In our efforts to maintain strength for our physical bodies we must get enough rest, have a healthy diet and be consistent with our exercise regiment. Otherwise, we can become weak and ineffective.  Even with all of our efforts to maintain physical strength, life and the gravity of aging can rob us of that needed strength. 
But when it comes to our journey of faith we find ourselves attempting to use our physical strength to overcome a spiritual conflict. There are spiritual battles going on behind every physical conflict we are faced with. We are incapable of overcoming any spiritual conflict with our human strength or resources. 
We can do all things through Christ who strengthens us! (Phil. 4:13) But that requires that we acknowledge that when we are weak that is when we are strong. When we admit that we are incapable of doing anything outside of the grace of God, it's then that He reveals the surpassing greatness of His power that's within us. 
We must put on the full of armor of God which is represented by the fullness of who Jesus Christ is in us! It is Christ in us that is the hope of glory! It is Christ in us that is the power that overcomes! It is our faith in the Christ who is in us that is the victory! 
Are you living in the power and authority of Christ in you or are you still fighting the battles in your own strength?