Hi. Having worked with several physical therapists and providing training to them, I totally agree with Karin that the skill-sets for being a good therapist and being a good trainer are different. While the knowledge base that you bring to the table for things such as anatomy and physiology is top-shelf as a licensed physical therapist, the skill-set in program design and training is different than what you may be used to, or trained for with a healthy versus rehabilitation client. Certification as a personal trainer or group exercise instructor is a must. While I agree with some of the other posters here about ACE, and ACSM I feel that they are leaving out another GOLD STANDARD certifying agency – NSCA (National Strength and Conditioning Association) which is a leader in both sports conditioning and personal training certifications. They provide much of the research based knowledge and information espoused by the training community and are another good option to those given by others here.