Muscle Description Functions Deltoids Large, three-headed, triangular muscle of the shoulder Rotate and life the arm Biceps Two-headed inner arm muscle extending from the deltoid to below the elbow Lift and curl the arm and to pronate (twist downward) the wrist Trapeziu s Flat triangular muscle of the upper back and neck Lift the shoulder girdle and help turn the head Triceps Three-headed outer arm muscle extending from the deltoid to below the elbow Straighten the arm and supinate (twist upward) the wrist Latissim us Dorsi Large triangular muscles that extend from under the shoulders down to the small of the back Pull the shoulders downward and to the back Forearm Muscles on the outside and inside of the lower arm Control the actions of the hand and wrist Gluteus Large, powerful muscles of the buttocks Maintains the trunk in an erect posture Hamstrin gs The thigh flexors at the rear of the upper leg Curl the leg back Calves Muscles at the rear of the Flex the foot