Forty-year-old weak knees may be related to lack of exercise, knee injury, low blood potassium, etc.. 1. Lack of exercise: when patients keep lying or sitting position for a long time and lack of muscle exercise, it can lead to weak muscle strength and serious lack of lower limb strength, which can cause weak and tender knee joints. 2. Knee joint injury: when patients have meniscus, ligament, joint capsule and other parts of the injury, it can lead to a decrease in the strength and stability of the knee joint, so that the patient appears to have weak knees, and at the same time, it can be accompanied by pain in the knee joint. 3. Low blood potassium: the lack of potassium ions in the blood can lead to fatigue and weakness, and the first symptom can be the lack of muscle power in the lower limbs, which makes the patient feel that his knees are weak and have no strength, and when it is serious, there can be cardiac discomfort. There are many diseases that cause weak knees in forty-year-old patients, it is recommended to go to the hospital in time to improve the relevant examination, to avoid delaying the condition.