The information provided here is not a replacement for a doctor. You shouldn't use it for the purpose of self-diagnosing or self-medicating but rather so you can have a more informed discussion with a professional TCM practitioner.
In Chinese Medicine weak back can be treated by one herbs that we have on record. Here is the list below.
Because it is a herb specifically indicated to treat weak back as can be seen on Eucommia Leaf's page.
Its main actions are: Strengthens the Liver and the Kidneys