services a PTA provides

Physical therapy assistants assist physical therapists in providing care to patients. Their regular work is to provide exercise, instruction, electrical stimulation, mechanical traction and observing the radiology images. They also serve massaging and train the patients on gait and balancing. Physical therapy assistants also record the patient's responses to treatment. The following site would give you more details. http://physicaltherapycolleges.org/
During the beginning of their practice physical therapy assistants work under the direction and supervision of physical therapists. But gradually with experience they learn to perform the work independently.