Both legs appear double soft weakness, may be caused by the following reasons: 1, there may be some lumbar spine disease compression of bilateral lower limb nerve roots and lead to some pulling of the nerve pain, numbness, weakness and other symptoms, commonly found in the lumbar intervertebral disc herniation, lumbar spinal stenosis or lumbar vertebral spondylolisthesis caused by the. 2, it may be that the day before the symptom, do a long period of strenuous exercise, resulting in localized Lactic acid accumulates in the muscles, and the muscles stimulated by lactic acid will also have localized symptoms such as soreness, pain, and fatigue. But this situation is generally rest 2-3 days after lactic acid metabolism, the symptoms will naturally disappear. 3, there may be double leg muscle strain or aseptic inflammation, this situation is common in the long-term standing, walking or lower limbs to do sports more common.