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Dynamic Electromyographic Analysis of the Throwing Shoulder with Glenohumeral Instability.

·J Bone Joint Surg Am·1988·575 citations·Shoulder & Elbow
PubMed
SummaryAbstract on PubMed →

This study used dynamic intramuscular EMG to compare shoulder muscle activation during live pitching in 15 throwers with confirmed anterior glenohumeral instability versus 12 healthy throwing controls. It defines which muscles fire abnormally and during which phases, providing a neuromuscular map for rehabilitation design.

Study Snapshot

Design
Prospective case-control
Setting: Single center, Inglewood, California
Funding: None
Objective
Whether shoulder muscle EMG patterns differ in throwers with glenohumeral instability vs. normals
Outcome(s)
EMG activity levels across 8 shoulder muscles during 5 pitching phases
Subjects
27 male throwers
  • 15Unstable
  • 12Normal controls
Inclusion
  • Skilled thrower with chronic anterior instability
  • Positive anterior apprehension sign
  • Instability confirmed surgically
Exclusion
  • Impingement
  • Degenerative arthritis

Key Findings

  • The internal rotators — pectoralis major, subscapularis, and latissimus dorsi. Were markedly underactive in unstable throwers during late cocking and acceleration (p < 0.05).
    –These are precisely the phases when the shoulder is at maximum external rotation and most vulnerable to anterior subluxation.
    –Reduced internal rotation force allows persistent external rotation, directly increasing stress on the anterior capsule and labrum.
  • Serratus anterior was lower throughout all pitching phases, with the greatest gaps during acceleration and follow-through (p < 0.05).
    –Reduced scapular protraction leaves the glenoid behind the forward-flexing humerus, concentrating force on the anterior labrum and capsule.
    –The authors also note this deficient scapular rotation may narrow the subacromial space, linking serratus dysfunction to the rotator cuff tendinitis commonly seen in throwers.
  • Biceps and supraspinatus were mildly more active in unstable shoulders, interpreted as a compensatory attempt to compress the humeral head against the glenoid when ligamentous restraints are lax.
  • The infraspinatus showed a paradoxical pattern:
    –Early cocking and follow-through: increased activity
    –Late cocking: decreased activity
    –This pattern does not fit clean compensatory inhibition and may add to relative instability during the most dangerous phase.
  • The deltoid was the only muscle showing no significant difference between groups. Consistent with its role in elevation rather than glenohumeral stability during throwing.
Board PearlIn throwers with anterior instability, internal rotators and serratus anterior are markedly underactive during late cocking — target these muscles in every rehabilitation program.

Clinical Relevance

A throwing athlete with anterior instability does not have a purely ligamentous problem. The dynamic stabilizers have failed too, and this paper maps exactly which ones and when. The subscapularis, pectoralis major, latissimus dorsi, and serratus anterior are all markedly underactive at the precise moment they are needed most: the transition from late cocking into acceleration.

This is the neuromuscular basis for why internal rotator strengthening and serratus anterior retraining are non-negotiable in any rehabilitation program for these athletes, whether conservative or postoperative. Before returning a thrower to sport after instability repair, verify that eccentric internal rotator control and scapular protraction have been specifically trained and tested.

This paper also explains why impingement and instability coexist in throwers. Deficient serratus anterior function narrows the subacromial space, so the rotator cuff tendinitis you see in these athletes is partly a downstream consequence of scapular dyskinesis, not simply a separate diagnosis.

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|

Dynamic Electromyographic Analysis of the Throwing Shoulder with Glenohumeral Instability.

·J Bone Joint Surg Am·1988·575 citations·Shoulder & Elbow
PubMed
SummaryAbstract on PubMed →

This study used dynamic intramuscular EMG to compare shoulder muscle activation during live pitching in 15 throwers with confirmed anterior glenohumeral instability versus 12 healthy throwing controls. It defines which muscles fire abnormally and during which phases, providing a neuromuscular map for rehabilitation design.

Study Snapshot

Design
Prospective case-control
Setting: Single center, Inglewood, California
Funding: None
Objective
Whether shoulder muscle EMG patterns differ in throwers with glenohumeral instability vs. normals
Outcome(s)
EMG activity levels across 8 shoulder muscles during 5 pitching phases
Subjects
27 male throwers
  • 15Unstable
  • 12Normal controls
Inclusion
  • Skilled thrower with chronic anterior instability
  • Positive anterior apprehension sign
  • Instability confirmed surgically
Exclusion
  • Impingement
  • Degenerative arthritis

Key Findings

  • The internal rotators — pectoralis major, subscapularis, and latissimus dorsi. Were markedly underactive in unstable throwers during late cocking and acceleration (p < 0.05).
    –These are precisely the phases when the shoulder is at maximum external rotation and most vulnerable to anterior subluxation.
    –Reduced internal rotation force allows persistent external rotation, directly increasing stress on the anterior capsule and labrum.
  • Serratus anterior was lower throughout all pitching phases, with the greatest gaps during acceleration and follow-through (p < 0.05).
    –Reduced scapular protraction leaves the glenoid behind the forward-flexing humerus, concentrating force on the anterior labrum and capsule.
    –The authors also note this deficient scapular rotation may narrow the subacromial space, linking serratus dysfunction to the rotator cuff tendinitis commonly seen in throwers.
  • Biceps and supraspinatus were mildly more active in unstable shoulders, interpreted as a compensatory attempt to compress the humeral head against the glenoid when ligamentous restraints are lax.
  • The infraspinatus showed a paradoxical pattern:
    –Early cocking and follow-through: increased activity
    –Late cocking: decreased activity
    –This pattern does not fit clean compensatory inhibition and may add to relative instability during the most dangerous phase.
  • The deltoid was the only muscle showing no significant difference between groups. Consistent with its role in elevation rather than glenohumeral stability during throwing.
Board PearlIn throwers with anterior instability, internal rotators and serratus anterior are markedly underactive during late cocking — target these muscles in every rehabilitation program.

Clinical Relevance

A throwing athlete with anterior instability does not have a purely ligamentous problem. The dynamic stabilizers have failed too, and this paper maps exactly which ones and when. The subscapularis, pectoralis major, latissimus dorsi, and serratus anterior are all markedly underactive at the precise moment they are needed most: the transition from late cocking into acceleration.

This is the neuromuscular basis for why internal rotator strengthening and serratus anterior retraining are non-negotiable in any rehabilitation program for these athletes, whether conservative or postoperative. Before returning a thrower to sport after instability repair, verify that eccentric internal rotator control and scapular protraction have been specifically trained and tested.

This paper also explains why impingement and instability coexist in throwers. Deficient serratus anterior function narrows the subacromial space, so the rotator cuff tendinitis you see in these athletes is partly a downstream consequence of scapular dyskinesis, not simply a separate diagnosis.

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