First of all, it's hypothetical. If only for the fact that using a wheelchair in real-life actually greatly increases difficulty getting around. The point is that if you don't lose your muscles, they get weaker. Happens with anyone that has an accident that causes them to be unable to use their muscles for a prolonged amount of time. See also: Astronauts needing to exercise while in space or back on earth because they've not had to use many of their muscles as much due to lack of gravity.