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That would depend....

Are you also wearing trousers?

Lol yup. Serious inquiries only!

I was serious. If you're not wearing trousers then I'm not sure about you training me.

To be honest, I'm not that bothered what t-shirt you wear. But from a marketing perspective, if you are training though a gym then you should be wearing whatever the gym uniform is.

If you are freelancing then you should be wearing whatever makes your clients feel good about themselves. Wear something that fits their needs.

So for some people a muscle shirt might be appropriate, for others a loose shirt that actually hides your physique might be better. And for others something that gives them a giggle might be what it takes to get them into the zone and train well.

If you are going to train people, then everything is about them, your ego plays no role in it. Use that as a guideline and you'll do fine :-)