Left leg knee soreness and weakness may be due to cold, squatting for a long time and other physiological factors. It may also be a symptomatic manifestation of some diseases, such as degeneration of the knee joint, low potassium, nerve compression, rheumatic immune diseases, etc., and it is necessary to consult a doctor in time to find out the cause. 1. Physiological factors: When working in a cold environment for a long time, the knee joint is cooled, or squatting for a long time, the tissues around the knee joint are subjected to undesirable stimuli, and a feeling of soreness and weakness will occur, which can usually be improved by resting and warming the knee joints and other self-conditioning methods. 2. Knee degeneration: Due to hyperplasia and degenerative changes in the left knee joint, the strength of the quadriceps muscle will decrease, and soreness and pain will occur during knee activities. 3. Low potassium: As potassium ions are involved in neuromuscular conduction, the diastole and contraction of muscles will be affected by low potassium, which will lead to weakness and tenderness of the left knee. 4. Nerve compression: When there is lumbar disc herniation, the nerve root will be compressed, which will cause the weakness of the left knee joint. 5. Rheumatic immune disease: the more common is rheumatoid arthritis, will cause synovial hyperplasia of the left knee joint, which will lead to joint swelling, tenderness and weakness, and may lead to deformity in severe cases. To sum up, after the left knee joint is sore and weak, exclude physiological factors, consult a doctor in time, identify the cause of the disease and follow the doctor’s instructions to standardize the treatment.