Final answer:
Contraction of the triceps brachii (long head) mainly results in extension of the elbow and can assist in adduction and extension of the arm at the shoulder joint.
Step-by-step explanation:
The contraction of the triceps brachii (long head) is primarily involved in the extension of the elbow, and because the long head originates on the scapula, its contraction can also assist in shoulder joint movements - specifically the adduction and extension of the humerus at the glenohumeral joint. This occurs as the triceps brachii works to counterbalance the pulling force of muscles on the opposite side of the arm. When the triceps brachii contract, the forearm and hand are moved posteriorly (in an anatomical position), resulting in extension at the elbow joint. This movement is in the sagittal (anterior-posterior) plane.
Should we consider movements at the glenohumeral joint, the triceps long head, due to its origin on the scapula, can have a small role in shoulder movements. When the triceps brachii contract, they contribute to the extension of the arm at the shoulder. Furthermore, because of its attachment to the scapula, the long head can also assist in the adduction of the arm, drawing the humerus towards the midline of the body.