The reasons why dogs suddenly have weakness in their hind legs in winter:

1. Long-term calcium deficiency. Calcium deficiency cannot be caused in a day or two. Only if the calcium intake is insufficient for a long time, the dog's body will show calcium deficiency, and one of the symptoms is weakness in the legs.

2. Excessive exercise. If the dog exercises excessively for a long time and exceeds the body's capacity, it will cause certain damage to the legs.

3. Trauma. It is also possible that the dog is suddenly unable to stand and walk due to trauma or fracture, and the hind legs are particularly weak.

4. Low blood sugar. If the dog has not eaten for a long time, the body's blood sugar will be very low, which will make the body weak. It is recommended to feed an appropriate amount of glucose and feed it regularly and quantitatively.

5. Kidney failure. Due to chronic renal failure, dogs lose a large amount of protein in their urine, resulting in a reduction in muscle mass and weakness in the hind legs.

6. Arthritis. Dogs suffering from spondylitis or arthritis may also experience weakness in their hind legs and should be sent to the hospital for treatment in time.