They grow stiff in the joints. They get in a rut. They go to seed.
Just getting from the airport straight to the gym is gonna be your best bet to knock off that rust and wake yourself up and get that blood flowing. You sit in a car or a plane cramped up, you lose a lot of blood flow and get that swelling in your legs. There are all kinds of dangers in sitting still too long.
There's some muscle group around your shoulders that seizes up during the perfection dance and doesn't let go until you are asleep, or alone. Or maybe it never really lets go at all.
The general definition of being muscle-bound is that you have so many muscles that you can't move freely. I don't know of any bodybuilder in that category; in fact, many of them are quite active in other sports.
The only time I have a problem is when I have to get in a vehicle after we play and sit there in a cramped position for a couple of hours to drive to the next place. Then I get super stiff.
In some ways, cramp is worse than having a broken leg.
Botox, trust me I've been tempted - but I resist! Think about what happens to your muscles - and your skin - if you're sick and don't move for a few days.
I've got plenty of arthritis. But if you keep moving, it won't bother you that much. That's why old guys stiffen up. They forget they have to get out of their chairs and do something. You let the moss grow over, it's your own fault.