Kinesiology and Biomechanics of Elbow and Forearm

Elbow and Forearm Complex Overview

General Overview of the Elbow Complex

• The elbow and forearm complex includes several key joints and components vital for the movement and function of the upper limb.

• Key Joints:

  • Humeroulnar

  • Proximal Radioulnar

  • Humeroradial

  • Distal Radioulnar

• The Humeroulnar, Humeroradial, and Proximal Radioulnar joints are contained within the same joint capsule (Figure 6-11, 6-27).

Functions of the Elbow Complex

• The elbow complex functions primarily to:

  • Adjust the length of the upper limb by facilitating elbow flexion and extension.

  • Adjust the orientation of the palm, allowing for various functional tasks (e.g., grasping, pushing).

Osteology of the Elbow

Distal Humerus Characteristics

• Trochlea:

  • Located on the medial aspect of the distal end of the humerus.

  • Shaped like a pulley with a saddle configuration (convex anterior/posterior and concave medial/lateral).

  • Features a medial and lateral lip separated by a groove.

  • The medial lip is typically more prominent and extends further distally (Figure 6-2, 6-4).

• Capitulum:

  • Rounded, hemispheric ovoid surface located on the lateral aspect of the distal end of the humerus.

• Epicondyles:

  • Medial Epicondyle:

  • Prominent medial projection providing attachment sites for ligaments and muscles.

  • Lateral Epicondyle:

  • Less prominent than medial epicondyle, also serves as an attachment site.

Osteology of the Ulna

• Olecranon Process:

  • Flat, blunt proximal end; insertion point for the triceps brachii.

• Coronoid Process:

  • Anterior projection/ridge from the proximal aspect of the ulna serving as a site for ligament and muscle attachments. (Figure 6-7).

• Trochlear Notch:

  • Large crescent-shaped concavity between the olecranon and coronoid processes.

  • Features a longitudinal crest at the center.

• Radial Notch:

  • Small concavity on the lateral surface just distal to the trochlear notch, serving as a site for attachment.

• Tuberosity of Ulna:

  • Roughened area at the base of the coronoid process allowing attachment for the brachialis muscle.

• Ulnar Head:

  • Rounded, slightly expanded distal end, mostly covered by articular cartilage.

• Styloid Process:

  • Sharp pointed tip at the distal end (Figure 6-5).

Osteology of the Radius

• Head:

  • Cap-like expansion at the proximal end with a shallow concavity covered by articular cartilage.

  • Contacts the ulnar radial notch (Proximal Radioulnar joint) (Figure 6-5).

• Bicipital (Radial) Tuberosity:

  • Roughened area on the anterior/medial surface just distal to the radial head.

• Radial Neck:

  • Narrow section connecting head and tuberosity.

• Ulnar Notch:

  • Concavity on medial surface of the distal radius.

• Radial Styloid Process:

  • Prominent, pointed projection from the lateral aspect at the distal end.

Elbow Joint Arthrology

Joint Composition

• The elbow joint consists of two primary joints:

  • Humeroulnar (HU): Humeral trochlea with ulnar trochlear notch.

  • Humeroradial (HR): Proximal surface of the radial head with the capitulum of the humerus.

• Movement Plane:

  • Sagittal plane allowing flexion and extension.

• Axis of Movement:

  • Medial-lateral axis running through the lateral epicondyle.

Carrying Angle

• The axis of motion is angled in a superior lateral direction due to the inferior projection of the medial lip of the trochlea.

  • This configuration causes the humerus to deviate laterally at the elbow (Cubital Valgus) at approximately 15° in full elbow extension.

  • There is great variability in this angle ranging from 5° (Varus) to 30° (Valgus) (Figure 6-9).

Joint Capsule

• The HU and HR joints are enclosed within a thin articular capsule lined with synovial membrane, enveloping the humeroulnar, humeroradial, and proximal radioulnar joints (Figures 6-10, 6-11).

Elbow Capsular Ligaments

Medial Collateral Ligament (MCL) - 3 Segments

1. Anterior Fibers:

   • From the anterior aspect of the medial humeral epicondyle to the medial aspect of the coronoid process.

   • Strongest and stiffest band, resisting valgus stress.

   • Most fibers are taut in full extension, some fibers remain taut in full flexion, providing stability throughout the range of motion (ROM).

2. Posterior Fibers:

   • Extends from the posterior aspect of the medial epicondyle to the medial surface of the olecranon.

   • Becomes taut in extremes of elbow flexion.

3. Transverse Fibers:

   • From the olecranon process to the coronoid process; provides little stability because both attachments are on the ulna.

Lateral Collateral Ligament

• From the lateral humeral epicondyle, splitting into two bands, all resisting varus forces:

  • Radial Collateral Ligament:

  • Fibers blend with the annular ligament.

  • Lateral Ulnar Collateral Ligament:

  • Attaches to the supinator crest.

Joint Stability

• Stability is provided by a combination of:

  • Bones, ligaments, and capsule

  • Muscle actions

  • Mechanoreceptors in ligaments aiding in proprioception and detection of safe limits of passive tension.

  • Intracapsular air pressure: defined as the volume of air divided by volume of space, reduced when the capsule is less stiff and there is an inverse relationship between pressure and vacuum (stiffness).

Kinematics of the Elbow Joint

General Movement Characteristics

• The elbow allows for flexion and extension with limitations from:

  • Paralysis, contracture, inflammation, tight joint capsule, skin tightness.

• Flexion and extension functions:

  • Flexion enables lifting, pulling, and bringing the hand to the face/body.

  • Extension aids in reaching and pushing.

• Full Range of Motion:

  • Ranges from 5° of hyperextension to 130° of flexion.

• Functional Range of Motion:

  • From 30° to 130° of flexion.

Humeroulnar Joint Arthrokinematics

• Flexion:

  • The trochlear notch rolls and glides anteriorly over the trochlea; requires the full anatomical length of the posterior capsule, extensor muscles, ulnar nerve, and posterior fibers of MCL (Figure 6-17).

• Extension:

  • The trochlear notch rolls and glides posteriorly over the trochlea; requires the full anatomical length of the anterior capsule, flexor muscles, anterior skin, and anterior fibers of MCL.

  • Note that the motion described in figures can be reversed accordingly.

Humeroradial Joint Arthrokinematics

• Flexion:

  • The radial head rolls and glides anteriorly over the convex capitulum.

• Extension:

  • The radial head rolls and glides posteriorly with little to no contact between surfaces in full extension, influenced by the pull of flexor muscles (Figure 6-18).

Interosseous Membrane

• Fibrous connective tissue structure connecting the interosseous borders of the radius and ulna.

• It allows for the connection between the radius and ulna without limiting pronation and supination.

• Functions:

  • Provides surface area for muscle attachment.

  • Collagen bundles are directed distal and medial from the radius to the ulna, allowing for efficient force transfer:

  • From the hand to the radius to ulna to humerus.

Compression Forces

• Approximately 80% of weight-bearing compression forces on the hand are transmitted laterally through to the radius.

• Portions of these forces are distributed to the ulna via the interosseous membrane, allowing more even distribution of forces through the humeroulnar (HU) and humeroradial (HR) joints rather than solely at the HR joint (Figure 16-21).

• Major muscles attached to the radius pull proximally, and the interosseous membrane facilitates force distribution to protect HU and HR joints from wear and tear.

Distraction Forces

• The fibers of the interosseous membrane are oriented to resist distractive forces.

• When carrying loads, fibers/bands of the interosseous membrane may become slack, and forces generated by oblique cords, annular ligament, and surrounding muscles (e.g., brachioradialis, pronator teres) may counterbalance this distraction (Figure 6-22).

Forearm Arthrology

Pronation and Supination

• The forearm allows for the rotation of the radius around its longitudinal axis through both proximal and distal radioulnar joints, enabling the hand to rotate independently of the ulna or humerus.

• Limitations in pronation/supination can be compensated by glenohumeral rotation.

• In open kinetic chain (OKC), the radius rotates around a fixed ulna (Figure 6-24).

Proximal Radioulnar Joint Characteristics

• The joint features a convex periphery of the radial head articulating with a fibro-osseous ring and shares a joint capsule with HR and HU joints.

• Quadrate Ligament:

  • A band from distal to the radial notch to the medial radial neck, aiding joint stability.

• Annular Ligament:

  • A ring-like fibrous band that attaches to the edges of the radial notch of the ulna, encircling the radial head to hold it against the radial notch, lined with articular cartilage, and attached to the joint capsule, radial collateral ligament, and supinator muscle.

Distal Radioulnar Joint Characteristics

• Articular Disc:

  • Triangular fibrocartilage structure extending from the rim of the ulnar notch laterally to a depression at the base of the ulnar styloid, with anterior and posterior borders continuous with the capsule.

• Ulnocarpal Complex (Triangular Fibrocartilage Complex, TFC):

  • Composed of the articular disc and other connective tissues that stabilize the distal radioulnar joint; disruptions can lead to instability and limit pronation/supination (Figure 7-9).

Forearm Kinematics

General Characteristics

• Neutral Position:

  • Defined as midway between full pronation and supination.

• Full Anatomical Range of Motion:

  • Approximately 75° for pronation and 85° for supination.

• Functional Range:

  • From the mid-position, roughly 100° with 50° for both pronation and supination (Figure 6-27).

Pronation and Supination Mechanics

• Requires motion across both proximal and distal radioulnar joints.

• Limitations at one joint affect the full range of motion.

• Supination Mechanics:

  • For the proximal radioulnar joint, the radial head spins within the fibro-osseous ring.

  • For the distal radioulnar joint, the concave ulnar notch of the radius rolls and glides posteriorly (Figure 6-28).

• Pronation Mechanics:

  • For the proximal radioulnar joint, the radial head spins within the fibro-osseous ring.

  • For the distal radioulnar joint, the concave ulnar notch of the radius rolls and glides anteriorly (Figure 6-29).

Closed Kinetic Chain (CKC) Pronation/Supination

• In a closed kinetic chain, the humerus and ulna rotate around the fixed hand and radius.

  • Starting from a fixed hand in full supination:

  • Pronation occurs due to lateral rotation at the shoulder joint.

  • At the proximal radioulnar joint: the fibro-osseous ring spins around the radial head.

  • At the distal radioulnar joint, the ulnar head rolls anterior and glides posterior (Figure 6-31).

Muscles of the Elbow

Overview of Muscle Groups

• Muscles with distal attachments on the ulna flex and extend the elbow without affecting pronation and supination.

• Muscles with distal attachments on the radius can flex/extend the elbow and/or pronate/supinate the forearm.

Elbow Flexor Muscles

• Muscles that pass anterior to the medial/lateral axis through the elbow joint include:

  • Primary Elbow Flexors:

  1. Biceps Brachii:

     • Originates from supraglenoid tubercle and inserts at the radial tuberosity.

     • Displays maximal EMG activity during elbow flexion with forearm in supination; minimal during flexion while maintaining pronation.

  2. Brachialis:

     • Originates from the distal anterior aspect of the humerus and attaches to the ulnar tuberosity.

     • Known as the “workhorse” for elbow flexion due to its large cross-sectional area and ability to generate force irrespective of forearm position (Figure 6-35).

  3. Brachioradialis:

     • Originating from the lateral supracondylar ridge of humerus and attaching to the lateral surface of the distal radius.

     • Involved in quick elbow flexion maneuvers against high resistance with stabilization influence; max contraction at full elbow flexion and neutral forearm position.

Biomechanical Considerations of Elbow Flexion

• Typical elbow flexion torque is observed to be greater in the dominant limb compared to the non-dominant limb, particularly with respect to extension, pronation, and supination.

• Flexion Torque Ratio:

  • 70% greater than elbow extension torque, with ratios of approximately 20-25% for supination vs. pronation, attributed to increased flexion moment arm of the biceps brachii and brachioradialis in supination.

• Maximum torque for all primary elbow flexors occurs at around 90° of flexion, based on the combined effects of muscle force potential and moment arm length (Figure 6-38).

Law of Parsimony

• The nervous system activates the least number of muscles/muscle fibers necessary at a given task, adhering to the efficiency principle:

  • Lower force demands typically involve one-joint muscles, which are more energy-efficient.

  • Multi-joint muscles engage additional segments to neutralize undesired motions at joints crossed by the muscle, thus resulting in complex movement arrays that are less efficient energetically.

Biceps Brachii Multi-Joint Influence

• Examines the impact of shoulder and elbow positioning on the biceps during flexion:

  • With Shoulder Flexion:

  • Biceps length is approximately 30 cm at rest, ~23 cm at 45° shoulder flexion and 90° elbow flexion, producing a velocity of 7 cm/sec.

  • With Shoulder Extension:

  • When the biceps flexes the elbow, the posterior deltoid contracts to extend the shoulder, reducing biceps shortening to about 25 cm, with a contraction velocity of 5 cm/sec (Figure 6-39).

• Maximum force output is greater as contraction velocity approaches zero, indicating isometric conditions lead to greater force production.

Utility of Reverse Action

• Muscle contraction affects both ends; the fixed end moves toward the more fixed distal end.

• Normal elbow flexion results in movement where the forearm is pulled toward the arm (Figure 6-50).

• Utilizing the fixations—by anchoring the distal forearm— causes the arm to move toward the forearm, useful for individuals with C6 tetraplegia.

Elbow Extensor Muscles

• The muscles that pass posterior to the medial/lateral axis through the elbow joint include:

  • Primary Elbow Extensors:

  1. Triceps Brachii:

     • Originates from the infraglenoid tubercle and attaches to the ulna, it generates considerable torque with the long head providing the largest volume among muscles crossing the elbow joint.

  2. Anconeus:

     • Assists in elbow extension primarily during light duty activities.

• Torque Production:

  • Maximum triceps torque is produced at 90° of elbow flexion, and leverage is greater with the elbow in full extension. However, torque production depends more on muscle length rather than sheer leverage (Figure 6-42).

Forearm Supinator Muscles

• 1° Muscles:

  • Biceps Brachii, Supinator

• 2° Muscles:

  • Extensor Carpi Radialis Longus (ECRL), Extensor Carpi Radialis Brevis (ECRB), Extensor Pollicis Longus (EPL), Extensor Indicis, Brachioradialis (neutral position).

• The Supinator muscle has approximately double the cross-sectional area of the pronator muscles and produces 25% more isometric force.

Supinator Muscle Characteristics

• Originates from the lateral humeral epicondyle, radial collateral ligament, and annular ligament, and inserts at the anterior surface of the proximal radius.

• Activated during lower-level supination activities, regardless of elbow position and speed (Figure 6-45).

Biceps Brachii in Supination

• Originates from supraglenoid tubercle and inserts at the radial tuberosity.

• In pronation, the tendon wraps around the radial head, demonstrating the greatest effect during elbow flexion at 90°.

• At lesser angles, a smaller portion of the force is directed toward supination, effective in high-demand resisted supination activities (turning screws, door knobs) (Figure 6-46).

Supination Torque Calculation

• Calculated using torque equation: $T = B imes IMA$, where B is the force in Newtons and IMA is the moment arm.

• e.g., $T_o = 500 N imes 1 cm$ resulting in a torque of 500 Nm.

• In a 90° elbow flexion state, supination torque can be determined as $T<em>s = T</em>o imes ext{sine}(30°)$ yielding modifications in torque values depending on the elbow position (Figure 6-46).

Supination Activity

• The activation of biceps for shoulder and elbow flexion is offset by the activity of triceps:

  • As the triceps insert into the ulna, they stabilize elbow flexion tendencies without interfering with supination (Figure 6-47).

Forearm Pronator Muscles

• 1° Muscles:

  • Pronator Teres, Pronator Quadratus

• 2° Muscles:

  • Flexor Carpi Radialis (FCR), Palmaris Longus.

Pronator Teres Characteristics

• Originates from the medial humeral epicondyle and coronoid process, attaching to the anterior surface of the midpoint of the radius.

• Functions primarily as a pronator while also serving as a secondary elbow flexor during high-demand pronation tasks (like pitching a baseball); it works synergistically with the triceps to nullify the elbow flexion tendency.

Pronator Quadratus Characteristics

• Situated on the anterior surface of the distal ulna to distal radius, serving as a one-joint muscle.

• Actively fires consistently for pronation and is less influenced by force demands or forearm positioning.