Well, if you already have back issues then learning to sleep crumpled up in the front seat of an ambulance while at a post for sometimes hours, then ems probably isn't the best choice of profession for you. You mentioned that there are people around to help lift patients and equipment, ya, sometimes that's true, but sometimes it is just you and your partner. Getting your gurney into some broken down old single wide mobile home with no ramp and tight quarters is hard enough. Then throw a patient on your gurney and your equipment and your back hates you at that point. Another thing to consider is your physical relationship to your patient, a lot of times you are bent over or kneeling down trying to assist your patient, not the easiest on your back.
If you really want to do this job, talk to your doc and have him/her get you on a physical therapy program to get your back in top shape before you jump in to ems.