It's can be due to compression of the nerve root at the L4 spinal segment, a spinal cord injury affecting that segment, or secondary to a stroke affecting the higher centers in the brain that are responsible for controlling the muscles that raise the foot up. there are a few possible causes.

Many causes, amongst them Multiple Sclerosis. Remedy depends on diagnosis.

What is a drop foot? Injury to the anterior leg muscles or nerve along the lateral knee (peroneal nerve) can impair ones ability to elevate the foot. Weakness or inability to lift the foot is frequently referred to as a ‘foot drop' and can significantly impair normal walking. How is foot drop treated? Treatment is determined by the cause of the impairment. A foot drop produced by pressure or trauma to the motor nerve are treated with regard to the degree of injury and potential for spontaneous recovery. Mild pressure or stretching of the peroneal nerve can produce a temporary impairment of local circulation that interrupts normal nerve conduction. This is referred to as neuropraxia and usually resolves within days to weeks with return of good function. More substantial trauma can lead to destruction of the nerves electrical conduction cells (axons). If their biologic insulation and tube like pathways remain undamaged then axon regeneration and spontaneous recovery can be anticipated within several months. This process of nerve injury and regeneration is referred to as axonotemesis. When all the nerve components are damaged then an obstructing scar develops and little to no recovery will develop without surgical intervention. Impaired nerve function can also result from local compression. The surgical release of constrictive tissue (nerve decompression) can increase circulation within the nerve improving its ability to conduct electrical signals. Severely damaged nerve segments can also be reconstructed using segments of expendable sensory nerves. The damaged segments are identified with intraoperative nerve conduction studies. The nonfunctional segments are removed and replaced with healthy nerve grafts. The grafts are secured into place using an operating microscope and stitches finer than a hair (microneurosurgery) and provide a biologic guide for regenerating nerve cells. Drop Foot (Foot Drop) and Steppage Gait (Footdrop Gait) Drop Foot and Foot Drop are interchangeable terms that describe an abnormal neuromuscular (nerve and muscle) disorder that affects the patient's ability to raise their foot at the ankle. Drop foot is further characterized by an inability to point the toes toward the body (dorsiflexion) or move the foot at the ankle inward or outward. Pain, weakness, and numbness may accompany loss of function. Walking becomes a challenge due to the patient's inability to control the foot at the ankle. The foot may appear floppy and the patient may drag the foot and toes while walking. Patients with foot drop usually exhibit an exaggerated or high-stepping walk called Steppage Gait or Footdrop Gait. Drop Foot/Foot Drop: A Symptom Drop foot is not a disease but a symptom of an underlying problem. Depending on the cause, drop foot may be temporary or permanent. Often drop foot is caused by injury to the peroneal nerve deep within the lumbar and sacral spine. The peroneal nerve is a division of the sciatic nerve. The peroneal nerve runs along the outside of the lower leg (below the knee) and branches off into each ankle, foot, and first two toes. It innervates or transmits signals to muscle groups responsible for ankle, foot, and toe movement and sensation. Peroneal Nerve: Causes of Injury The peroneal nerve is susceptible to different types of injury. Some of these include nerve compression from lumbar disc herniation (e.g. L4, L5, S1), trauma to the sciatic nerve, spondylolisthesis, spinal stenosis, spinal cord injury, bone fractures (leg, vertebrae), stroke, tumor, diabetes, lacerations, gunshot wounds, or crush-type injuries. Drop foot is found in some patients with Amyotrophic Lateral Sclerosis (ALS), Multiple Sclerosis (MS), and Parkinson's Disease. Sometimes the peroneal nerve becomes injured when stretched during hip or knee replacement surgery. Proper Diagnosis of Underlying Cause The diagnostic process includes a comprehensive assessment of the patient's symptoms, past and current medical histories, physical and neurological examinations, imaging studies such as MRI (magnetic resonance imaging), and EMG (electromyogram). The spine specialist must determine the cause of drop foot before formulating a treatment plan.