NOTES for Psa 16:5
David says that even when he is confident his path is right, his steps at times are not very firm: his legs tremble. A human being is a complex creature, and resolve in one thing does not cancel fear in another. Thus, the decision to serve one's neighbors does not cancel the fear of pain. And in this prayer David asks God to strengthen him precisely in what he cannot handle by himself. It is very important that we become aware of our weakness, our inability to cope with life without God, and open it to God in prayer. Then God can establish us and grant us His strength, because God's power is made perfect in our weakness (see 2 Cor. 12:9).