The humerus is the single bone forming the upper arm, extending from the shoulder to the elbow. It plays a fundamental role in the upper limb, serving as a crucial connector that enables a wide array of movements and provides support for the arm’s structure.
Key Structural Role
The humerus creates a vital link between the shoulder girdle and the forearm. Proximally, its rounded head articulates with the glenoid cavity of the scapula, forming the glenohumeral joint.
Distally, the humerus connects with the two bones of the forearm, the radius and the ulna, to form the elbow joint. The trochlea of the humerus articulates with the trochlear notch of the ulna, while the capitulum articulates with the head of the radius.
Enabling Arm Movement
The connections of the humerus at both the shoulder and elbow joints enable the extensive range of motion characteristic of the human arm. At the shoulder, the ball-and-socket glenohumeral joint, formed by the humerus and scapula, facilitates movements such as flexion (raising the arm forward), extension (moving it backward), abduction (moving it away from the body), and adduction (moving it toward the body). It also permits internal and external rotation, as well as circumduction, which is a circular movement combining these actions. The humerus functions as a lever for these motions.
The humerus’s distal end contributes to the hinge-like movements of the elbow joint. This articulation primarily allows for flexion, which bends the forearm towards the upper arm, and extension, which straightens the arm. Beyond these, the humerus’s connection with the radius and ulna also facilitates the rotational movements of the forearm: pronation, turning the palm downward, and supination, turning it upward. These combined movements highlight the humerus’s direct involvement in the arm’s dynamic capabilities.
Anchor for Muscles and Tissues
Beyond its role in forming joints, a primary purpose of the humerus is to provide extensive attachment sites for numerous muscles, tendons, and ligaments. These soft tissues are crucial for generating the movements observed in the arm. The humerus features distinct landmarks such as its shaft, epicondyles, and tubercles, which serve as specific points for these tissues to originate or insert.
For instance, the four muscles of the rotator cuff—the supraspinatus, infraspinatus, teres minor, and subscapularis—attach to the greater and lesser tubercles of the humerus, enabling various shoulder rotations and stability. Other large muscles like the pectoralis major, latissimus dorsi, and teres major insert into the intertubercular groove on the humerus, contributing to powerful arm movements. The deltoid muscle, responsible for lifting the arm, attaches to the deltoid tuberosity on the humerus shaft.
Muscles that control elbow flexion and extension, such as the biceps, triceps, brachialis, and brachioradialis, either originate from or insert onto the humerus or use it as a stable base for their actions. The robust structure of the humerus is essential for withstanding the considerable forces exerted by these muscles during movement and daily activities. These attachments are fundamental to translating muscle contractions into the precise and powerful actions of the arm.