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Overhand Throwing Mechanics and the Glenohumeral Joint

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Glenohumeral Injury - T. Kobayashi 1Overhand ThrowingMechanicsand the Glenohumeral JointOverhand ThrowingMechanicsand the Glenohumeral JointTracey KobayashiCity College of San FranciscoDepartment of Physical Education and DanceThe structure of the shoulder complex makes it susceptible to injury duringthrowing, particularly when throwing in a fatigued state. This presentationcovers mechanical factors related to the overhead throw, and touches on someimplications for causes of chronic shoulder pain as well as suggestedprecautions.Glenohumeral Injury - T. Kobayashi 2Gross StructureGross StructureReview of Gross Structure (name bones & muscles) - Bones: scapula; clavicle; humerus - Joints: glenohumeral; scapulothoracic; acromioclavicular; sternoclavicular - Rotator Cuff muscles provide dynamic stability to the joint via mainlycompressive forces.Glenohumeral Injury - T. Kobayashi 3Joint StructureJoint StructureGlenohumeral Joint -- ball & socket configuration allows three-dimensionalmobility - shallowness of the glenoid fossa and large humeral head aid mobility at theexpense of stability - vacuum effect assisted by the glenoid labrum, balance of muscular forces &joint fluid provides stability - surrounding joint capsule and ligaments provide static stability - surrounding muscles provide dynamic stabilityAcromioclavicular and Sternoclavicular Joints: stabilized primarilythrough ligamentsAcromial Arch (acromion and coracoacromial ligament): inhibits upwardmovement of the humeral headThrowing issues, as we’ll see later, arise due to instability of the glenohumeraljointGlenohumeral Injury - T. Kobayashi 4Shoulder Angle ConventionsShoulder Angle ConventionsSome conventions for describing shoulder movements and position withinfluence on the throwing motion:o Flexion / Extensiono Internal / External Rotationo Horizontal Abduction/AdductionGlenohumeral Injury - T. Kobayashi 5Pitching PhasesPitching PhasesPitching Phases (from Stodden, et al, 2005).o Wind-up: ends at knee upo Stride: ends at foot contacto Arm Cocking: ends at maximum external rotation Critical instant - maximum internal Torque occurs during this phase Max External Rotation 150o - 210oo Arm Acceleration: ends after releaseo Arm Deceleration: ends after maximum internal rotation Critical instant - max compressive forceo Follow-throughGlenohumeral Injury - T. Kobayashi 6Anatomical IssueHumeral Head DistractionAnatomical IssueHumeral Head DistractionAnterior Translation• Tensile failure of supportingmusculature during deceleration• Rotator Cuff• Biceps Brachii• Caused by rotator cuff and/orbiceps fatigue -> increasedligament loadsGenerally, most shoulder pain in pitchers and other overhand throwing athletesis related to humeral head distraction, usually anterior translation. Distractioncan be attributed largely to tensile failure of the supporting musculature(rotator cuff - mid inferior area of supraspinatus most common - and bicepsbrachii).This failure is caused by fatigue due to high loads. Excessive external rotationduring cocking -> increased eccentric loads on rotator cuff -> fatigue ->increased demand on the Inferior Glenohumeral ligament, the primaryconstraint to external rotation in abduction. Rotator cuff compresses humeralhead in cavity for additional support. As rotator cuff fatigues, it exerts lesscontrol, increasing ligament loads.Role of biceps: provides elbow flexion torque and glenohumeral compression.Max elbow flexion torque must occur early enough before compression or itwill be required to exert a higher maximum force. Biceps fatigue can strainthe ligament structures.Glenohumeral Injury - T. Kobayashi 7• Tearing• Grinding• “Pinching”• Soft Tissue StrainAnatomical IssueHumeral Head DistractionAnatomical IssueHumeral Head DistractionEffectsDistraction Force > 100% body weight may put rotator cuff and labrum at riskRepetitive Trauma -> chronic pain - Tearing: compressive force from translation, joint laxity & high (380 N)anterior force during cocking. - Grinding: rapid internal rotation with above. - Labrum “trapped” between humeral head and glenoid rim (acute) - Soft tissue strain: strain of gh joint restraints concurrent with humeral headsubluxationGlenohumeral Injury - T. Kobayashi 8• Internal rotation torque during cocking• Compressive force during decelerationKinematics and KineticsTwo Critical InstantsKinematics and KineticsTwo Critical InstantsFactors Relating to Shoulder DistractionInternal rotation torque during cocking - least stable in abduction combined with external rotation - shoulder position @ max ext rotation (184 + 14o): greater external rotation - high peak external rotation & abduction torquesCompressive force during arm deceleration - posterior & compressive forces resist distraction (biceps) - horizontal abduction torque resists horizontal adduction - external rotatory torque resists internal rotationChanges in throwing mechanics during a game session (Barrentine et al) - @ foot contact: +5o abduction; +5o horizontal adduction; +8o externalrotation - arm cocking and acceleration: -4o external rotation; - @ release: -4o abductionGlenohumeral Injury - T. Kobayashi 9ApplicationsApplicationsMax Throws/Game2 + 0.6106 + 1617-182 + 0.491 + 1615-162 + 0.476 + 1613-142 + 0.568 + 1811-122 + 0.652 + 158-10GAMES/ WKMAX /GAMEAGE89 + 2262 + 2145 + 2527 + 2217-1877 + 2062 + 2338 + 2325 + 2015-1670 + 2056 + 2036 + 2130 + 2213-1458 + 1855 + 2335 + 2027 + 2011-1251 + 1943 + 1634 + 1621 + 188-104 DAYREST3 DAYREST2 DAYREST1 DAYRESTAGERecommended Rest for Min ThrowsGlenohumeral Injury - T. Kobayashi 10ApplicationsApplicationsRecommended Ages for Learning Various Pitches17 + 2Screw ball16 + 2Fork ball16 + 2Slider15 + 3Knuckle ball14 + 2Curve ball10 + 3Change-Up8 + 2FastballAgePitchGlenohumeral Injury - T. Kobayashi 11ApplicationsApplications• Glenohumeral internal rotation deficitin those with impingement.• Higher maximum external rotationassociated with injury/pain,CuesInternal rotation: throwing shoulder -8.5o in normal pitchers.Glenohumeral Injury - T. Kobayashi 12ApplicationsApplications• Conditioning• Muscular & General Endurance• Muscular Strength• Range of Motion• Training• Teach proper throwing mechanics• Keep age-related progression in mind• Don't throw to fatiguePreventionAppropriate Conditioning & TrainingSpecific Conditioning - Shoulder exercises -Glenohumeral Injury - T. Kobayashi 13PreventionPreventionCAVEAT: no one


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