Strength training is only in part about muscular strength and your muscular strength only in part comes from your ability to contract your muscles. The mental strength to workout regularly and the emotional strength to detach from daily events that pull you down influence your muscular strength and they are non-physical.
Your kidney function and your nerve function directly impact your ability to contract your muscles. In severe cases of kidney failure and nerve damage, these relationships become clear. In less than severe conditions, you can measure the impact of your hydration level and your ability to contract your muscles. In various nerve impingement conditions like like Carpal tunnel syndrome, the progression of the nerve damage and muscular atrophy could be observed and measured.
Before we go deeper into the non muscle related strength training lets define the meaning of strength within the laws of physics. Physical strength is our muscular ability to exert force on physical objects. Often in strength training, your exertion of force moves something. That something could be your body weight, dumbbells or an elastic band. You can also exert force without moving an object like pushing a wall.
Strength training implies progression of your ability in increase the amount of force you exert. In body weight training going from modified push-ups on your knees, to full push ups to handstand push-ups is a progression in how much force you generate. The same applies in lifting heavier dumbbells or working with elastic bands with heavier tension.
Without progression to heavier loads, you will not increase the demand on your nervous system and your muscles to restructure and adapt. Adaptions in your nerve signals and your muscle are responsible for your increased strength. Strength training without progression is meaningless.