Flexors

Flexors are muscles that produce flexion, decreasing the angle between bones at a joint. In anatomy the term refers to muscles that bring the bones closer together, typically along the limbs, and may also describe tendinous structures that assist bending. Flexion is usually opposed by the extensors, which straighten a joint.

Upper limb: Forearm flexors reside in the anterior compartment of the forearm. They commonly share a common flexor origin on the medial epicondyle of the humerus. Key muscles include flexor carpi radialis, flexor carpi ulnaris, palmaris longus, flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus, and pronator teres. They primarily flex the wrist and/or the fingers, and many are innervated by the median nerve, with the flexor carpi ulnaris and part of flexor digitorum profundus supplied by the ulnar nerve.

Lower limb: Hip flexors such as iliopsoas (iliacus and psoas major), rectus femoris, tensor fasciae latae, and sartorius bend the hip and contribute to thigh flexion. Knee flexors are the hamstrings: biceps femoris, semitendinosus, and semimembranosus, which bend the knee and also assist in hip extension. The gastrocnemius crosses the knee and ankle and can assist knee flexion when the foot is dorsiflexed.

Clinical and functional notes: Flexors enable gripping, walking, and many daily movements. Damage to flexor muscles, tendons, or their nerves can impair bending of joints. The term “flexor” is used generically for any structure that bends a joint and is distinguished from “extensor” muscles that straighten joints.