It's relative.
The hard work of an orchestra isn't so much physical as intellectual. Professional musicians are so practiced that the notes on the page are second nature to them. Everything is a scale or a rhythm or a fingering pattern that's been met and practised hundreds, if not thousands of times during their career.
Playing those notes at exactly the right time, tempo and dynamic in conjunction with forty or fifty other musicians is another matter. Some pieces are harder to do this with than others, for various reasons both technical and due to the ability of the players and conductor (it's why a school orchestra will never play 'Happy Birthday' quite as well as the Berlin Philharmonic).
Watching my conductor as he leads us through Leroy Anderson's 'Christmas Festival' or 'Bugler's Holiday', though, I would guess that's where most of the physical work is done - that and the percussionists as they race back and forth along the back row.