Biomechanics of Upper Limb

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Shoulder Complex - The Mobile Anchor

  • Glenohumeral Stability:
    • Static: Labrum, capsule, GH & CH ligaments, negative intra-articular pressure.
    • Dynamic: Rotator cuff (📌 SITS), biceps long head, deltoid, scapular stabilizers.
  • Rotator Cuff (📌 SITS): Key for movement & dynamic stability.
    • Supraspinatus: Initiates abduction (first ~15°).
    • Infraspinatus & Teres Minor: External rotation.
    • Subscapularis: Internal rotation.
  • Scapulohumeral Rhythm: Coordinated GH & scapulothoracic (ST) movement. Overall ratio GH:ST approx. $2:1$.

    ⭐ The overall ratio of glenohumeral to scapulothoracic movement during arm elevation is approximately 2:1.

    • Muscles: Serratus anterior, trapezius.
  • Force Couples: Synergistic muscle actions.
    • Deltoid (up) & Rotator Cuff (down/compressive) → stable abduction.
    • Trapezius & Serratus Anterior → scapular upward rotation.
  • Common Movements:
    • Abduction: Deltoid, Supraspinatus.
    • Flexion: Ant. Deltoid, Coracobrachialis, Pectoralis Major (clavicular). Scapulohumeral rhythm during shoulder abduction

Elbow & Forearm - Hinge & Swivel

  • Joints & Movements:
    • Humeroulnar: Hinge (flexion/extension).
    • Humeroradial: Hinge & pivot (flexion/extension, pro/supination).
    • Proximal Radioulnar: Pivot (pronation/supination).
  • Carrying Angle: (Cubitus valgus)
    • Males: 5-10°; Females: 10-15°.
    • Clears hips during ambulation. Carrying angle of the elbow
  • Stability:
    • Valgus: Medial Collateral Ligament (MCL/UCL).
    • Varus: Lateral Collateral Ligament (LCL) complex.
  • Key Muscles:
    • Flexors: Biceps brachii, brachialis, brachioradialis.
    • Extensors: Triceps brachii, anconeus.
    • Pronators: Pronator teres, pronator quadratus.
    • Supinators: Supinator, biceps brachii.
  • Forces: High in lifting (compression) & throwing (MCL tension).

⭐ The normal carrying angle of the elbow is typically 5-10 degrees in males and 10-15 degrees in females, facilitating objects to be carried away from the thighs.

Wrist & Hand - Dexterity's Dance

  • Wrist Joint: Radiocarpal & Ulnocarpal complex (TFCC critical for ulnar stability).
    • 📌 Carpal Bones: Some Lovers Try Positions That They Can't Handle (Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate).
  • Carpal Kinematics: "Dart thrower's motion" - coupled flexion-ulnar deviation & extension-radial deviation for functional tasks.
  • Hand Arches: Longitudinal & Transverse (proximal, distal) allow cupping & grip adaptation. Anatomy of hand arches and carpal bones
  • Grip Types & Muscle Roles:
  • Tendon Pulley System: Annular (A1-A5) & cruciform (C1-C3) ligaments prevent flexor tendon bowstringing, optimizing mechanical efficiency.

⭐ The scaphoid bone acts as a crucial mechanical link between the proximal and distal carpal rows, and is the most commonly fractured carpal bone (often from FOOSH).

Clinical Applications - When Forces Fail

  • Rotator Cuff Impingement/Tears: Subacromial space ↓, repetitive overhead motion → supraspinatus compression. Abnormal scapulohumeral rhythm. Normal vs. Impinged Rotator Cuff
  • Shoulder Instability: Humeral head excessive translation. Static/dynamic stabilizer failure. Force > stabilizing capacity.
  • Tennis Elbow (Lateral Epicondylitis): Repetitive wrist extension/supination. Eccentric ECRB overload.

    ⭐ Tennis elbow (lateral epicondylitis) is primarily caused by repetitive eccentric overload of the extensor carpi radialis brevis (ECRB) origin.

  • Golfer's Elbow (Medial Epicondylitis): Repetitive wrist flexion/pronation. Common flexor origin overload.
  • Carpal Tunnel Syndrome: ↑ carpal tunnel pressure → median nerve compression. Repetitive wrist movements. Cross-section of carpal tunnel with median nerve
  • Ulnar Nerve Entrapment:
    • Elbow (Cubital Tunnel): Compression/traction in cubital tunnel; elbow flexion ↑ pressure.
    • Wrist (Guyon's Canal): Compression in Guyon's canal; repetitive trauma/pressure.

High‑Yield Points - ⚡ Biggest Takeaways

  • Shoulder joint: Most mobile, least stable; rotator cuff (SITS) crucial for dynamic stability.
  • Scapulohumeral rhythm: Essential 2:1 ratio for full shoulder abduction.
  • Elbow: A hinge joint (flexion/extension); normal carrying angle is 5-15° valgus.
  • Forearm rotation: Pronation/supination via proximal & distal radio-ulnar joints.
  • Wrist (radiocarpal joint): Enables flexion-extension & radial-ulnar deviation.
  • Grip strength: Relies on extrinsic flexors & intrinsic hand muscles.
  • Upper limb joints function as lever systems, optimizing force and range of motion.

Practice Questions: Biomechanics of Upper Limb

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A 35-year-old tennis player presents with pain on the outer aspect of the elbow that worsens with gripping activities and wrist extension. The pain is localized over the lateral epicondyle. This condition most likely involves:

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Flashcards: Biomechanics of Upper Limb

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When the knee is extended, the _____ bundle of ACL is tight, and the AM bundle is moderately lax.

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When the knee is extended, the _____ bundle of ACL is tight, and the AM bundle is moderately lax.

PL

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