Arm Muscles: Understanding the Brachialis and Brachioradialis

What is the Muscle Between the Bicep and Tricep?

The primary muscle located deep to the biceps brachii and anterior to the triceps brachii is the brachialis, a dedicated and powerful elbow flexor. Additionally, the brachioradialis, a significant muscle of the forearm, lies laterally and contributes to the bulk often associated with the "middle" arm region.

The Brachialis Muscle: Anatomy and Function

When one considers the muscles of the upper arm, the biceps brachii and triceps brachii often dominate the conversation. However, tucked away beneath the biceps, acting as a direct and unyielding workhorse for elbow flexion, is the brachialis muscle. This muscle is arguably the most crucial elbow flexor in the human body, playing a pivotal role in nearly every pulling and lifting motion.

• Anatomical Location: The brachialis originates from the anterior surface of the humerus (upper arm bone), specifically from its lower half. It then inserts onto the coronoid process and tuberosity of the ulna, one of the two bones in the forearm. Unlike the biceps, the brachialis has no attachment to the radius, the other forearm bone, which is key to understanding its unique function.
• Primary Action: Pure Elbow Flexion: The brachialis is often referred to as the "true" elbow flexor because its sole action is to bend the elbow joint. It is not involved in supination (rotating the palm upwards) or pronation (rotating the palm downwards) of the forearm, nor does it cross the shoulder joint. This makes it a consistent and reliable contributor to elbow flexion, regardless of forearm position. It acts as the "workhorse" of elbow flexion, providing a significant portion of the force for this movement.

Distinguishing the Brachialis from the Biceps Brachii

While both the brachialis and biceps brachii are elbow flexors, their anatomical attachments and functional roles differ significantly:

• Biceps Brachii: This more superficial muscle has two heads (long and short) originating from the scapula (shoulder blade) and inserting primarily onto the radial tuberosity of the radius. Because it crosses both the shoulder and elbow joints, and attaches to the radius, the biceps is involved in:
  • Elbow flexion (especially with the forearm supinated)
  • Forearm supination (powerful)
  • Weak shoulder flexion
• Brachialis: As discussed, the brachialis originates from the humerus and inserts onto the ulna. Its direct attachment to the ulna means it is solely responsible for elbow flexion and is unaffected by forearm rotation. This makes it active in all forms of elbow flexion, whether the forearm is supinated, pronated, or in a neutral grip. It's particularly dominant when the forearm is pronated, as this position largely deactivates the biceps' ability to flex the elbow effectively.

The Brachioradialis: Another Important Neighbor

Another muscle that contributes significantly to the bulk and function of the "middle" arm region, particularly on the lateral (thumb side) aspect of the forearm, is the brachioradialis.

• Anatomical Location: The brachioradialis originates from the lateral supracondylar ridge of the humerus and inserts onto the styloid process of the radius. While it's primarily a forearm muscle, its origin on the humerus means it crosses the elbow joint.
• Primary Action: The brachioradialis is also an elbow flexor, particularly effective when the forearm is in a neutral (thumb-up) position. It also assists in bringing the forearm to a neutral position from either full pronation or supination. Its contribution to elbow flexion increases with faster movements and heavier loads.

Training Implications and Practical Application

Understanding the distinct roles of the brachialis, biceps brachii, and brachioradialis is crucial for anyone aiming for comprehensive arm development, strength, and injury prevention.

• Targeting the Brachialis: To specifically emphasize the brachialis, exercises that minimize the biceps' contribution or place it in a mechanically disadvantaged position are ideal.
  • Pronated Grip Curls (Reverse Curls): By using an overhand grip, the biceps' supination function is negated, forcing the brachialis to take on a greater load for elbow flexion.
  • Hammer Curls: While also engaging the brachioradialis, the neutral grip places the biceps in a less optimal position for flexion, increasing brachialis activation.
  • Preacher Curls (Pronated Grip Variation): The fixed arm position can further isolate the elbow flexors, and a pronated grip will direct more stress to the brachialis.
• Targeting the Brachioradialis:
  • Hammer Curls: This exercise is excellent for developing the brachioradialis, contributing to the "thickness" of the lower arm.
  • Reverse Curls: While primarily for the brachialis, the brachioradialis also assists.
• Balanced Arm Development: For strong, well-developed arms, it's essential to include exercises that work all three major elbow flexors (biceps, brachialis, brachioradialis) through various grip positions (supinated, pronated, neutral). This ensures balanced strength, improved aesthetics, and functional capacity.
• Injury Prevention and Rehabilitation: A thorough understanding of these muscles helps trainers and therapists identify weaknesses or imbalances that could lead to conditions like golfer's or tennis elbow, or general elbow pain. Strengthening the often-overlooked brachialis can contribute to overall elbow joint stability and resilience.

Conclusion

While often overshadowed by the more prominent biceps brachii, the brachialis muscle is the unsung hero and the primary answer to the question of what lies between the biceps and triceps. Its unwavering commitment to pure elbow flexion makes it a foundational muscle for arm strength. Coupled with the significant contribution of the brachioradialis on the lateral forearm, these muscles collectively create the powerful and versatile arm we rely on daily. Recognizing and specifically training these muscles is key to achieving optimal arm strength, function, and muscularity.