Final answer:
The joint action at the shoulder during biceps and triceps exercises is flexion and extension, where the biceps help in flexing and the triceps in extending the elbow joint.
Step-by-step explanation:
When performing biceps and triceps exercises, the joint action that should occur at the shoulder is flexion and extension. The biceps brachii muscle aids in flexing the forearm and also helps in flexing the arm at the shoulder. When the biceps contract, they pull the forearm up, reducing the angle at the elbow joint, producing a flexing motion. In contrast, when the triceps contract, they extend the elbow by increasing the angle between the ulna and the humerus, helping to straighten the arm. This alternation of contraction between the biceps and triceps allows for a full range of motion during exercises that target these muscles.