The common causes of swelling and thickening of calves are long-term exercise, phlebitis of the lower extremities, varicose veins of the lower extremities and so on. 1. Long-term exercise: Long-term exercise can lead to thickening of calves and hardening of muscles. If the amount of exercise is too large, it may cause calf muscle spasm and swelling. 2. Phlebitis of the lower limbs: Phlebitis of the lower limbs can cause circulatory disorders in the lower limbs. Thrombophlebitis of the deep lower limbs manifests itself as swelling of the affected limbs, which is aggravated by exertion. When there is wandering superficial lower extremity phlebitis, subcutaneous hard knots or stripes can be touched, and it is painful when pressed. 3. Varicose veins of lower extremity: Varicose veins of lower extremity will be manifested as swelling of calf and bruises under the skin. Swelling and thickening of calves may also be caused by other reasons, and you should go to the hospital in time to find out the cause of the swelling and targeted treatment.