When was the last time you needed a different air mixture in order to breathe properly? The air is the air.
When was the last time you needed to vary your macronutrients in order to eat healthfully? The protein you need is the protein you need.
If the stimulus to a muscle is sufficient, it is sufficient. And if the stimulus is severe enough to cause adaptation, it stimulates adaptation.
As an example, if you are shot with a bullet, it will pierce your skin and impale you - every time. You will never adapt and become resistant to the bullet. The stimulus to too severe to become resistant to it.
Many fitness experts would have you believe that after a time, an exercise that has stimulated a positive tissue adaptation will no longer stimulate any further benefit because the body has become used to the same stimulus and resists change. They then claim that in order to make continued gains in that muscle or muscle group, a different exercise is required that the body in not used to.
This is nonsense.
If the stimulation to a given muscle is severe enough to recruit and fatigue the fibers deeply enough to cause a positive adaption, it will always cause such adaptation until you reach the limit of your genetic potential.
And yes, genetic potential is an unknown.
If you are working a muscle or group of muscles intensely enough and increasing the resistance even by a fraction at each session, assuming the exercise is a bio-mechanically correct exercise, you're on the right track. Keep it up.
I am NOT saying you shouldn't do different exercises. I am NOT saying you can't alter the order of your routine to spice it up and keep it interesting. Variety is the spice of life and training.
But the notion that your body gets used to an exercise and refuses to adapt because you have been doing it for too long is poppy-cock of the highest order.
I'm off to the Club Industry fitness show at the Jacob Javitz Center in NYC now. I will undoubtedly be bombarded with huge amounts of fitness fantasies. Fun!