The flexor carpi radialis is a muscle found in the palm side of the forearm. One of eight muscles located here, it is one of the most superficial in that it is close to the skin. Situated alongside the palmaris longus, pronator teres, and flexor carpi ulnaris, this muscle is involved in flexion of the forearm, or curling the palm side of the wrist inward toward the elbow. It also is an abductor of the wrist, meaning that it waves the hand laterally toward the thumb side.
A long and narrow muscle, the flexor carpi radialis is centrally located on the anterior forearm. It originates on the medial epicondyle of the humerus, one of the two rounded bony prominences at the lowest point on the large bone in the upper arm. The medial epicondyle, which is the prominence closest to the trunk of the body, can easily be palpated about an inch above the elbow on the inside of the upper arm.
From here the muscle runs lengthwise down the center of the forearm, just to the outside of the flexor digitorum superficialis. It connects at the bottom of the second metacarpal on the anterior or palm side. The second metacarpal lies beneath the index finger and is one of five long bones contained within the palm of the hand. Connecting the flexor carpi radialis muscle to this bone is a tendon, which is easily visible upon flexing the wrist. One of two large tendons seen here, it is the one nearest the thumb.
As its name suggests, the flexor carpi radialis is a flexor of the wrist or radiocarpal joint, contracting in the forearm to pull the wrist forward. This muscle can be exercised indirectly by performing dumbbell biceps curls, during which it assists in gripping the weight and in maintaining isometric flexion of the wrist. In other words, it helps the wrist to stay straight and not bend backward, or extend, throughout the curl.
Another important function of the flexor carpi radialis is abduction of the wrist. Abduction and its opposing movement adduction are the actions of waving the hand side to side at the wrist. As this muscle is located nearer to the lateral or thumb side of the arm, it is responsible for abduction, or bending the wrist sideways so that the angle between the thumb and forearm decreases. An exercise that works the muscle in abduction is the hammer-grip biceps curl, in which one holds the dumbbell sideways with palm facing inward while curling the weight.