Shoulder instability

Sean Fyfechecks out the evidence for a link between poor scapular control and compromised performance

As sports science knowledge progresses, the role of sports injury practitioners is broadening to include greater input to performance enhancement. This is perfectly logical and reasonable. If we understand how certain biomechanical weaknesses, such as shoulder instability, can affect an athlete’s performance, we can work with other specialists to maximise rehabilitation and injury prevention programmes.

Here, we look at the research evidence on shoulder instability not so much from the point of view of injury rehab but to try and assess whether and how instability might affect performance, even in the asymptomatic athlete. Armed with such knowledge, we should be able to help the sports support team to avoid shoulder injuries in throwing athletes.

It also helps us to appreciate that the athlete who wishes to return to pre-injury performance levels after an instability-related shoulder injury must address important changes in muscle sequencing, activation and strength.

Chronic shoulder instability is the term we use when there is excessive movement (translation) at the glenohumeral (main shoulder) joint. Instability can be symptomatic (painful), or asymptomatic. Much has been written about instability leading to a multitude of injuries such as tendinitis or impingement in overhead athletes; but we know less about whether and how shoulder instability affects performance during overhead sports. There are still gaps in the research, but we can learn a lot by drawing intelligent inferences from the available literature.

Most of the research on the throwing motion has been done on baseball pitchers. It is universally agreed, however, that we can apply these findings to all overhead throwing sports, despite subtle mechanical variations in technique. These sports include volleyball, tennis or the quarterback in American football.

Biomechanics of throwing

During the throwing motion the shoulder undergoes a complex interplay of concentric and eccentric glenohumeral and scapulothoracic muscle contractions, coupled with lengthening and shortening of passive joint structures. There are six stages to the throwing motion.

1. Wind-up:Readying phase, in which the shoulder is in slight internal rotation and abduction.

2. Early cocking:Shoulder moves into 90 degrees abduction and 15 degrees of shoulder horizontal abduction. Early in this stage, the deltoid is active; late in this stage, activity increases in the supraspinatus, infraspinatus and teres minor muscles.

3. Late cocking:This is the first stage when substantial load is placed on the shoulder. The shoulder reaches maximal external rotation (170-180 degrees), the scapula is retracted and the shoulder moves into 15 degrees of horizontal adduction. As a result of tension in the anterior capsular/liga- mentous complex, the humeral head trans- lates posteriorly (moves back) in the glenoid. Activity in infraspinatus, supraspinatus and teres minor reaches its maximum halfway through this phase and eccentric muscle activity of subscapularis occurs at the end of the phase. The rotator cuff muscle activity controls rotation, but also increases joint compression, which limits any sliding effect within the joint (translation). At the end of the late cocking phase, pec major, latissimus dorsi and serratus anterior are recruited to create a maximal horizontal adduction and internal rotation moment.

4. Acceleration:The glenohumeral joint is explosively rotated back to 90 degrees of external rotation – the point of release (or contact) – while abduction is maintained. The body moves forward and opens up, while the scapula protracts to maintain a stable platform for the glenohumeral joint. At the beginning of acceleration, the anterior musculature changes from eccentric to concentric muscle contraction, while the posterior muscles change from concentric to eccentric. Early in this stage, there is marked activity in triceps and pec major, later on in latissimus dorsi and serratus anterior. As the shoulder joint unwinds, the humeral head moves centrally from its previous posterior position in the glenoid.

5. Deceleration:Beginning at release (or contact) and going through to zero degrees of rotation, this stage involves a slowing of the arm and a dissipation of the energy that hasn’t been released to the ball. This is the phase in which the shoulder must withstand the most load. All muscles contract eccentrically to increase joint compression and resist posterior and inferior shear forces.

6. Follow-through:The final phase allows the body and arm to re-balance. Abduction is maintained at 100 degrees, adduction increases to 60 degrees and rotation to 30 degrees. The body follows through with trunk and hip flexion and trunk rotation.

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Changes in the dominant throwing arm

Overhead athletes develop significant muscular differences between their dominant throwing arm and their non-dominant arm.

* Testing at 90 degrees abduction reveals a weakness in external rotation in the dominant shoulder.

* Adduction strength is significantly greater in the dominant shoulder.

* The medial rotators of the dominant shoulder develop greater eccentric strength at the end of range of external rotation when compared to the internal rotators contracting concentrically(1).

* The dominant shoulder develops greater internal rotation strength at speed(2,3).

So the main joint movement change that throwing athletes undergo in their dominant arms include excessive external rotation and decreased internal rotation with an overall decrease in shoulder mobility.

As always, individual variations will exist and we know through clinical assessments and quantitative research that excessive muscle imbalances can lead to shoulder injury. It is also widely accepted that increased joint translation leads to fatigue of the rotator cuff muscles.

Instability and performance

To see how instability affects throwing performance we need an objective measure to gauge performance. The most obvious one is power. I have been unable to find any literature linking throwing power and instability. However, because we understand what determines power (namely, force production over time), we can use the literature to make inferences about how instability affects power.

Force production can be determined by the strength of muscular contraction. The contribution of timing to power lies in the correct amount and sequencing of muscular contractions. Let’s keep in mind force production and timing as factors that will affect power as we take a look at the research.

* Warner et al(4)found significant differences among three different populations with shoulder injuries (asymptomatic, anterior stability and impingement) between internal and external rotator ratios for both peak torque and total work. The asymptomatic subjects demonstrated 30% greater internal rotator strength in the dominant shoulder.

* Ebaugh et al(5) looked at the changes produced in shoulder movement with fatigue from repeated overhead throwing. They found increased elevation and external rotation of the scapula, more clavicular retraction and less external rotation during arm elevation. EMG activity readings showed signs of fatigue in all muscles except the lower trapezius. Infraspinatus and deltoid muscles were most affected. In the context of a sport such as volleyball, where performance relates to repetitive throwing over a prolonged period, these results are significant.

* A study by Glousman et al(6)of skilled throwing athletes with glenohumeral instability used dynamic intramuscular electromyography synchronised with high speed photography to analyse pitching in baseball. They compared their results with another study in which healthy throwing athletes performed the same movements. The athletes with instability showed a mild increase in activity of the biceps, supraspinatus and deltoid. During early cocking and follow-through, infraspinatus was more active. By contrast, it was less active in the late cocking stage, which places more load on the glenohumeral joint. Pec major, subscapularis, latissimus dorsi and serratus anterior all exhibited markedly decreased activity.

Because we know that instability can lead to impingement (Warner has reported(4) that 68% of shoulders presenting with instability also demonstrate signs of impingement) it is reasonable to look at the research on how impingement can affect throwing performance.

* Overhead throwing athletes with signs of impingement demonstrate deficits in their protraction strength and decreased lower trapezius muscle activity with retraction(7).

* Ben-Yishay et al(8)showed muscular inhibition leading to strength deficits in patients with impingement.

What it all adds up to

All this research provides us with quite a good understanding of how muscle function is affected with instability. We can reasonably make the following inferences to work out how instability might affect performance. 1. The sequencing of muscle contractions around the scapulothoracic and gleno-humeral joints is altered. This will definitely have implications for power and accuracy.

2. The two main force-producing muscles of the shoulder girdle for the throwing motion are pec major and latissimus dorsi. These are both powerful internal rotators and adductors. Among athletes with instability, the research shows a decrease in EMG activity in these muscle groups during the throwing motion and a decrease in internal rotation strength.

3. Important stabilising muscles, particularly the ideal function of serratus anterior and infraspinatus, are affected and this will only lead to further instability and subsequent inhibition.

References

1.Scoville CR, Arciero RA et al, ‘End range eccentric antagonist/concentric agonist strength ratios: a new perspective in shoulder strength assessment’ J Orthop Sports Phys Ther1997 Mar; 25(3):203-7

2. Meister K, ‘Injuries to the Shoulder in the Throwing Athlete’Am J Sports Med28 (2)

3.Wilk KE, Andrews JR et al, ‘The strength characteristics of internal and external rotator muscles in professional baseball pitchers’ Am J Sports Med1993 Jan-Feb; 21(1):61-6

4.Warner JJ, Micheli LJ et al, ‘Patterns of flexibility, laxity and strength innormal shoulders and shoulders with instability and impingement’ Am J Sports Med1990 Jul-Aug; 18(4):366-75

5.Ebaugh DD, McClure PW et al, ‘Effects of shoulder muscle fatigue caused by repetitive overhead activities on scapulothoracic and glenohumeral kinematics’ J Electromyogr Kinesiol2005 Aug 23

6.Glousman R, Jobe F et al, ‘Dynamic electromyographic analysis of the throwing shoulder with glenohumeral instability’ J Bone Joint Surg Am1998 Feb; 70(2):220-6

7.Cools AM, Witvrouw EE et al, ‘Isokinetic scapular muscle performance in overhead athletes with and without impingement symptoms’ J Athl Train2005 Jun; 40(2):104-110

8.Ben-Yishay A, Zuckerman JD et al , ‘Pain Inhibition of shoulder strength in patients with impingement syndrome’. Orthopaedics1994 Aug; 17(8):685-8