A 49-year-old man sustains a dislocation of his left elbow that is successfully reduced and splinted. He misses his scheduled follow-up appointments and returns 6 weeks later. He is immediately enrolled in a course of vigorous physical therapy. At a repeat visit at 6 months, examination reveals that he lacks 40 degrees of elbow extension, and has flexion to 80 degrees. He is taken to the operating room for surgical release. Figures A and B are diagrams depicting the ligamentous attachments about the elbow. To restore elbow flexion, in addition to releasing the articular capsule, which ligament should be released?