Kinesiology and Biomechanics: Elbow Complex

9.1 Describe all bony & soft-tissue structures that compose the elbow complex.
9.2 Discuss both osteokinematic (gross, angular) and arthrokinematic (roll / glide / spin) motions at every elbow-complex joint.
9.3 Explain the elbow’s carrying angle.
9.4 Define the Law of Parsimony and relate it to elbow-extensor recruitment patterns.
9.5 Identify each muscle crossing the elbow, detailing its isolated action, synergistic role, and stabilising function.
9.6 Apply muscular roles to common functional tasks of daily life, work, and sport.

Positions the hand anywhere in 3-D space.
• Lengthens the reach (elbow extension).
• Shortens the reach to bring objects toward the trunk / mouth (elbow flexion).
Permits axial rotation of the forearm so the hand can function in all planes.
Provides a rigid base for powerful wrist- & hand-generated grip forces.

Humerus
• Medial & lateral epicondyles
• Supracondylar ridges
• Trochlea (articulates with ulna)
• Capitulum (articulates with radius)
• Fossae
– Coronoid fossa (anterior)
– Radial fossa (anterior)
– Olecranon fossa (posterior)
Ulna
• Olecranon process
• Coronoid process
• Trochlear notch
• Radial notch (for proximal radioulnar articulation)
• Ulnar head & styloid process (distal)
Radius
• Fovea & radial head (disc-shaped)
• Radial tuberosity
• Interosseous border (attachment for interosseous membrane)
• Radial styloid process
Distal forearm
• Ulnar notch of radius
• Articular disc (triangular fibro-cartilage complex)
• Styloid processes of both bones

Humeroulnar joint
• Flexion: coronoid process glides & rolls into coronoid fossa.
• Extension: olecranon process locks into olecranon fossa.
Humeroradial joint
• Flexion: radial head slides into radial fossa.
• Extension: little to no contact between radius & humerus.

Radial (Lateral) Collateral Ligament Complex
• Resists varus stress (distal segment forced medially).
• Adds rotational stability.
Ulnar (Medial) Collateral Ligament Complex
• Resists valgus stress (distal segment forced laterally).
Annular Ligament
• Encircles radial head; vital for proximal radioulnar rotation & stability.
Quadrate ligament & Oblique cord (secondary stabilisers of proximal radioulnar joint).
Interosseous membrane
• Transmits force from radius → ulna & maintains forearm syndesmosis.
Dorsal & Palmar radioulnar ligaments (distal joint capsules).

Primary plane: sagittal.
Joint type: modified hinge (allows tiny terminal rotation).
Motion ranges
• Active flexion: $0^\circ \to 145^\circ$
• Passive flexion: $0^\circ \to 160^\circ$
• Functional ADL range: $30^\circ \to 130^\circ$
• Full extension: neutral (straight) to $5^\circ$ hyper-extension.
Arthrokinematics (ulna on humerus)
• Flexion: trochlear notch rolls & glides anteriorly.
• Extension: trochlear notch rolls & glides posteriorly.

Flexion/extension axis is not perfectly horizontal; it slopes infero-medially.
Creates an oblique resting angle between humerus & ulna — the carrying angle (≈ $15^\circ$ ).
Valgus: distal segment (forearm) positioned away from midline.
Varus: distal segment angled toward midline ("gun-stock deformity" or cubitus varus).

Proximal joint
• Radial head + capitulum + annular ligament + radial notch of ulna.
Distal joint
• Ulnar head + ulnar notch of radius + articular disc.
Both joints move as a linked pair → radius pivots about a stationary ulna.
Supination
• Palm up; ROM $85^\circ \text{–} 90^\circ$ .
• Radius rolls & slides posteriorly (laterally) around ulna.
Pronation
• Palm down; ROM $75^\circ \text{–} 80^\circ$ .
• Radius rolls & slides anteriorly (medially).

Biceps Brachii
• Long head assists shoulder flexion; both heads supinate and flex elbow.
• Selectively active when forearm is supinated during flexion.
• Peak torque between $90^\circ$ – $100^\circ$ elbow flexion.
• Works most efficiently with the shoulder in extension (length–tension advantage).
Brachioradialis
• Maximal force between $100^\circ$ – $120^\circ$ flexion.
• Because it inserts on the radius, it can assist with pronation or supination to mid-position.
• Its compression component enhances elbow stability.
Brachialis
• "Workhorse" due to largest cross-sectional area; generates force in every forearm position (pronated, neutral, or supinated).

Triceps Brachii
• Long head crosses shoulder → also extends & adducts GH joint.
• Provides body support during closed-chain tasks (e.g., push-ups, transfers).
• Capable of high-velocity, high-power concentric or eccentric work.
Anconeus
• Small CSA → minor contributor to extension torque.
• Important for lateral elbow joint stability and joint capsule tensioning.

Nervous system recruits the minimal number of muscle fibres needed to accomplish a motor task.
Low-force elbow extension → anconeus + medial head of triceps only.
As external demand increases → lateral head, then long head of triceps are progressively added.
Advantages
• Energy conservation
• Fine-motor control before gross-motor power

Supinator (primary for low-load tasks)
Biceps Brachii (recruited when moderate-to-high force or speed is required)
• Greatest supination torque when elbow flexed ≈ $90^\circ$ (optimal mechanical line of pull).
Brachioradialis (secondary)
• Most effective rotating the forearm back to mid-position from either extreme.

Pronator Teres
• Median nerve passes between its two heads; potential entrapment site ("pronator syndrome").
Pronator Quadratus
• Always active during pronation; primary stabiliser of distal radioulnar joint.

Forearm pronation/supination usually couples with glenohumeral internal/external rotation to expand total UE rotational ROM.
Proximal shoulder-complex muscles (rotator cuff, scapular stabilisers) fix the humerus so elbow flexors can bring food or objects to the mouth.
During powerful gripping, elbow flexors & extensors co-contract, stiffening the elbow to transmit force distally without unwanted motion.

Annular-ligament compromise (e.g., "nurse-maid’s elbow" radial-head subluxation) disrupts proximal radioulnar stability.
Excess valgus stress (throwing athletes) strains the medial collateral ligament → potential for "Tommy John" injuries.
Excessive varus (cubitus varus) often results from malunited supracondylar fractures in childhood.
Median-nerve compression within pronator teres may mimic carpal-tunnel symptoms.
Understanding the Law of Parsimony aids clinicians in designing progressive resistance programmes that minimise undue fatigue while targeting specific heads of triceps.