Analysis of test questions: (1) Skeletal muscles are composed of tendons and abdominal muscles. Skeletal muscles usually span one or several joints and are connected with adjacent bones through tendons. Skeletal muscle has the characteristic of contraction when stimulated. The stimulation of human skeletal muscle comes from the excitement of nerve. When stimulated, muscles contract, which affects the movement of attached bones around joints. When doing gravity upward movement, both biceps brachii and triceps brachii contract.

(2) Joints act as fulcrums in motion.

(3) The outside air reaches the human chest through respiratory movement. After oxygen diffuses into capillaries around alveoli, it reaches the heart through blood circulation, passes through the left atrium and left ventricle of the heart in turn, and finally reaches skeletal muscle cells. In the mitochondria of bone cells, oxygen decomposes organic matter and releases energy for life activities.

(4) The completion of any action is under the control of the nervous system, and with the assistance of other systems, skeletal muscles pull the bones around the joints.