Our oldest son does nothing but play baseball. He doesn’t have a girlfriend either.
You have one muscle, therefore it only does one thing. That's it. It is single mindedly doing that one thing. It is intently and earnestly doing that one thing.
The reading is a bit weird. It's the kun'yomi readings, where 一 is ひと and 筋 is すじ. Look back on the vocab words that use these readings to remember this one.