; 13:216-22. 18. Pink M, Jobe FW, Perry J, Kerrigan J, Browne A, Scovazzo ML. The normal shoulder during the backstroke: an EMG and cinematographic analysis of twelve muscles. Clin J Sports Med 1992;2:6-12. 19. Pink M, Jobe FW, Perry J, Kerrigan J, Browne A, Scovazzo Ml. The normal shoulder during the butterfly swim stroke: an EMG and cinematographic analysis of twelve muscles. Clin Orthop (in press).
1. Atwater AE. Biomechanics of overarm throwing movements and of throwing injuries. Exerc Sport Sci Rev 1979;7:43-85.
20. Pink M, Perry J, Jobe FW, Browne A, Scovazzo Ml, Kerrigan J. The normal shoulder during freestyle swimming: an EMG and cinematographic analysis of twelve muscles. Am J Sports Med 1991;19:569-75.
2. Bartlett lR, Storey MD, Simons BD. Measurement of upper extremity torque production and its relationship to throwing speed in the competitive athlete. Am J Sports Med 1989;17:89-91.
21. Roberts EM. Cinematography in biomechanical investigation. In: Cooper JM, ed. Committee on Institutional Cooperation Symposium on Biomechanics, Indiano University, 1970. Chicago: Athletic Institute, 1971.
3. Basmajian JV, Deluca DJ. Muscles alive: their functions revealed by electromyography. Baltimore: Williams and Wilkins, 1985:265-89.
22. Saha AK. Mechanics of elevation of glenohumeral joint. Acta Orthop Scand 1973;44:668-78.
4. Bassett RW, Browne AO, Morrey BF, An KN. Glenohumeral muscle force and moment mechanics in a position of shoulder instability. J Biomech 1990;23:405-15. 5. Elliott B, Grove JR, Gibson B, Thurston B. A three-dimensional cinematographic analysis of the fastball and curveball pitches in baseball. Int J Sports Biomech 1986;2:20-8. 6. Feltner M, Dapena J. Dynamics of the shoulder and elbow joints of the throwing arm during a baseball pitch. lnt J Sports Biomech 1986;2:235-59. 7. Gainor BJ, Piotrowski F, Puhl J, Allen We, Hagen R. The throw: biomechanics and acute injury. Am J Sports Med 1980;8: 114-8.
23. Sisto DJ, Jobe FW. The operative treatment of scopulothoracic bursitis in professional pitchers. Am J Sports Med 1986;14:192-4. 24. Sisto DJ, Jobe FW, Moynes DR, Antonelli DJ. An electromyographic analysis of the elbow in pitching. Am J Sports Med 1987; 15:260-3. 25. Stacey E. Pitching injuries to the shoulder. Athletic J 1984;64:44-7. 26. Tullos HS, King JW. Throwing mechanism in sports. Orthop Clin North Am 1973;4:709-20. 27. Turkel SJ, Panio MW, Marshall Jl, Girgis FG. Stabilizing mechanisms preventing anterior dislocation of the glenohumeral joint. J Bone Joint Surg [Am] 1981;63A: 120817.
The upper extremity is vulnerable to in;ury during the baseball pitch because of the repetitious nature of the action, the extremes in range of motion, and the high angular velocities and torques generated at the shoulder and elbow. Hence this study was designed to describe the muscle-firing patterns through fine-wire electromyography in 29 muscle bellies in the upper extremities of skilled pitchers during the fastball pitch. The results demonstrated that the muscles functioned with precise timing for [oint stabilization to prevent injury, [oint activation to transfer forces to the ball, and [oint deceleration to dissipate forces after ball release. The synchrony of reciprocal and sequential muscle contraction necessary to accomplish these functions was clearly evident. This study provides a better understanding of the coordinated sequence of muscle activity during the throwing motion; this understanding is crucial to the development of exercise protocols and surgical procedures used for treatment and prevention of shoulder and 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 a projectile, the ball. Four parameters determine the 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 high level of neuromuscular control. Synchrony of muscle contraction is vital to the motion. Effectiveness also requires performance at a level that maximally stresses the anatomic elements involved. At work is a delicate balance between mobility and stability. The fine line that separates maximum performance and injury is often crossed, and shoulder and elbow injuries in pitchers are not uncommon.* Understanding the biomechanics of pitching is impo rtant to prevent and treat these injuries. For the past 12 years the Biomechanics Laboratory at Centinela Hospital and the KerlanJobe Clinic have collected electromyographic data on muscular activity during the throwing motion in athletes. The early reports were on f rom the Biomechani cs Lab oratory, Centinela Ho spital M edical Center, Inglewo od , Colif. Reprint requests: Marilyn Pink, MS, PT, Centinela Hospital M edical Center , Biomechanics Labo ratory, 555 E. Hardy St., Inglewood, CA 90301. ' References 7, 8, 10- 12, 16, 23-26.
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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 not initially available. This study was designed to describe a comprehens ive, integrated, and quantified data base of electromyographic activ ity in 29 muscles of the shoulder girdle and upper extremity in a large sample of uninjured, skilled pitchers during the fastball pitching motion.
SUBJECTS AND METHODS Fifty-six uninjured, skilled pitchers, who were active at the college or professional level, were evaluated at the Biomechanics Laboratory of Centinela Hospital in Inglewood, California. Indwelling electromyography was used to record act ivity in 29 separate muscles or muscle bellies of the shoulder girdle and dominant upper extremity during the fastball pitch. These included muscles responsible for scapular rotation (7 muscles), 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 pitcher during a single pitching session . Occasionally pitchers volunteered to come back for a second session, but none came back for a third session . Different pitchers had different combinations of the eight muscles monitored . No pitcher had all
15
16 DiGiovine et 01.
START
J. Shoulder Elbow Surg. January /February 1992
HANDS APART
FOOT DOWN
MAXIMAL EXTERNAL ROTATION
BALL RELEASE
FINISH
Figure 1 Six phases of pitching. 29 muscle bellies monitored. Recordings for each muscle were obta ined in an average of 13 pitchers (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 stimulation was then performed to confirm placement, while muscle activity wos observed on an oscilloscope. The electrodes were connected to a baHery-powered transmiHer belt pack to prevent restrictions in body movements. To correlate electromyograph ic act ivity with specific movements in the pitching sequence, a 16 mm motion picture camera was used to film each pitch at the rate of 400 or 450 frames per second. The electromyographic signals were telemetered to a computer for conversion to digital format and quantified by integration of 2500 samples every second . Excluding noise, the peak 1-second signal during a maximal manual muscle test (MMT) was selected as 100% activity. All other muscular activity was assessed every 20 msec and expressed as a relative percentage of the MMT. To facilitate generalized comparisons a range of 0% to 20% was considered low activity; 21% to 40%, moderate activity; 41% to 60%, high act ivity; and greater than 60%, very high activity. The film was reviewed on a stop-action projector. Single-frame viewing allowed each pitch to be accurately d ivided into six phases: windup, early cacking, late cocking, acceleration, deceleration, and follow-through (Fig. 1). As previously defined, follow-through began when the ball left the fingers and continued until motian in the pitching arm ceosed .P- 14.26 Deceleration now describes the first one third of this phase, and follow-through is defined as the second two thirds of this phase. Synchronization between the pitching phases and the
electromyographic data was achieved by use of electronic markers, wh ich were placed on both the motion picture film and the electromyographic recording . Electromyographic activity was averaged with in each phase of the pitch for each muscle and each pitcher. Recorded measurements of the act iv ities for every phase in each muscle were averaged among pitchers and expressed as a mean and standard deviation.
RESULTS Scapular rotators. In the upper trapezius, activity was very high during early cocking. In the middle trapezius, act ivity was high in late cock ing . In all portions of the trapez ius, activity was very high during acceleration. In the lower trapezius, 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 very high 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 during deceleration. During follow-through, activity was moderate to high in the serratus anterior and low to moderate in the rest of the scapular rotators (Fig. 2; Table I). Deltoids. The three portions of the deltoid were highly active during early cocking. In all portions of the deltoid, act ivity decreased to moderate levels during late cocking and acceleration. The only deviation from this paHern was the very high level of activity seen in the posterior deltoid during acceleration . All three portions of the deltoid exhibited high levels of act ivity during deceleration and low levels of activity during follow-through (Fig. 3; Table I). Rotator cuff. The rotator cuff muscles as a group demonstrated moderate activity during
Electromyography of upper extremity in pitching
Volume 7 Number 7
17
Acceleration Deceleration Follow-through Scapular muscles Upper trapezius Middle trapezius Lower trapezius Serratus anterior (sixth rib) Serratus anterior (fourth rib) Rhomboids Levator scapula Glenohumeral muscles Anterior deltoid Middle deltoid Posterior deltoid Supraspinatus Infraspinatus Teres minor Subscapularis (lower third) Subscapularis (upper third) Pectoralis major Latissimus dorsi Elbow and forearm muscles Triceps Biceps Brachialis Brachioradialis Pronator teres Supinator Wrist and finger muscles Extensor carpi radialis longus Extensor carpi radialis brevis Extensor digitorum communis Flexor carpi radialis Flexor digitorum superficialis Flexor carpi ulnaris
64 ::!: 43 ::!: 39 ± 44 ::!:
53 22 30 35
11 11 13 11
18:!: 16 7:!: 5 13 ± 12 14 ± 13
10
20 ± 20 40 ± 22 106 ::!: 56
50 ± 46
35 ± 24 41 ± 26 35 ± 14 72 ± 54
11 11
7± 8 6± 5
37 ::!: 51 ::!: 38 ::!: 69 ±
29 24 29 32
69 ::!: 71 ::!: 76 ± 60 ±
31 32 55 53
53::!: 35:!: 78 ± 51 ±
22 17 33 30
14 ::!: 15 ::!: 25 ± 32 ±
12 14 15 18
34 ± 7
41
24
71 ± 35 77 ± 28
45 ± 28 33 ± 16
14 ± 20 14 ± 13
27 ± 36 ± 68 ± 51 ± 31 ± 54 ± 56 ±
47 ± 59 ± 60 ± 39 ± 37 ± 84 ± 41 ±
±
16 14 18 16 16 12 11
15 ± 9± 6± 13 ± 11 ::!: 5 ::!: 7±
12 8 5 12 9 6 9
40 ± 44 ± 42 ± 60 ± 30 ± 23 ± 26 ±
11
7
8
37 ± 26 99 ± 55
115 ± 82
60 ± 36
16 ± 15
14 13
6 ± 6 11 ± 13 56 ± 27 12 ::!: 10 33 ± 33 50 ± 37
54 ± 24 88 ± 53
29 ± 18 59 ± 35
31 ± 21 24 ± 18
13 18 13 13 14 13
4± 8± 8± 5± 14 ± 9±
6 9 5 5 16 7
17 ± 22 ± 17 ± 35 ± 18 ± 38 ±
89 ::!: 20 ± 20 ± 16 ± 85 ± 55 ±
54 ± 44 ± 49 ± 46 ± 51 :+: 59 :+:
23 32 29 24 21 31
22 ± 18 16:!: 14 13 ± 17 22 ± 29 21 ± 21 22 ± 19
13
11
8
53 ± 24 72 ± 37
30 ± 20
43 ± 24
22 ::!: 14
15
17 ± 17 47 ± 26 75 ::!: 41
55
35
43 :+: 28
24 ± 19
14
21 ± 17 37 ± 25 59 ± 27
35 ± 35
47 ::!: 25
24 ± 18
12 11
13 ± 9 16 ± 6
24 ± 35 47 ± 33 20 ± 23 47 ± 52
120 ± 66 80 ± 66
79 :+: 36 71 :+: 32
35 ± 16 21 ± 11
27 ± 18 41
112 ± 60
77 ± 42
24 ± 18
10
8
±
±
±
5
20 19 26 31 18 15 22
17 14 13 20 15 20
28 ::!: 30 12 ± 17 28 ± 27 49 ± 29 74 ± 34 71 ± 42 62 ::!: 19
37 ± 32 26 ± 20 18 ± 26 31 ::!: 24 39 ::!: 28 54 ± 38
±
25
±
19 22 66 46 28 50 31
40 16 22 12 39 31
34 19 28 43 20 52 23
21 ± 16 ± 13 ± 10 ± 20 ± 25 ± 25 ±
16 13 11 9 16 21 18
'Means and standard deviations, expressed as a percentage of the maximal manual muscle test.
early cocking, very high activity during late cocking, high activity during acceleration, moderate to very high activity during deceleration, and low activity during follow-through. This general pattern was interrupted by exceedingly high activity in the upper third of the subscapularis during late cocking and acceleration. Activity was high in the supraspinatus in early cocking, but the activity level was similar to that of the rest of the rotator cuff in the other phases of the pitch (Fig. 4; Table I).
Internal rotators. The activity of the humeral internal rotators was most notable in the late cocking and acceleration phases-activity was high to very high (Fig. 5; Table I). Elbow flexors and extensors. The triceps exhibited very high activity during the acceleration phase but low to moderate activity in other phases. In the elbow flexors, activity was low to moderate in every phase of the pitch. The brachioradialis distinguished itself from the other elbow flexors; its activity was moderate in early
18 DiGiovine et 01.
J. Shoulder Elbow Surg. January / February 1992
120 100 80
60 ldJ~_~
40
20 0-
OllOW TH RO UG H
L ATE COCKING
MU SCL ES
0
_
UP TRAP
_
MID TRAP
o
SER 6T H
o
RHO M BO IDS _
LOW TRAP
_
SER 4TH
LE VATO R SCAP UL A
Figure 2 EMG activity of scapular rotator muscles.
60
120 100 80 40
20 O --lL-- . . . ::;==-- ..L-- ---=;==--- ..L----=;==--- L..-- -=r==---- L.-- -=;==------/ EARLY CO CKING
L AT E COC KING
ACCEL ERATION
DECE LERAT ION
FO L LOW TH ROU G H
MU SCL ES _
ANTER IOR DELTOID
_
M ID DL E DELTO ID
o
POSTERIOR DELTOI D
_
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 high during deceleration (Fig. 7; Table I).
Wrist and finger extensors and flexors. In the wrist and finger extensors, activity was high during early cocking and very high during
late cocking (Fig. 8; Table I). Activity of the wrist and finger flexors was very high during acceleration and deceleration (Fig. 9; Table I).
DISCUSSION The discussion will be approached through the distinct phases of the baseball pitch. Muscle activity above 40% MMT (high and very high activity) will be addressed as key to understand-
Electromyography of upper extremity in pitching
Volume 7 Number 7
19
120 100 80 60 40 20
o
J'-----=r=--"--.::y=--L_---=;=-_-L-_....:;::=-~C__..:::;=--/
EA RLY COC K I NG
L ATE ACCELERATION COCKIN G
r--------- -
-
DECEL ERATI ON
- - - - - - - - - --
F O L LOW THR O UGH
-
-
-
-
-
-
--,
MUS CLE S _
INFRASPINATUS
_
SUBSCAP UP 1/3
TERES MINOR
0
0
SUBSCAP LOW 1/3
SUPRASPINATUS
Figure 4 EMG activity of rotator cuff muscles.
12 0 100 80 60 40 20
O JL_ -.:;:..
L...-_-.-_-----L_-,,--_-L-_-,.--_ _c __--,--_-----'
EARLY CO C K ING
MUS CL ES _
o
PECTOR ALIS MA J OR
_
L AT IS SIM US DORSI
SUBSCAP LO WER 1/3
_
SUBS CAP UPPER 1/ 3
Figure 5 EMG activity of internal humeral rotator muscles.
ing the muscle mechanics during the fastball pitch. Windup. During windup the activity of the muscles was below 21 % MMT in all cases. The low activity in the upper extremity reflected the
lack of critical events related to performance or to injury potential. Early cocking. During this phase the trapezius and serratus anterior actively positioned the glenoid for the humeral head as the arm
20
DiGiovine et 01.
1. Shoulder Elbow Surg. January / February 1992
120 100 80 60 40 20 O --"--
---,-
-
"---
-.---
--L-
----,r --
-L--
---,-
-
"---
-.---
-J
MUSC L ES _
TRICEPS
_
BI C E PS
0
BRA CH IALI S
_
BRA CHIORADI ALIS
Figure 6 EMG activity of elbow flexor and extensor muscles.
120 100 80 60 40 20 O --"--
-,--
--'::....--
EARLY COC KING
, --
...L.--
-,--
--''---
LAT E ACCELERATION COC KIN G
. -_ -L.._ ---.-_ -----'
DECELERATION
F O LL OW THR O UGH
MU SCL ES -
PRONATOR TERES
_
SUP INATOR
Figure 7 EMG activity of forearm pronator and supinator muscles.
moved into 104° obduction." These scapular muscles formed a force couple to upwardly rotate and protract the scapula, and the deltoids and supraspinatus abducted the arm. If the scapula is not positioned for the abducting and rotating humerus, impingement can occur. The synergism of the deltoids and supraspinatus has been noted in other overhand sportS. 1 8 - 2 0 The supraspinatus, which inserts
closer to the joint axis than does the deltoid, assisted with humeral abduction while it kept the humeral head congruent with the glenoid and stabilized or protected the joint. If the deltoids were to abduct the humerus without the supraspinatus, they would be in a mechanical position to lever the head out of the joint. Saha 2 2 described the humeral muscles as drivers or steerers. In this case the deltoids were
Volume 7 Number 7
Electromyography of upper extremity in pitching
EMG 120 .... 100 80 60
40 20
ACf~VrrY'(
21
¥oMMf )
- - - - - - - - - - - - ---,
--If-+----.
O -"'---,---L.---,-----L.--r--L-_,--_.L-_-:;=~_/
EARLY COC KING
LATE COCKING
MUSCLES -
ECRL
ECRB
0
EDC
Figure 8 EMG act ivity of wrist a nd finger extensor muscles. fCRL, Extensor car pi radialis longus ; fCRB, extensor carpi rad ialis brevis; f OC, extensor dig itorum commun is.
120 100 80 60
40 20
o
-¥----r--..L.----r--L----,---~e.--.---_----L:....__
_._---./
FOLLOW THR O UGH
MUSCL ES _
FCR
FDS
DFCU
Figure 9 EMG activity of wrist and finger flexor muscles. FCR, Flexor carpi radial is; F05, flexor digitorum superficialis; FCU, flexor carpi ulna rus.
the drivers that positioned the arm in space and the supraspinatus was the steerer that delicately fine -tuned the position of the head in the gleno id. This relationship of the deltoids and supraspinatus separated the supraspinatus from the rotators of the rotator cuff and functionally
aligned the supraspinatus with the deltoids for humeral abduction . The extensor carp i radial is longus and extensor carpi radial is brevis were the only other muscles in the upper extremity that revealed high activity during early cocking . During this
22 DiGiovine et 01.
J. Shoulder Elbow Surg. January / February 1992
SGHl
Su bac apul.r I.
Supr•• pl atus
IGHl
Peel.
T. r••
m.jor
mino r
T.r •• ma 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 slight flexion to a position of extension. The wrist extension occurred as the arm was abducting and the palm was facing the ground. Hence the wrist extension was done against gravity with the weight of the ball. Late cocking. During late cocking the humerus maintained its level of abduction but moved from 18° to 11° horizontal adduction and 46° to 170° external rototion.? In that there was no further humeral abduction, the scapula did not need additional elevation. Yet it did need to provide a stable base for the rapid external rotation of the humerus. The middle trapezius, rhomboids, and levator scapula have all been shown to be scapular retractors that function isometrically at the end of the rcnqe.!? Thus these are key muscles in providing scapular stabilization. The serratus anterior was the key muscle opposing the retractors while stabilizing and protracting the scapula. In addition to forming a force couple for stabilization, these muscles may have helped to "tip" the scapula so that the glenoid offered maximum congruency for the externally rotating humeral head. An unstable or improperly positioned scapula could be the source of a potential injury, because the
scapuloglenohumeral rhythm and synchrony would be offset and impingement could occur. As the humerus ceased to increasingly abduct, the deltoids were less active during late cocking. The active glenohumeral muscles included the rotator cuff along with the pectoralis major and latissimus dorsi. Turkel et al. 2 7 described the relative position of the muscles and ligaments around the glenohumeral joint in a position of 90° abduction, neutral horizontal adduction/abduction, and 90° external rotation (Fig. 10). As seen in Fig. 10, the subscapularis, pectoralis major, and latissimus dorsi are all positioned anterior to the glenohumeral joint. These muscles form what we have named the" anterior wall." The anterior wall provides stability to the anterior aspect of the joint. At the end of late cocking when the humerus is externally rotated to 170°, the anterior aspect of the joint is vulnerable to instability and subluxation. It is now understood that if subtle instability and subluxation are left unheeded, they can lead to impingement and rotator cuff tearing.' 0 Thus, if the anterior wall muscles are not firing, the possibility of injury is increased. In addition, during this phase the humerus was horizontally adducted 29°.6 The
Volume 7 Number 7
Electromyography of upper extremity in pitching
pectoralis major undoubtedly contributed to that motion. The posterior ratator cuff muscles were also quite active during late cocking. The infraspinatus and teres minor were actively externally rotating the humerus. Their posterior placement also offered a posterior restraint to the anterior subluxation. A weakness of either of these muscles would diminish the posterior restraint. The supraspinatus was the least active of the rotator cuff muscles. As mentioned previously, the supraspinatus has demonstrated a synergy with the deltoid. During this phase the arm maintained rather than increased its level of elevation. Hence activity of the supraspinatus was lower in late cocking than in early cocking but still high. Also, in the position of extreme external rotation, the supraspinatus was rotated posteriorly. At this point the supraspinatus would be in a relatively ineffective position with less superior compressive force. The superior compressive force, which was abdicated by the supraspinatus, may have been in 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 some compression and support for the superior portion of the anterior wall. The only elbow / forearm muscle that was active during late cocking was the supinator. The literature has shown that during this phase the forearm is in supination. Pronation does not begin until 10 msec before ball releose." Thus the function 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. By cocontracting these muscles the pitchers were able to have a stable base from which they subsequently launched the ball. Acceleration. During acceleration the humerus internally rotated approximately 100° in 0.05 second and the elbow extended an average of 54°.6 Angular velocities of 6100° per second'v 17 created a humeral internal rotation torque of 14,000 inch-pounds." The highest shoulder joint compression force (860 Newtonmeters}? was observed at this time. A stable scapula is needed as a fulcrum for the high angular velocities and torques. All of the scapular muscles demonstrated very high activity as they filled this function. As the humerus was internally rotating, the posterior deltoid was optimally positioned to be the primary humeral horizontal abductor, as
23
noted by its very high electrical activity. The supraspinatus was also highly active as it again demonstrated its relationship to the deltoid. At this point the teres minor and the infraspinatus had differing levels of activity-high in 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 experience with the electromyographic manual muscle testing has revealed higher levels of activity in the teres minor when the humerus is abducted or extended. In addition, electromyographic investigations in other sports have shown separate functions for the infraspinatus and teres minor. 1 8 - 2 o The activity in the teres minor is similar to the activity 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 internal rotation, the teres minor provides a stabilizing posterior restraint. The direction of the fibers of the teres minor give it an extension component, which may be the reason it performs this function. The large angular velocity during acceleration is the result of energy that is transferred from the trunk, with augmentation by the latissimus dorsi and pectoralis major. Bassett et ol." have shown that although both the latissimus dorsi and the pectoralis major can potentially generate large internal rotation torques about the 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 the greater 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 muscles were the only ones in the upper extremity to have a positive correlation between peak torque developed in isokinetic testing and pitching velocity." The subscapularis, especially the upper portion, also exhibited very high activity during this phase and functioned with the latissimus dorsi and the pectoralis major. The subscapularis apparently functioned as the steering muscle to precisely position the humeral head in the glenoid; this is similar to the relationship of the supraspinatus with the deltoid. This alignment of the subscapularis with the latissimus dorsi has been observed in other sports activities. 1 8 - 2 o
24 DiGiovine et 01.
J. Shoulder Elbow Surg. January / February 7992
Elbow extension, which began in late cocking, continued into the acceleration phase and reached an angular velocity of 2200 per second."- 17 Extension of the elbow took place by means of two mechanisms, contraction of the triceps initially and forward momentum of the forearm secondarily.": 6, 21 In a pitcher with a complete radial nerve block, the elbow will initially flex to 155 in the late cocking phase. 2 1 This occurs as the torque generated by the rotating body and arm exerts a centripetal force on the inertia of the forearm, hand, and ball, causing them to collapse toward the body as the late cocking phase ends. Very high triceps activity is mainly responsible for resisting this centripetal flexion torque at the elbow,"- 6, 21 Although triceps-activated elbow extension may not be directly responsible for increased ball velocity, it maintains elbow position and thereby provides the most effective moment arm for other more forceful body rotations to propel the ball. During acceleration there is a large valgus stress at the elbow. The only muscles of the lower arm with very high activity were those that originate 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, but the supinator also needed to be active to control the degree of rapid pronation. Motion of the wrist during acceleration took place from hyperextension to slight extension just before ball relecse.!? The extensor carpi radialis brevis was mainly responsible for the slight extension as it extended the wrist in a neutral position, as opposed to the radial deviation component of the extensor carpi radialis longus. Deceleration. During the deceleration phase the excess kinetic energy that was not transferred to the ball was safely dissipated by controlled deceleration of the upper extremity. Deceleration has been measured to be a negative 500,000 sec/ sec at the shoulder and elbow."" An external rotation torque of 15,000 inch-pounds is experienced by the humerus." In general, opposing muscles around the shoulder, elbow, and wrist fired simultaneously to control the deceleration. In the scapula the trapezius, serratus anterior, and rhomboids all demonstrated high or very high activity. All three heads of the deltoid were 0
0
0
/
active, most notably, the middle and posterior heads because they were positioned more antagonistically to the motion than was the anterior head. The teres minor demonstrated the highest level of activity of all the glenohumeral muscles. This is a carryover of its activity as described in the acceleration phase and is clinically substantiated by the posterior rotator cuff pain, which can be isolated to the teres minor and reproduced in a deceleration motion. The latissimus dorsi was more active than the pectoralis major during deceleration. This is logical, because the pectoralis major loses its mechanical advantage once the humerus drops below 90 0 elevation. The subscapularis also had 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 finger muscles showed high activity, and the wristflexors had very high activity. This is reasonable, since these smaller, more distal joints would have much kinetic energy to dissipate. Follow-through. Follow-through seems to be a noncritical motion, since all shoulder girdle and upper extremity muscles exhibited activity below 42% MMT. During this phase of the pitch, all of the kinetic energy has dissipated, and the trunk is beginning to extend, allowing the pitcher to field his position.
CONCLUSIONS The muscles of the upper extremity during the baseball pitch contract in a highly coordinated manner, balancing the requirements of stabilization with rapid motion. The purpose of stabilization is to protect the musculoskeletal elements and maximize ball velocity by providing the most effective moment arms for the transfer of torques. The purpose of rapid motion is readily apparent; it influences ball velocity, accuracy, and spin. Every muscle group, and indeed every muscle, had a unique role. As a group the scapular muscles were important, because they first functioned to optimally position the glenoid against the humeral head, then provided a stable base for the rapid humeral motion, and finally controlled the deceleration of the scapula after ball release. The three components of the deltoid were active to position the arm. The supraspinatus functioned with the deltoid as it fine-tuned the position of the humeral head against the glenoid. The infraspinatus and teres minor both externally rotated the humerus in late cocking.
Volume 7 Number 7
Electromyography of upper extremity in pitching
During acceleration and deceleration they had differing activities. The teres minor, unlike the infraspinatus, maintained a high level of activity, providing a posterior restraint to limit humeral head translation. This may have clinical relevance, because posterior cuff pain in baseball pitchers can frequently be isolated to the teres minor. The subscapularis, pectoralis major, and latissimus dorsi were part of the components of the" anterior wall," which afforded anterior stability during the time of maximal humeral external rotation. All muscles of the lower arm, which originate on the medial epicondyle of the humerus, demonstrated very high activity during the time of valgus stress to the elbow. They were likely adding a dynamic component to elbow stability. The critical role of the deceleration phase to help dissipate the kinetic energy that was not imparted to the ball is now evident; a lack of muscular control at this time would undoubtedly lead to injury. On the other hand, the windup and follow-through phases demonstrated a lack of consequential events in the upper extremity. This study has provided information on both the coordinated sequence of muscular events and their relative magnitudes in the upper extremity during pitching. This knowledge is critical to the development of exercise protocols and surgical procedures that are used for the prevention and treatment of shoulder and elbow injuries resulting from throwing motions in athletes.
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