Triceps Brachii: Anatomy, Primary, and Secondary Mechanical Functions

What is the mechanical function of the triceps?

The triceps brachii is the primary extensor of the elbow joint, responsible for straightening the arm. Its long head also plays a significant role in shoulder extension and adduction, making it a crucial muscle for a wide array of pushing, pressing, and stabilizing movements.

Anatomy of the Triceps Brachii

The term "triceps" literally means "three heads," referring to the three distinct origins of this powerful muscle located on the posterior aspect of the upper arm. These three heads converge into a common tendon that inserts into the olecranon process of the ulna, a prominent bony projection at the back of the elbow.

• Long Head: Originates from the infraglenoid tubercle of the scapula (shoulder blade). Due to its origin across the shoulder joint, it is the only head that influences both the shoulder and elbow.
• Lateral Head: Originates from the posterior surface of the humerus, superior to the radial groove. It lies superficial to the medial head and is particularly active during movements requiring high force.
• Medial Head: Originates from the posterior surface of the humerus, inferior to the radial groove. It is the deepest of the three heads and is often considered the "workhorse" of the triceps, active in most elbow extension movements, even those requiring low force.

All three heads are innervated by the radial nerve (C6, C7, C8), which enables their coordinated action.

Primary Mechanical Function: Elbow Extension

The most fundamental mechanical role of the triceps brachii is elbow extension. This action involves straightening the forearm at the elbow joint, effectively increasing the angle between the humerus (upper arm bone) and the ulna/radius (forearm bones).

• Concentric Action: When the triceps contracts and shortens, it pulls the ulna towards the humerus, causing the elbow to extend. This is observed when pushing an object away from the body, performing a triceps pushdown, or extending the arm during a bench press.
• Eccentric Action: As the triceps lengthens under tension, it controls the flexion of the elbow, acting as a brake. This is crucial for safely lowering a weight or decelerating the arm, such as during the lowering phase of a push-up or overhead press.
• Isometric Action: The triceps can also contract without changing length to stabilize the elbow joint, maintaining a static position, such as holding an object with an extended arm.

This primary function is vital for all pushing and pressing movements, where the arm must be straightened against resistance.

Secondary Mechanical Functions

While elbow extension is the triceps' primary role, the long head's unique origin allows for additional, albeit secondary, contributions to shoulder joint mechanics.

• Shoulder Extension: The long head of the triceps can assist in extending the arm backward at the shoulder joint. This action brings the humerus posteriorly, as seen during the pull-down phase of a lat pulldown or during the follow-through of a throwing motion.
• Shoulder Adduction: The long head also contributes to adduction, which is the movement of the arm towards the midline of the body. This occurs in conjunction with other adductors like the latissimus dorsi and pectoralis major, especially when the arm is abducted (raised away from the body).
• Joint Stabilization: The triceps, especially its long head, plays a critical role in stabilizing both the elbow and shoulder joints. By maintaining tension across these joints, it helps to ensure proper alignment and prevent excessive movement, particularly during dynamic and high-force activities.
• Antagonistic Role: The triceps acts as an antagonist to the biceps brachii and brachialis, which are the primary elbow flexors. This antagonistic relationship is essential for controlled movement, allowing for smooth and precise actions of the forearm.

Biomechanical Role in Common Movements

Understanding the mechanical function of the triceps is key to appreciating its role in everyday activities and athletic endeavors.

• Pushing Movements: The triceps is the prime mover in exercises like the bench press, overhead press, push-ups, and dips. In these movements, it powerfully extends the elbow to push weight away from the body or to push the body itself.
• Throwing and Striking: In sports like baseball, javelin, or boxing, the triceps is crucial for the acceleration and follow-through phases. Its rapid extension of the elbow contributes significantly to the velocity and power imparted to an object or a punch.
• Everyday Activities: Simple tasks such as pushing open a door, reaching for an object on a high shelf, getting up from a chair, or propelling a wheelchair heavily rely on the triceps' ability to extend the elbow and stabilize the arm.

Clinical and Training Relevance

A comprehensive understanding of the triceps' mechanical functions is indispensable for both clinical professionals and fitness enthusiasts.

• Strength Training: Targeting the triceps with specific exercises (e.g., triceps pushdowns, overhead triceps extensions, close-grip bench presses) is essential for developing upper body pushing strength and muscular hypertrophy. Understanding the role of each head can help in exercise selection for balanced development.
• Injury Prevention and Rehabilitation: Weakness or dysfunction in the triceps can contribute to elbow instability or shoulder issues. Rehabilitative exercises often focus on restoring triceps strength and control after injuries like triceps tendinopathy (inflammation of the triceps tendon) or elbow fractures.
• Performance Enhancement: For athletes, optimizing triceps strength and power directly translates to improved performance in sports requiring pushing, throwing, or striking actions.
• Functional Movement: Strong and well-functioning triceps are fundamental for maintaining independence and performing daily tasks, particularly as individuals age.

Conclusion

The triceps brachii is a multifaceted muscle whose primary mechanical function is the powerful extension of the elbow joint. Its long head further contributes to shoulder extension and adduction, making it a key player in both upper body strength and dynamic movements. From everyday pushing to high-performance athletic endeavors, the triceps' ability to straighten and stabilize the arm is indispensable, highlighting its critical importance in human movement and physical capability.