Sudden swelling and hardening of the calves may be caused by sports strain, renal swelling, lower extremity deep vein thrombosis and other reasons. You should consult a doctor in time to find out the cause of the disease. 1. Exercise strain: due to excessive exercise, overload exercise can cause lactic acid accumulation in the body, muscle strain, which will lead to calf swelling, local muscle hardness; severe strain will also appear capillary rupture, increased exudation. 2. Renal edema: If you suffer from acute nephritis, chronic nephritis or nephropathy, the decrease in renal blood flow, sodium retention, increased capillary permeability, excessive loss of protein in the urine will lead to a decrease in plasma colloid osmolality, tissue aggregation, which will lead to edema, resulting in a sudden swelling of the calves, hardness. 3. Lower extremity deep vein thrombosis: it is a venous return obstacle caused by abnormal coagulation of blood in deep veins. It often occurs in the lower limbs, mostly in people who have less activity or are bedridden. When there is thrombus in the deep veins, the blood will also circulate through the superficial veins, and then superficial varicose veins will appear, which will show that the veins in the lower limbs are more obvious than before, and it can lead to sudden swelling and hardness of the calves. There may be other reasons for sudden swelling and stiffness of the calves, so you should consult your doctor to find out the exact cause and get appropriate treatment if necessary.