There is a scientific training concept called "Specific Adaptation To Imposed Demands". It means that whatever you do with your body, if it is done regularly, your body will adapt to it to reduce the amount of effort needed to perform the same task. Without SAID we would never get any better at most of the physical activities we do, for work or for enjoyment. Want to run a marathon? It's never going to happen if your body doesn't adapt so those first 5 miles don't kill you every time. Same for performing a good tennis serve, driving a golf ball or cycling the Paradise Loop (in Main, CA. It's a beautiful 44 mile ride from San Francisco!).
However, there is a flip side to this body mechanism. It means that unless you vary the way you're exercising, your body gets used to it and your workouts become less and less effective with each repetition.
You see this alot in the gym with people doing their cardio "workouts". They'll choose one piece of equipment and do only that equipment every workout, for the same amount of time with the same effort. No matter what your level of effort, if you don't mix it up, if you don't challenge your nervous system, your body will get used to it and you'll be spinning your wheels like a hamster.
This also goes for any other way you train. I have a client who does lots of yoga and GYROTONIC® but doesn't do any weight training. I decided to take her out to the training floor and do a weight training workout with her. The lightest weights were wiping her out. She kept saying "I can't do this." and "Oh, this is too hard." to which I responded "You're doing it right now." and "Good job." Although she felt like she was in pretty good shape from doing yoga for such a long time, her muscles couldn't handle the most basic weight training.
Is she unhappy with me for making her do something she found very uncomfortable and not pleasant? I'll let her words to the talking:
Thanks, You are a good and conscientious trainer, constantly challenging me. Hard work.Thanks M.
She will grow stronger and her body will very quickly get used to the workout I made up for her. In 4 weeks, I'll be changing her workout. She'll hate doing it but she'll trust me and she'll be glad when her body feels and performs better at all times... all because I wouldn't let her doing what she was comfortable doing.