Explanation:
The ulnar collateral ligament (UCL), particularly its anterior bundle, is the primary restraint to valgus stress at the elbow. This ligament is especially important in overhead throwing athletes such as baseball pitchers, where repetitive valgus forces are generated during the late cocking and acceleration phases of throwing. Chronic stress can lead to attenuation or rupture of the UCL, resulting in pain, instability, and decreased throwing velocity. Surgical reconstruction of this ligament, commonly known as 'Tommy John surgery', has become a well-established treatment for high-level athletes with symptomatic instability. Reference: Jobe FW et al., 'Reconstruction of the ulnar collateral ligament in athletes', Journal of Bone and Joint Surgery (1986).