Forearm Movement: Understanding Supination, Its Muscles, and Exercises

What is the external rotation of the elbow muscle?

The term "external rotation of the elbow" is an anatomical misnomer. The elbow joint primarily performs flexion and extension. Rotational movements, correctly termed supination (outward rotation) and pronation (inward rotation) of the forearm, occur at the radioulnar joints, which are distinct from the humeroulnar joint (true elbow).

Understanding Elbow vs. Forearm Rotation

To accurately address the query, it's crucial to distinguish between the movements of the elbow joint and the forearm.

• Elbow Joint (Humeroulnar Joint): This is a hinge joint formed by the humerus and ulna. Its primary actions are:
  • Flexion: Bending the elbow (e.g., bringing hand to shoulder).
  • Extension: Straightening the elbow (e.g., pushing hand away).
• Radioulnar Joints (Proximal and Distal): These are pivot joints that allow the radius to rotate around the ulna. These joints are responsible for:
  • Supination: The outward rotation of the forearm, turning the palm upward or forward (e.g., turning a doorknob clockwise with the right hand). This is often what people mistakenly refer to as "external rotation of the elbow."
  • Pronation: The inward rotation of the forearm, turning the palm downward or backward (e.g., turning a doorknob counter-clockwise with the right hand).

Therefore, when discussing "rotation of the elbow," the correct anatomical term for the outward turning movement is forearm supination.

The Primary Muscle for Forearm Supination: The Supinator

The Supinator muscle is the primary muscle responsible for forearm supination, particularly when the elbow is extended or during slow, unresisted movements.

• Anatomy: It is a broad, two-headed muscle located in the posterior compartment of the forearm, deep to other muscles.
  • Origin: Lateral epicondyle of the humerus, radial collateral ligament, annular ligament, and the supinator crest of the ulna.
  • Insertion: Anterior, lateral, and posterior surfaces of the proximal third of the radius.
• Action: It wraps around the radius, and when it contracts, it pulls the radius to rotate laterally, resulting in supination of the forearm.
• Innervation: Radial nerve (posterior interosseous nerve branch).

Synergistic Muscles in Forearm Supination

While the supinator is the primary muscle, others assist, especially when the movement is resisted or when the elbow is flexed.

• Biceps Brachii: This powerful muscle, known for elbow flexion, is also a strong supinator, particularly when the elbow is flexed to 90 degrees or more. Its tendon inserts on the radial tuberosity, and its line of pull becomes more effective for supination as the elbow flexes.
  • Anatomy: Originates from the scapula (short and long heads) and inserts onto the radial tuberosity and bicipital aponeurosis.
  • Action: Flexes the elbow and supinates the forearm.
  • Innervation: Musculocutaneous nerve.

Muscles NOT Involved in "Elbow External Rotation" (but often confused)

It's important to clarify that muscles responsible for shoulder external rotation are entirely different and act on the glenohumeral (shoulder) joint, not the elbow or forearm. These include:

• Infraspinatus
• Teres Minor
• Posterior Deltoid

Confusing these movements can lead to ineffective training or misunderstanding of anatomical function.

Functional Importance of Supination

Forearm supination is a fundamental movement critical for countless daily activities and athletic endeavors:

• Eating: Bringing a spoon or fork to the mouth.
• Drinking: Lifting a glass to the lips.
• Tool Use: Turning screwdrivers, opening doorknobs.
• Sports: Throwing (e.g., baseball pitch), racket sports (e.g., tennis serve), weightlifting (e.g., supinated grip bicep curls).
• Balance and Coordination: Fine motor control of the hand and wrist.

Exercises to Strengthen Supinators

To strengthen the muscles responsible for forearm supination, exercises should focus on rotating the forearm against resistance.

• Dumbbell Supination:
  • Hold a light dumbbell with an overhand (pronated) grip, palm facing down.
  • Rest your forearm on a table or your thigh, allowing your hand to hang off the edge.
  • Slowly rotate your forearm outward, turning your palm upward (supinating) as far as comfortable.
  • Control the return to the starting position.
• Cable Supination:
  • Using a cable machine with a D-handle, grasp the handle with a pronated grip.
  • Perform the supination movement, focusing on the rotation of the forearm.
• Bicep Curls (Supinated Grip):
  • While primarily a bicep exercise, performing bicep curls with a fully supinated grip (palms facing up throughout) emphasizes the supinating action of the biceps.
  • Hammer curls (neutral grip) and pronated grip curls will reduce the supination demand on the biceps.

Common Misconceptions and Clarifications

The most common misconception is the conflation of "elbow rotation" with "forearm rotation." Always remember:

• Elbow: Flexion and Extension only.
• Forearm: Supination and Pronation (via the radioulnar joints).
• Shoulder: External and Internal Rotation (via the glenohumeral joint).

Precision in anatomical terminology is vital for effective communication, accurate diagnosis, and targeted exercise programming.

Conclusion

While there isn't a single "external rotation of the elbow muscle" in the literal sense, the movement most closely aligned with this description is forearm supination. The primary muscles responsible for this crucial action are the Supinator and, particularly when the elbow is flexed, the Biceps Brachii. Understanding these distinctions is fundamental for anyone interested in human movement, from fitness enthusiasts to clinical professionals, ensuring that exercise and rehabilitation efforts are anatomically sound and effective.