Biomechanics of Upper Limb

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).

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.
• 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.
• 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.
• 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.
• 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.