Use your model to talk about how muscles move when you stretch your arms. How about stretching your arms?

The muscles in the front and back of the arm are involved in this sport. When the arm bends, the muscles in the front of the arm contract, and the muscles in the front of the arm expand and contract. When the arm is extended, the back of the arm is relaxed.

The two ends of the same skeletal muscle are fixed to two different bones across the joint. Skeletal muscle has the characteristic of stimulated contraction. When skeletal muscle is stimulated by nerve to contract, it will affect the bones attached to it and move around the joints, so the body will move. However, skeletal muscle can only contract and pull bones, but not push them away. Therefore, the completion of an action is always accomplished by the cooperation of elbow flexion and elbow extension. When structure ① contracts biceps brachii, ② contracts biceps brachii. When carrying heavy objects, ① biceps brachii and ② triceps brachii contract. When hands droop naturally, ① biceps brachii and ② triceps brachii relax. The exercise system consists of bones and skeletal muscles, and bones are connected by 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 known as fulcrum role), and skeletal muscles play a dynamic role. So the above example shows that two groups of muscles need to cooperate with each other to do any action.