A 52-year-old man is brought to the emergency room after being found in the park where apparently he had lain overnight after a fall. He complains of severe pain in the left arm. Physical examination suggests that he has a broken humerus, which is confirmed radiologically. The patient can extend the forearm at the elbow, but supination appears to be somewhat weak; the hand grasp is very weak when compared with the uninjured arm. Neurologic examination reveals an inability to extend the wrist ("wrist drop"). Since these findings point to apparent nerve damage, the patient is scheduled for a surgical reduction of the fracture. Two sets of movements occur at the elbow: flexion-extension and supination-pronation. The number of degrees of freedom at the radiohumeral joint is …