"Strengthen the hands and knees" sounds like part of a physical therapy syllabus - but the text speaks not of physically disabled people. Rather, a helping hand, often rejected, becomes feeble. And knees, when faced with difficult situations, become weak. But it is really a problem of a fearful (literally "hurried") heart. Isaiah speaks to us who have this problem, more crippling than a broken leg. God, through him, tells us to be strong, unbound, not afraid.
Why shouldn't we fear? God's answer: He is coming with vengeance!? Let's look at the context: Israel was besieged; God's wrath was against not Israel but her enemies. He slew 185,000 Assyrians for Israel, to deliver His people! Now look at our besieging armies - more spiritual than physical. The creation is groaning, a dangerous place burdening humanity with fear. Fear gives way to paralysis. But God tells us not to fear!
Jesus, while on earth, signaled the arrival of God's invading kingdom by reversing physical and spiritual paralysis. As a result of His life-sacrifice, you are a new creation by grace through faith, never to be forsaken by your Father! God has come; He will come again. Therefore actively proclaim and participate in the Gospel of Jesus Christ, God's Son, Savior.
Soli Deo gloria.