The weakness and tenderness of the right leg may be related to the following causes: 1. There may be joint-related lesions of the right lower extremity, and for this symptom, the most common is the lesion of the right knee joint, such as the possible existence of right patellar chondromalacia and osteoarthritis of the right knee joint. This type of disease can manifest itself as weakness and soreness of the entire right lower limb, and can be accompanied by significant pain around the knee joint. 2. This symptom may be caused by peripheral nerve lesions, and is more commonly associated with lumbar disc herniation and sciatic neuritis on the right side. 3. The brain can have a stroke or other diseases. The above-mentioned diseases can also cause unilateral or single limb loss of muscle strength, and feeling weak and tender. 4, may be caused by autoimmune related diseases, such as the presence of myasthenia gravis, Green-Barre syndrome and a series of diseases.