It states (68:35-36) עוז נותן הוא ...לאלקים עוז תנו לעם ותעצומות, "Give strength to Hashem... It is He who grants might and power to the nation..." One of the tzaddikim of our generation asked that it seems like a contradiction. First, we say that we give strength to Hashem, keviyachol, and then it states that Hashem gives strength to His nation!
He answers that people often excuse themselves with the excuse, "I don't have strength. I can't." We tell them, לאלקים עוז תנו, do what you can, and then Hashem will give you strength. Hashem will help you.
We can also explain לאלקים עוז תנו, "Give strength to Hashem," in the following manner: Obviously, Hashem doesn't need our strength. Nevertheless, the system Hakadosh Baruch Hu placed in the world is that there needs to be a דלמטה התעוררותא, arousal from people down below. Hashem wants to see us make an effort, and then He will help us succeed.
