The deltoid is a triangular muscle located in the shoulder. It is responsible for abduction, flexion, and extension of the arm. The deltoid is connected to the clavicle, scapula, and humerus. Its name comes from the Greek letter delta, which is shaped like a triangle. The TIBIA is a long bone located in the lower leg. It connects the knee to the ankle and is the second largest bone in the body. It is also known as the shinbone.