The basic structure of joint includes articular surface, articular capsule and articular cavity. The articular surface is divided into articular head and articular fossa, and there are tough ligaments inside and outside the articular capsule. The bones that make up a joint are at least composed of a joint head and a joint fossa. When carrying heavy objects, the biceps brachii and triceps brachii are contracted. Because the ligaments of joints are very strong, they also play an important role in carrying heavy objects. When the fingertip of a finger accidentally touches a high-temperature object, there will be a hand-shrinking reflex. The nerve center of this reflex is located in the spinal cord, and the process of hand retraction reflex is equivalent to elbow flexion, so the triceps brachii relaxes and the biceps brachii contracts.

So the answer is: (1)2.

(2) Ligament

(3) Three relaxed and six contracted the spinal cord.