I had one which was specially measured in an Orthopaedic Clinic. If you are lifting people then you should have a Risk Assessment from the Company who is paying you. They should also have shown you how to lift correctly and one should never lift a person alone unless they can assist safely.
Maybe go along to your local physiotherapist or a good Gym Clinic for advice on lifting, posture, foot wear advice. Learning to breath correctly can help in between clients. How is the seat in your car for example, comfy.?