I only used that analogy in the sense that if you use a function repeatedly, whether that's the motion of an arm or advanced Calculus, you're more likely to retain the use of that function. If you cease using a function completely, you tend to lose the same sort of fluidity that you previously enjoyed - again, whether that's through muscular atrophy or simply forgetting. Subsequent to that loss, it generally takes a certain amount of 'rehab' (exercises, perhaps gradually increasing in complexity) to regain that ability.
The 'Brain Gym' games, from what I've seen, focus on keeping the player thinking, even on some low level. Me, I prefer the Sunday Times crossword and Sudoku. In ink