Hey,

I've been working with web designers and developers to help with SEO for over decade now. Your question is common.

The short answer is... Google treats sub-domains as separate sites. So, if you go with a sub-domain for each trainer it will be like a fresh new site with no authority, which is not very good for the trainers. Nor will it bring authority to the root domain because these are fresh sites. Having trainers as sub-domains will be the same as having them on a separate domain.

My recommendation is to keep your trainer profiles as sub-directories if it's at all possible.

I hope this is still relevant and the above input helps a little. Feel free to book a call if you need to discuss more this or other cases.

Thanks!
Alex.