The product of millions of years of evolution, the arm is a remarkable machine, beautifully demonstrating the fusion of form and function. When our ancestors outgrew walking on all fours, the arm adapted into the highly specialized extremity we know today. The precise arrangement of muscles connecting the bones of the arm finely regulate its motion and allow us to execute complex movements like pitching a baseball or cooking a meal.
Elbow Movement
The elbow is a hinge joint, and its motion is controlled by two muscle groups: extensors and flexors. The triceps are the main muscles of elbow extension, allowing us to straighten our arms. The primary flexors of the elbow are the biceps but also include the brachialis, brachioradialis, and pronator teres. Flexion and extension should be considered the essential motion of the elbow.
Wrist Movement
Capable of motion more complicated than the elbow, the wrist is designed to move in multiple directions. The arrangement of muscles in your wrist and forearm allows precise positioning of your hand, an important feature of the arm for decidedly human behaviors, such as the use of tools or utensils. Muscles originating in your forearm stretch long distances, extending their tendons out into your fingers, thus controlling flexion and extension of your wrist and fingers.
Upper Arm and Shoulder
The bone that connects your forearm to your shoulder is known as the humerus. Muscles from your shoulder, chest and back project connections onto this bone, creating multiple planes of motion. For instance, when you raise your arms up from your sides, as you would while doing jumping jacks, this motion is known as abduction of the humerus. Similarly, when you drop your arms to the side, you are adducting the humerus. These two motions are opposites and require the activation of separate muscle groups.
Pronation and Supination
Besides the simple act of flexion and extension, several muscles in the arm are capable of rotating bones of the arm around a longitudinal axis. For instance, when you rotate your hand and arm to collect your change after a purchase, you have just supinated your forearm. Similarly, rotating your arm in the opposite direction to pocket the change requires pronation of the forearm. Your humerus and forearm are both capable of these motions, adding another dimension to the variety of motions the human arm can execute.
References
- "Illustrated Essentials of Musculoskeletal Anatomy"; Kay Sieg and Sandra Adams; 2009
- "DeGowin's Diagnostic Examination: The Spine, Pelvis and Extremities"; R. F. LeBlond, et.al.; 2010
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