Exercise epiphany

My left arm had been hurting for some time. I've been helping a lot with my 2 grandchildren aged 9 months and 2 1/2 years now. Always holding them with my left arm, while I use my right to do things. And then it occurred to me that I was using my arm muscle in one way all the time. I pushed the opposite way and felt immediate relief. Wow!

Now my arm is okay. Sometimes I forget to do the opposite exercise, and it starts hurting a little.

Then I started thinking about my legs. I'm always climbing stairs in this 3-story house we live in. And my legs are pretty strong (carrying 190 lbs.), but they never seem to get really strong so that I don't tire out before running up 2 flights of stairs. And it occurred to me I'm not doing the opposite exercise for my legs either. So I've started doing opposites with an elastic cord and doing leg lifts too like for karate. It seems like my legs feel better too, but it might be all in my head.