What nerve is primarily affected in cases of weakness of forearm flexion and supination?

In cases of weakness of forearm flexion and supination, the musculocutaneous nerve is primarily affected due to its critical role in innervating the flexor muscles of the forearm, particularly the biceps brachii and the brachialis. The biceps brachii is not only responsible for flexing the elbow but also plays a vital role in supination of the forearm.

When the musculocutaneous nerve is damaged, the ability to perform flexion at the elbow and supination of the forearm significantly decreases, leading to noticeable weakness. This nerve travels from the brachial plexus and supplies the muscles in the anterior compartment of the arm, making it essential for these movements.

Other nerves, while involved in arm and hand function, do not primarily affect both flexion at the elbow and supination to the same extent. For instance, the median nerve primarily innervates muscles involved in wrist flexion and finger movements, but not significantly in elbow flexion. The radial nerve is mainly responsible for wrist and finger extension, affecting forearm supination but not flexion. The ulnar nerve largely serves the muscles of the hand and does not contribute to elbow flexion or forearm supination