Understanding Voluntary Muscles: Biceps, Quadriceps & Deltoids

Biceps brachii - located on the front of the upper arm, this muscle is responsible for bending the elbow

Quadriceps femoris - located on the front of the thigh, this muscle is responsible for straightening the knee

Deltoid - located on the shoulder, this muscle is responsible for lifting the arm away from the body, as well as rotating the arm inward and outward