It sounds like you already know what will be done in the study. If that's not true, make a phone call and find out exactly what's expected. Mention your concern and see what they say.
Researchers examining patients with low back pain report that repeated exams don't increase patients' pain. It seems that back pain is not as variable or changeable as we used to think.
Some scientists think the benefits of noninvasive research of this type outweigh any risks to the patient. They base this view on results of many studies already done. Some volunteers may have to drop out of the study because of increased pain or symptoms. The chances of this happening are very low and often can't be predicted ahead of time.