I would highly recommend Pablo Nejamis: http://personaltrainerinba.com. I worked with him for over 6 months, based on a recommendation from another expat who has worked with him for 3+ years. He is very knowledgeable and attentive, customizing your workout based on your personal needs and goals. He speaks both Spanish and English. I met with him at the barrio Notre Megatlon, although there may be other available location options.