(1) The exercise system consists of bones and skeletal muscles, and bones are connected with bones. Joint is the main form of bone connection. In exercise, the nervous system plays a regulatory role, bones play a lever role, joints play a fulcrum role (also called fulcrum role), and skeletal muscles play a dynamic role.

(2) The two ends of the same skeletal muscle are respectively fixed on two different bones across joints. Skeletal muscle has the characteristic of stimulated contraction. When skeletal muscle contracts under nerve stimulation, it will affect the bones attached to it and move around the joints, so the body will move. Skeletal muscles can only contract and pull the bones but not push them away, so the completion of an action is always completed by the cooperation of elbow flexion and elbow extension.

So the answer is: (1) skeletal muscle.

② ④ Diastolic period? Triceps brachii contraction