Lumbar disc herniation can cause weakness in the legs because the herniated lumbar disc mainly compresses the nerve roots and the dural sac at the site of herniation, the main function of the nerve that is to innervate the sensation and muscle movement of the body. Severe compression and herniation of the nerve root or dural sac will result in muscle weakness in the corresponding nerve root innervation area. When a lumbar 4/5 disc herniates, it often compresses the lumbar 5 nerve root, which innervates the dorsal extension of the bunion, and most commonly also compresses the lumbar 5 root. Prolonged and more severe compression will result in decreased dorsiflexion strength of the bunion, and compression of the lumbar 4 nerve root will result in weak dorsiflexion of the ankle joint. There is a clinical grading of muscle strength, and a professional spine surgeon will determine the location of the compressed nerve root and the severity of the compression. In conclusion, a herniated lumbar disc can cause muscle weakness, but muscle weakness due to a herniated lumbar disc needs to be judged by a specialist spine surgeon depending on the location and severity of the compression, and also needs to be differentiated from muscle weakness due to peripheral nerve compression. The identification of weakness caused by lumbar disc herniation or weakness caused by peripheral nerve entrapment requires appropriate physical examination and electromyography.