The defender remains on the outside of Martin's plans this season after failing to get a summer move away.
"We've had a few playing already but I never force anyone to play in the under 21s," Martin told the Daily Echo.
"I think we can have a conversation about it and always make sure they understand why we want them to, but it's also their prerogative to say no.
"They're not being brought here to play for the under 21s. Some people think that's right, some people think that's wrong but they're men - they're human beings.
"They have a choice. Sometimes I'll tell them I think it's for the best they play to try and get them back in the team, to keep fitness or coming back from injury.
"They always have a say in it - so the guys who have played it's always been their choice, they've been given the choice and they've made the choice to play."