Sudden hardness and pain in the calf may be a muscle spasm in the calf, which may be caused by dehydration, muscle fatigue, etc.; it may also be epilepsy, tetanus and so on. 1. Dehydration, muscle fatigue: dehydration after strenuous exercise, untimely hydration leading to electrolyte disorders, muscle excitability changes; muscle fatigue leading to decreased motor coordination, unreasonable or excessive force, etc., will lead to the patient’s calves suddenly hard and painful. 2. Epilepsy: myoclonic seizures in epilepsy patients can be characterized by rapid, brief, electrocution-like muscle contractions, which can spread throughout the body or be limited to a certain muscle group or a limb, which can lead to sudden stiffness and pain in the calves of the patients. 3. Tetanus: Clostridium tetani multiplies in the wound and produces toxins that cause paroxysmal muscle spasms, causing patients to experience sudden stiffness and pain in the calf. There may be other reasons for sudden stiffness and pain in the calf, so it is recommended that the patient go to the hospital in time to clarify the cause of the disease and carry out standardized treatment.