Which of the following is a muscle that acts on two joints?

The triceps brachii is indeed a muscle that acts on two joints, which are the elbow and the shoulder. Its three heads—long, lateral, and medial—enable it to extend the elbow, while the long head also assists in extending and adducting the shoulder. This multi-joint functionality is significant in various movements, especially in activities that involve both shoulder and elbow actions, such as throwing or pushing.

In contrast, the tibialis anterior primarily acts at the ankle joint, playing a role in dorsiflexion of the foot without influencing any other joints. The upper trapezius primarily functions by elevating and retracting the scapula, also limited to one joint, the shoulder girdle. The anterior deltoid mainly acts on the shoulder joint, contributing to flexion and internal rotation but does not have the biarticular capabilities characteristic of the triceps brachii. Understanding the biomechanical roles of these muscles is crucial for effective training and rehabilitation strategies.