Thigh and calf muscle weakness may be caused by physiological factors such as excessive fatigue, or it may be caused by pathological factors such as muscle atrophy, lumbar intervertebral disc herniation, osteoporosis, etc. Generally, it may appear as clinical manifestations such as softness of the walking leg, easy to fall down, and instability of the knee joints. 1. Physiological factors: if strenuous exercise or prolonged heavy labor and other factors, can lead to fatigue of the leg muscles, the accumulation of lactic acid in the body, resulting in weakness of the lower limbs, accompanied by muscle pain, which can be relieved by appropriate rest. 2. Lumbar disc herniation: Due to the herniation of lumbar intervertebral discs, the nerve root is compressed, which can cause weakness of one side of the thighs and calves, accompanied by pain in the lower back and radiating pain in the lower limbs. 3. Osteoporosis: mostly seen in the elderly, long-term insufficient intake of calcium or reduced absorption can lead to osteoporosis, the main clinical manifestations of generalized fatigue, muscle weakness of the limbs, pain in the lower back and limbs and other symptoms. Thigh and calf muscle weakness may be caused by many other reasons, it is recommended that patients go to the hospital in a timely manner, after clarifying the cause of the disease, under the guidance of the doctor to give targeted treatment.