Which muscle primarily flexes the arm?

The biceps brachii is primarily responsible for flexing the arm at the elbow joint. It has two heads, the long head and the short head, that originate from different locations on the scapula and converge into a single tendon that attaches to the radial tuberosity of the radius. When contracting, the biceps brachii pulls the forearm upwards towards the shoulder, effectively flexing the arm.

While other muscles, such as the brachialis, also assist in flexion, the biceps brachii is more prominently involved and is often recognized as the main flexor due to its visibility and functionality during exercises like bicep curls. The triceps brachii, in contrast, is a primary extensor of the arm, while the psoas major is primarily involved in hip flexion and does not play a significant role in arm movement at the elbow. Understanding the specific roles of these muscles clarifies why the biceps brachii is accurately identified as the primary muscle for arm flexion.