David's cry to know his frailty isn't just about physical weakness; it's about understanding his very essence in God's sight.
The Hebrew word translated as "frail" here carries a sense of 'ceasing' or 'coming to an end.' David isn't just acknowledging he's not strong; he's recognizing his transient existence, his very being as something that will cease. He understands that before God, human life, even at its prime, is "vanity" – a vapor, a shadow. This isn't a complaint about God's harshness, but a realization that can strip away pride and self-reliance, leading to a humble dependence on God.