Or just: He's in good shape.

natürlich gibt es auch "fit body."

But it's a much less common way to express this idea. I agree with Bill, but you could use a verb:

You need to be in good shape / good physical condition ...
... to take this hike / attempt this event / etc.


To me, 'trained' suggests an additional element of experience and skill, not just physical condition:

Ballet dancers depend on their trained bodies to execute steps quickly, without thinking.

But it's a much less common way to express this idea. I agree with Bill, but you could use a verb:

You need to be in good shape / good physical condition ...
... to take this hike / attempt this event / etc.


To me, 'trained' suggests an additional element of experience and skill, not just physical condition:

Ballet dancers depend on their trained bodies to execute steps quickly, without thinking.

I agree with hm.