What is the upper arm and lower arm? An anatomical guide to distinction

September 29, 2025 •

4 min read

According to anatomical definitions, the arm is specifically the region between the shoulder and the elbow. Understanding this primary distinction is key to answering the common health and fitness query, what is the upper arm and lower arm?

Quick Summary

The upper arm, known as the brachium, is the segment between the shoulder and elbow, housing the single humerus bone. The lower arm, referred to as the forearm or antebrachium, is the region from the elbow to the wrist and contains two bones, the radius and ulna.

Key Points

Upper Arm (Brachium): The region between the shoulder and elbow, containing the single humerus bone and primary muscles like the biceps and triceps.
Lower Arm (Forearm): The area between the elbow and wrist, containing the radius and ulna bones, responsible for fine motor control.
Single vs. Paired Bones: The upper arm has one bone (humerus), while the lower arm has two (radius and ulna) that can rotate relative to each other.
Functional Specialization: The upper arm focuses on powerful movements like flexion and extension, whereas the lower arm controls the delicate movements of the hand and fingers.
Movement of Rotation: Pronation and supination (turning the palm up or down) are complex rotational movements made possible by the unique two-bone structure of the lower arm.

Unpacking the Upper Arm (Brachium)

The upper arm, or brachium, is the proximal portion of the upper limb. It extends from the shoulder joint to the elbow joint. This region is fundamentally defined by one long bone and a set of powerful muscles that enable a wide range of movements, particularly at the elbow.

The Bone of the Upper Arm: The Humerus

The humerus is the long bone of the upper arm. Its rounded head at the top fits into the glenoid cavity of the scapula (shoulder blade), forming the shoulder joint. At its lower end, the humerus articulates with the radius and ulna to form the elbow joint. This single, robust bone serves as a crucial anchor for the muscles that move both the shoulder and elbow.

Key Muscles of the Upper Arm

The muscles of the upper arm are divided into two primary compartments by the humerus itself, with one compartment at the front (anterior) and one at the back (posterior).

Anterior (Flexor) Compartment:

Biceps Brachii: The most well-known arm muscle, responsible for flexing the elbow and supinating (turning palm up) the forearm.
Brachialis: Lying beneath the biceps, this muscle is a major flexor of the elbow joint.
Coracobrachialis: Helps flex and adduct (bring closer to the body) the arm at the shoulder.

Posterior (Extensor) Compartment:

Triceps Brachii: This three-headed muscle is the primary extensor, or straightener, of the elbow joint.

Exploring the Lower Arm (Forearm or Antebrachium)

Extending from the elbow to the wrist, the lower arm is known anatomically as the forearm or antebrachium. This region is built for versatility, with two bones that cross over each other and a complex network of muscles that control the wrist, hand, and fingers.

The Bones of the Forearm: Radius and Ulna

Unlike the upper arm's single bone, the forearm contains two bones that run parallel to each other: the radius and the ulna.

Ulna: Located on the medial side of the forearm (the pinky-finger side). The olecranon process at its upper end forms the bony point of the elbow.
Radius: Located on the lateral side of the forearm (the thumb side). The radius rotates around the ulna, which is essential for movements like turning a doorknob.

Major Muscle Groups of the Forearm

The forearm muscles are grouped into anterior (flexor) and posterior (extensor) compartments.

Anterior (Flexor) Compartment: These muscles primarily flex the wrist and fingers.

Superficial Layer: Includes muscles like the flexor carpi ulnaris and palmaris longus.
Deep Layer: Consists of muscles such as the flexor digitorum profundus.

Posterior (Extensor) Compartment: These muscles extend the wrist and fingers.

Superficial Layer: Includes the brachioradialis and extensor carpi radialis longus.
Deep Layer: Includes the extensor pollicis longus and supinator.

Functional Differences and Importance

The distinct structures of the upper and lower arm correspond to their different functional roles. The upper arm, with its larger, simpler muscles and single bone, is designed for powerful movements at the shoulder and elbow. The lower arm, with its complex array of smaller muscles and two bones, provides the fine motor control necessary for the intricate movements of the wrist and hand. The ability of the radius to pivot around the ulna in the forearm is a prime example of this functional specialization, allowing for pronation and supination.

Upper Arm vs. Lower Arm: A Comparative Table


Feature	Upper Arm (Brachium)	Lower Arm (Forearm / Antebrachium)
Location	Between the shoulder and the elbow.	Between the elbow and the wrist.
Primary Bone(s)	Humerus (one bone).	Radius and Ulna (two bones).
Key Muscles	Biceps Brachii, Triceps Brachii, Brachialis.	Flexors and Extensors of the wrist and fingers.
Primary Function	Powerful elbow flexion and extension.	Fine motor control, wrist/finger movement.
Pronation/Supination	Limited role; primarily acts as a stable base.	Facilitates these rotation movements via radius pivoting around ulna.
Key Joints	Shoulder (Glenohumeral) and Elbow (Humeroulnar/Humeroradial).	Elbow (partially) and Wrist (Radiocarpal/Midcarpal).

Conclusion: A System of Specialized Parts

To answer what is the upper arm and lower arm?, one must understand that these are two distinct, yet interdependent, anatomical regions. The upper arm, anchored by the humerus and controlled by large muscles like the biceps and triceps, provides the power and leverage for basic arm movements. The lower arm, with its dual-bone structure and intricate muscle network, provides the dexterity and fine motor control for manipulating objects. Both parts are essential components of the overall function of the upper limb, with their specialized structures perfectly suited for their unique roles in lifting, carrying, and manipulating the world around us. For more detailed information on human anatomy, visit the National Center for Biotechnology Information (NCBI) website.

Frequently Asked Questions

Yes, the humerus is the single, long bone that runs from the shoulder to the elbow, forming the core structure of the upper arm.

The lower arm, or forearm, contains two bones: the radius and the ulna. The radius is on the thumb side, and the ulna is on the pinky side.

The upper arm is mainly for power movements like bending and straightening the elbow. The lower arm, with its complex muscle and bone structure, provides the finer dexterity for wrist, hand, and finger movements.

The main muscles of the upper arm are the biceps brachii on the front, which flexes the elbow, and the triceps brachii on the back, which extends it.

Understanding the distinction is crucial for proper injury diagnosis and targeted exercises. For example, a fracture of the humerus (upper arm) is treated differently than a fracture of the radius or ulna (lower arm).

The scientific or anatomical term for the lower arm is the forearm, or antebrachium. The upper arm is the brachium.

Yes, the types of injuries can vary based on the bones and muscle groups involved. Upper arm injuries often relate to the shoulder or elbow joint, while lower arm injuries frequently affect the wrist and hand, such as carpal tunnel syndrome or a Colles' fracture of the distal radius.