Albert Barnes Commentary Isaiah 35:3

"Strengthen ye the weak hands, and confirm the feeble knees." — Isaiah 35:3 (ASV)

Strengthen ye - That is, you who are the religious teachers and guides of the people. This is an address made by the prophet in view of what he had said and was about to say of the promised blessings. The sense is, strengthen and sustain the feeble and the desponding by the promised blessings; by the assurances (Isaiah 34:0) that all the enemies of God and his people will be destroyed, and that he will manifest himself as their Protector and send the promised blessings upon them.

Or it may be regarded as addressed to the officers and ministers of religion for when these blessings would come, and as an exhortation to them to use the influences, the promises, and the consolations that would attend the coming of the Messiah, to strengthen the feeble and confirm those who were faint-hearted.

The weak hands, and confirm the feeble knees - Strength resides mainly in the arms and in the lower limbs, or the knees. If these are feeble, the whole frame is feeble. Fear relaxes the strength of the arms and the firmness of the knees; and the expressions weak hands and feeble knees become synonymous with a timid, fearful, and desponding frame of mind.

Such were to be strengthened by the assurance of the favor of God and by the consolations which would flow from the reign of the Messiah. The Jews, who looked out upon the desolations of their country, were to be comforted by the hope of future blessings; those who lived in those future times were to be consoled by the assurances of the favor of God through the Messiah (compare the notes at Isaiah 40:1).