Back fork belongs to a symptom of back pain, which may be caused by acute sprain, muscle strain, instability of small joints of vertebrae, muscle spasm and so on. 1. Acute sprain: sudden lifting of heavy objects or lifting heavy objects, improper exertion caused by acute lumbar back sprain, will lead to back pain. 2. Muscle strain: prolonged weight-bearing, long-term sitting in a poor posture may cause local muscle ligament or nerve damage, resulting in back pain and other symptoms. 3. Vertebral small joint instability: in extreme bending or rotational activities, resulting in small joint dislocation in the chest and back, jamming the joint capsule, radiating pain or pinprick-like pain symptoms. 4. Muscle spasm: when the back is stimulated by cold air from outside, it may cause muscle spasm, which can also lead to back pain. Back fork, there may be other reasons, need to go to the hospital in time to check, clear cause.