I visited my mom in the hospital last night (yes she is in the hospital again [H1N1]; she is on the back side of it, but still has some breathing issues). The last time I visited she told me that she felt useless. That statement pierced my soul. Of course she can't move much of her body, she is very weak, she has to rely on people to do most everything for her - by all PHYSICAL accounts she is useless.
My encouragement to her was that in God's economy, she was not useless. While she may feel useless, God was not finished with her yet. The fact that she was alive was enough to prove that God still had a purpose for her - He still had a mission for her to accomplish.
While not really being sure what this mission is, I am convinced that as long as we are on this planet and able to function, the Great Commission in Matthew 28 applies to us. I encouraged my mom by saying that in this phase of her life she may experience her greatest ministry yet - having faith in the midst of very trying life circumstances; of holding onto the truth that God is at work through her even though life circumstances will not allow much physical movement.
This is what walking by faith looks like. Believing that God is at work even when we can see it or feel it. Believing that God is at work IN and THROUGH us, even when we can't "produce". Staying committed to Him and doing, thinking and living like Christ will put us in a position of being used by Him - whether we realize it, whether we feel it, whether we see it or not.