The possible causes of weakness and tenderness of the calves below the knee include overwork, hypokalemic periodic paralysis, nerve root or spinal cord compression, and so on. 1. Over-exertion: It may be caused by over-exertion, such as strenuous sports or prolonged heavy physical labor, which causes muscle strain in the lower limbs, and then the symptoms of calf weakness may appear. 2. Hypokalemic periodic paralysis: may be suffering from hypokalemic periodic paralysis. Patients with hypokalemic periodic paralysis may have muscle weakness in the limbs and weakness in the calves below the knee. 3. Nerve root or spinal cord compression: Numbness and weakness of the lower limbs may be caused by nerve root or spinal cord compression, resulting in nerve root or spinal cord injury, which can be seen in spinal cord cervical spondylosis, lumbar intervertebral disc herniation, lumbar spinal stenosis and other diseases. Weakness of the lower legs below the knee may also have other reasons, it is recommended to go to the regular hospital in time, improve the relevant examination, clear the cause of the disease, under the guidance of the doctor to carry out targeted treatment or treatment.